Formation of the glycan chains in the synthesis of bacterial peptidoglycan (2022)

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Volume 11 Issue 3

1 March 2001

Article Contents

  • Abstract

  • Introduction

  • Peptidoglycan polymerization in intact cells, membranes, crude cell walls, permeabilized cells, or protoplasts

  • Correlation between transglycosylation and transpeptidation

  • Peptidoglycan multimodular bifunctional polymerases

  • Monofunctional glycosyltransferases

  • Mechanism of the transglycosylation reaction

  • In vivo functioning of the peptidoglycan glycosyltransferases

  • Inhibitors of the transglycosylation reaction

  • Concluding remarks

  • Acknowledgments

  • Abbreviations

  • References

  • < Previous

Journal Article

Jean van Heijenoort

Jean van Heijenoort

Institut de Biochimie, Bat 430, Université Paris-Sud, Orsay, F-91405, France

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Glycobiology, Volume 11, Issue 3, 1 March 2001, Pages 25R–36R, https://doi.org/10.1093/glycob/11.3.25R

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01 March 2001

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    Jean van Heijenoort, Formation of the glycan chains in the synthesis of bacterial peptidoglycan, Glycobiology, Volume 11, Issue 3, 1 March 2001, Pages 25R–36R, https://doi.org/10.1093/glycob/11.3.25R

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Abstract

The main structural features of bacterial peptidoglycan are linear glycan chains interlinked by short peptides. The glycan chains are composed of alternating units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc), all linkages between sugars being β,1→4. On the outside of the cytoplasmic membrane, two types of activities are involved in the polymerization of the peptidoglycan monomer unit: glycosyltransferases that catalyze the formation of the linear glycan chains and transpeptidases that catalyze the formation of the peptide cross-bridges. Contrary to the transpeptidation step, for which there is an abundant literature that has been regularly reviewed, the transglycosylation step has been studied to a far lesser extent. The aim of the present review is to summarize and evaluate the molecular and cellullar data concerning the formation of the glycan chains in the synthesis of peptidoglycan. Early work concerned the use of various in vivo and in vitro systems for the study of the polymerization steps, the attachment of newly made material to preexisting peptidoglycan, and the mechanism of action of antibiotics. The synthesis of the glycan chains is catalyzed by the N-terminal glycosyltransferase module of class A high-molecular-mass penicillin-binding proteins and by nonpenicillin-binding monofunctional glycosyltransferases. The multiplicity of these activities in a given organism presumably reflects a variety of in vivo functions. The topological localization of the incorporation of nascent peptidoglycan into the cell wall has revealed that bacteria have at least two peptidoglycan-synthesizing systems: one for septation, the other one for elongation or cell wall thickening. Owing to its location on the outside of the cytoplasmic membrane and its specificity, the transglycosylation step is an interesting target for antibacterials. Glycopeptides and moenomycins are the best studied antibiotics known to interfere with this step. Their mode of action and structure–activity relationships have been extensively studied. Attempts to synthesize other specific transglycosylation inhibitors have recently been made.

Key words: peptidoglycan/transglycosylation/penicillin-binding proteins/monofunctional glycosyl-transferases/ lycopeptide and moenomycin antibiotics.

Accepted December 22, 2000;

Introduction

Peptidoglycan (or murein) is a continuous covalent macromolecular structure located on the outside of the cytoplasmic membrane of almost all eubacteria, and it is found exclusively in these organisms (Schleifer and Kandler, 1972; Rogers et al., 1980). Its main function is to preserve cell integrity by withstanding the internal osmotic pressure. It is also responsible for the maintenance of a defined cell shape and is intimately involved in the cell division process (Nanninga, 1998). Its absence from cells will in a hypotonic medium lead to swelling and to the rupture of the cytoplasmic membrane. Under certain conditions, cells lacking peptidoglycan can be maintained as protoplasts, or spheroplasts, but they lose their shape and cell division is impeded or greatly perturbed. The main structural features of this giant macromolecule are linear glycan chains interlinked by short peptides (Figure 1). The glycan chains are composed of alternating units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc), all linkages between sugars being β,1→4. The carboxyl group of each N-acetylmuramic acid residue is substituted by a peptide subunit, which is most often L-alanyl-γ-D-glutamyl-diaminopimelyl(or L-lysyl)-D-alanyl-D-alanine in nascent peptidoglycan and which subsequently loses one or both D-alanine residues in mature peptidoglycan. Neighboring glycan chains are interlinked either by a direct peptide linkage between a peptide subunit of a chain with one of another chain or by a short peptide bridge between two peptide subunits.

The biosynthesis of peptidoglycan has been investigated in various organisms, and an overall view valid for both Gram-positive and Gram-negative bacteria has emerged (Rogers et al., 1980; Ward, 1984; Bugg and Walsh, 1992; Matsuhashi, 1994; van Heijenoort, 1994, 1998). The first stage concerns the assembly of the disaccharide-peptide monomer unit via a series of UDP precursors and lipid intermediates (Figure 2). Six cytoplasmic steps (mediated by MurA to MurF) lead to the formation of the UDP-MurNAc-pentapeptide precursor from UDP-GlcNAc. Thereafter, the transfer of the phospho-MurNAc-pentapeptide moiety of UDP-MurNAc-pentapeptide to the membrane acceptor, undecaprenyl phosphate, is catalyzed by transferase MraY and yields lipid I. The addition of GlcNAc to lipid I by transferase MurG leads to lipid II, which carries the complete disaccharide peptide monomer unit: GlcNAc-MurNAc-L-Ala-γ-D-Glu-A2pm(or L-Lys)-D-Ala-D-Ala. In the course of the formation of the lipid intermediates the peptide subunit can undergo various modifications (amidation, addition of extra aminoacids, etc.). The final lipid intermediate is transfered by an unknown mechanism through the hydrophobic environment of the membrane to the externally located sites of incorporation of the monomer unit into growing peptidoglycan.

The second stage of peptidoglycan synthesis concerns the polymerization of the monomer unit on the outside surface of the cytoplasmic membrane and the binding of newly made material to the preexisting cell wall (Figure 2). The comparison of the structure of peptidoglycan with that of its precursors was the key in understanding the main aspects of the biochemical mechanisms underlying peptidoglycan polymerization. Two major types of membrane-bound activities are involved: glycosyltransferases1 that catalyze the formation of the linear glycan chains and transpeptidases that catalyze the formation of the peptide cross-bridges. In growing cells, polymerization reactions are accompanied by concomitant or subsequent structural modifications of peptidoglycan. Its closed covalent structure must continuously adjust to the requirements of surface growth and cell division. Each organism possesses a variety of specific peptidoglycan hydrolases and certain of them are responsible for the structural adjustments (Shockman and Höltje, 1994; Smith et al., 2000).

As targets of the β-lactam antibiotics and owing to their implication in certain mechanisms of resistance to these drugs, the transpeptidases have been extensively investigated. In particular, their active site has been well characterized (Ghuysen, 1991; Ghuysen and Dive, 1994). On the contrary, the transglycosylation step has been studied to a limited extent, although it is the initial key step in the assembly of the macromolecule from its monomer unit. This review is an attempt to bring together the essential molecular and cellular data with respect to the transglycosylation step. Early work dealing with the study of the polymerization reactions in various in vivo and in vitro systems is briefly summarized. Such approaches have been widely employed for studying the correlations between transglycosylation and transpeptidation, the attachment of newly synthesized material to preexisting peptidoglycan and the mechanisms of action of cell wall–targeted antibiotics. The main other topics of the review will concern: (1) the glycosyltransferases catalyzing the formation of the glycan chains; (2) the mechanisms of the transglycosylation reaction; (3) the in vivo functioning of the peptidoglycan glycosyltransferases; and (4) the inhibitors of the transglycosylation step.

Peptidoglycan polymerization in intact cells, membranes, crude cell walls, permeabilized cells, or protoplasts

In intact cells the synthesis of peptidoglycan has been followed by the incorporation of radiolabeled peptidoglycan constituents. Diaminopimelic acid has been used mostly in Escherichia coli (see references in Höltje and Schwarz, 1985 and Höltje, 1998; Wientjes et al., 1985; Mengin-Lecreulx et al., 1989; Prats and de Pedro, 1989), in a few other Gram-negative bacteria (Hebeler and Young, 1976; Rosenthal, 1979; Greenway and Perkins, 1985), and more rarely in Gram-positive bacteria (Mauck et al., 1971; Fuchs-Cleveland and Gilvarg, 1976). Lysine has been restricted to Gram-positive bacteria (Boothby et al., 1971; Mirelman et al., 1974), whereas N-acetylglucosamine has been used both in Gram-positive (Tynecka and Ward, 1975; Cheung et al., 1983; Sinha and Neuhaus, 1991; Pooley and Karamata, 2000) and Gram-negative (Martin and Gmeiner, 1979; Greenway and Perkins, 1985; Rosenthal, 1979; Rosenthal and Dziarski, 1994; Payie and Clarke, 1997) organisms. Less frequently have glutamate (Mauck et al., 1971; Hoffmann et al., 1974; Keglevic et al., 1974), alanine (Wise and Park, 1965; Waxman et al., 1980; Rosenthal, 1994 and references therein), glycine (Wise and Park, 1965), and acetate (Dezélée and Shockman, 1975) been used. Peptidoglycan contents can also be followed by chemical quantification of a specific constituent, such as A2pm (Mengin-Lecreulx and van Heijenoort, 1985). All these methods imply a separation of the polymerized material from its precursors.

In vitro peptidoglycan-synthesizing activity has been elicited with membrane preparations from various organisms (Chatterjee and Park, 1964; Meadow et al., 1964; Anderson et al., 1966; Izaki et al., 1966; Araki et al., 1966; Plapp and Strominger, 1970; Reynolds, 1971; Wickus and Strominger, 1972; Hammes and Neuhaus, 1974; Pellon et al., 1976; van Heijenoort et al., 1978; Kamio et al., 1982; Kraus et al., 1985). Crude cell walls from Gram-positive organisms retain to some extent the functional spatial interrelationships between the membrane and the cell wall and were found to be particularly appropriate for the in vitro study of the polymerization reactions (Mirelman and Sharon, 1972; Mirelman and Sharon , 1972; Ward, 1974; Ward and Perkins, 1974; Hammes and Kandler, 1976; Weston et al., 1977; Kalomiris et al., 1982). Ether- or toluene-permeabilized cells (Schrader and Fan, 1974; Mirelman et al., 1977; Brown and Perkins, 1979; Mirelman and Nuchamowitz, 1979; Giles and Reynolds, 1979; Maass and Pelzer, 1981; Metz et al., 1983; Kraus et al., 1985; Pisabarro et al., 1986) as well as protoplasts (Elliott et al., 1975; Rosenthal and Shockman, 1975) have also been used.

In these various systems nucleotide precursors UDP-GlcNAc and UDP-MurNAc-pentapeptide, one or the other being radiolabeled, have generally been employed as substrates, thereby implying a synthesis of the lipid intermediates prior to polymerization. In a few cases, the pentapeptide nucleotide was replaced by UDP-MurNAc-tri-, tetra-, or hexa-peptide (Izaki et al., 1968; Plapp and Strominger, 1970; Hammes and Kandler, 1976; Pisabarro et al., 1986). Isolated lipid II has also been used as substrate with membrane preparations (Strominger et al., 1966; Izaki et al., 1968; Taku and Fan, 1976; Kamio et al., 1982). An assay specific for transglycosylation was developed by using purified lipid II, E. coli membranes, and penicillin as transpeptidation inhibitor (van Heijenoort et al., 1978).

Incorporation of radioactivity has been taken as a measure of peptidoglycan polymerization and the resultingpolymerized material has been characterized in different ways: mobility on paper chromatography, solubility or insolubility in SDS or TCA, structural analysis, glycan chain length, and extent of cross-linking. It should be stressed that the efficiency of these systems is generally very low when compared to peptidoglycan synthesis in growing cells: For instance, the in vitro specific polymerizing activity accounted for 0.1 to 1% of the in vivo level in Staphylococcus aureus (Anderson et al., 1966) and for 2% in E. coli (Kraus et al., 1985). However, a high specific activity was reported with membranes from Gaffkya homari (Hammes and Neuhaus, 1974). Furthermore, the structure of the in vitro polymerized material can differ in many ways from that of native peptidoglycan (Kraus et al., 1985). It must thus be cautioned that results obtained with such systems could reflect imperfectly the true in vivo processes.

Correlation between transglycosylation and transpeptidation

In normally growing bacteria, transglycosylation catalyzing the formation of the glycan chains and transpeptidation catalyzing the cross-linking between peptide subunits are continuous, tightly coupled reactions. The problems of their correlation and of the attachment of newly synthesized material to preexisting peptidoglycan have been investigated with numerous in vivo and in vitro systems. The conclusions drawn from these studies (Rogers et al., 1980; Ward, 1984) can be briefly summarized as follows.

1.Transglycosylation can proceed independently from transpeptidation as exemplified by the formation of uncross-linked or low cross-linked soluble peptidoglycan in various cell-free systems and in protoplasts. Furthermore, the treatment of growing cells or cell-free peptidoglycan-synthesizing systems with β-lactam antibiotics, which are specific inhibitors of the transpeptidation reactions, results in the formation of soluble uncross-linked peptidoglycan material. In a few cases it has been shown that the soluble material can function as an intermediate in the synthesis of cell wall–linked peptidoglycan.

2.The question remains as to whether nascent peptidoglycan is present or not as soluble material in normally growing cells. It has been proposed that perhaps small amounts of nascent uncross-linked peptidoglycan are transiently synthesized prior to cross-linking (Ward, 1984). It should be stressed that when considering low levels of soluble peptidoglycan material it is difficult to distinguish between true nascent material and possible autolytic degradation products.

3.Conversely, a transpeptidation reaction without prior or concomitant formation of glycan chains has been reported only in very special cases (Kraus et al., 1985) or with model peptides (Lepage et al., 1997). Moreover, the specific inhibition of transglycosylation by moenomycin-type antibiotics does not lead to any polymerization by transpeptidation (Huber, 1979 and references therein; Suzuki et al., 1980). In peptidoglycan the glycan chains are generally quite longer than the peptide chains, and only some of the monomer units are involved in cross-bridges. This makes the addition of monomer units to preexisting peptidoglycan by transpeptidation and their subsequent interlinking by transglycosylation very unlikely.

Taken together, these facts suggest that in growing cells the polymerization of the monomer unit proceeds essentially by tranglycosylation, which precedes cross-linking to the cell wall. Transpeptidation is responsible not only for the formation of cross-bridges in the new material but also for the transfer of newly made material to preexisting peptidoglycan. It follows glycan chain formation or is at best concomitant with it. The possibility of some attachment of newly made material to old cell wall by transglycosylation has not been entirely ruled out (Mirelman and Sharon, 1972; Weston et al., 1977). The average length of the glycan chains synthesized in intact cells or in cell-free systems can vary from 10 to over 100 disaccharide units. The length distribution of the glycan chains was determined in E. coli (Harz et al., 1990).

Peptidoglycan multimodular bifunctional polymerases

Virtually all eubacteria possess a set of minor membrane proteins designated as penicillin-binding proteins (PBPs) that are the specific targets of the β-lactam antibiotics and that are involved in the late steps of peptidoglycan synthesis (references in Ghuysen, 1991; Ghuysen and Dive, 1994). They are detected by their ability to covalently bind radiolabeled penicillin and the stability of the penicillin-protein complexes has greatly facilitated their study. PBPs vary from species to species in number, size, amount, and affinity for β-lactam antibiotics. High-molecular-mass PBPs (HMM-PBPs) are essentially two-domain proteins that belong either to class A or class B, depending on the structure and the catalytic activity of their N-terminal domain (Goffin and Ghuysen, 1998). The C-terminal domain of both classes is responsible for transpeptidation activity and β-lactam antibiotics covalently bind to its catalytic center. In class A HMM-PBPs, the N-terminal domain is responsible for their glycosyltransferase activity, whereas in class B the N-terminal is presumably involved in interactions with other membrane proteins (Marrec-Fairley et al., 2000). Therefore, class A HMM-PBPs are bifunctional enzymes capable of catalyzing both transglycosylation and transpeptidation. About 30 class A HMM-PBPs from both Gram-positive and Gram-negative bacteria have now been identified on the basis of genome sequence data (Goffin and Ghuysen, 1998). Their non-penicillin-binding domains are a continuum of diverging sequences. The alignments of their amino acid sequences revealed five conserved motifs.

PBP1b from E. coli has been by far the most investigated class A HMM-PBP in terms of location, membrane topology, purification, and glycosyltransferase activity. The organization of this membrane-associated polymerase was analyzed by studying β-lactamase fusion proteins (Edelman et al., 1987) and by immunoelectron microscopy (den Blaauwen and Nanninga, 1990). PBP1b contains a 63-amino acid N-terminal cytoplasmic tail, followed by a 24-amino acid transmembrane domain and a 757-amino acid periplasmic region with the transglycosylation and transpeptidation domains clearly separated by an inert linker (Nakagawa et al., 1984; Lefèvre et al., 1997). The existence of a well-defined transglycosylation domain was futher substantiated by the production of truncated forms retaining glycosyltransferase activity (Nakagawa et al., 1984; Terrak et al., 1999). An additional membrane association site not apparent by hydropathy analysis was localized in the first 163 aminoacids of the periplasmic N-terminal glycosyltransferase domain of a form lacking the cytoplasmic and transmembrane domains (Wang et al., 1996). A similar situation was also more recently observed in the glycosyltransferase domain of PBP2a from Streptococcus pneumoniae (Di Guilmi et al., 1999). In the cell envelope of E. coli, PBP1b is present as a monomer and as dimers, one of which is closely associated with peptidoglycan (Zijderveld et al., 1995). Evidence has been brought by immunoelectron microscopy for its localization at the adhesion sites of the envelope (Bayer et al., 1990). Such a clustered distribution of peptidoglycan-polymerizing activity was further substantiated by fractionation procedures (Ishidate et al., 1986). The periplasmic localization of the transglycosylation domain is in agreement with a polymerization of the disaccharide-peptide monomer unit taking place on the outside of the cytoplasmic membrane.

To date, the overproduction and purification of class A polymerases with a biochemically characterized glycosyltransferase activity have been restricted essentially to the three present in E. coli: PBP1a (Tamura et al., 1980; Ishino et al., 1980), PBP1b (Tamura et al., 1980; Suzuki et al., 1980; Nakagawa et al., 1984; van Heijenoort et al., 1992; Terrak et al., 1999), and PBP1c (Schiffer and Höltje, 1999). The alignment of their sequences covering both the transglycosylation and transpeptidation domains revealed a clear homology between all three PBPs (Schiffer and Höltje, 1999). When assayed with lipid II as substrate and penicillin as transpeptidation inhibitor, their specific glycosyltransferase activity was greatly dependent on the presence of a solvent (CH3OH, DMSO, 1-octanol) and/or a detergent (decyl-PEG, sodium deoxycholate, etc.) required for the solubilization of both the substrate and the enzyme. Various properties of E. coli PBP1b, in particular kinetic parameters, were studied (Nakagawa et al., 1984; Terrak et al., 1999). In the best assay yet described (Terrak et al., 1999), the reaction proceeds with an efficiency of 39,000 M–1 s–1. The accessibility to reasonable amounts of lipid II remains a critical point for the development of enzymatic studies. Its preparation is a tedious affair that has been carried out only in a few cases (see references in: Umbreit and Strominger, 1972; Hara and Suzuki, 1984; van Heijenoort et al., 1992). Other class A polymerases have been purified: PBP1a and PBP2a from S. pneumoniae (Di Guilmi et al., 1998, 1999). However, appropriate glycosyltransferase assays remain to be developed for them. A polymerase purified from Bacillus megaterium that catalyzed the in vitro synthesis of uncross-linked peptidoglycan was tentatively identified as PBP4, but it is not yet known if it belongs to class A polymerases (Taku et al., 1982).

Monofunctional glycosyltransferases

A number of membrane-bound, non-penicillin-binding, monofunctional glycosyltransferases (Mgt) capable of catalyzing only the formation of uncross-linked peptidoglycan were found in E. coli (Hara and Suzuki, 1984), Micrococcus luteus (Park and Matsuhashi, 1984), S. aureus (Park and Matsuhashi, 1984), and S. pneumoniae (Park and Burman , 1985). They were solubilized, partially purified and characterized. They accounted for a large part of the in vitro measurable glycosyltransferase activity determined with lipid II as substrate. More recently, in a variety of bacterial strains (E. coli, Haemophilus influenzae, Klebsiella pneumoniae, Neisseria gonorrhoeae, Ralstonia eutropha, S. aureus) genes encoding monofunctional glycosyltransferases were detected on the basis of sequence homology with genes encoding class A HMM-PBPs (Spratt et al., 1996; Di Berardino et al., 1996; Paik et al., 1997). In E. coli overexpression of the the mgt gene led to a significant increased in vitro peptidoglycan-synthesizing activity (Di Berardino et al., 1996). A soluble form of the R. eutrophamgt gene product was obtained, thereby opening the way to a more easy approach to a structural analysis of a peptidoglycan glycosyltransferase domain (Paik et al., 1997).

Mechanism of the transglycosylation reaction

The exact mechanism of the transglycosylation reaction is still not known. In the periplasm the linear assembly of the glycan chains presumably proceeds by the repetitive addition of dissaccharide peptide units either at their reducing end (1 in Figure 3) or at their nonreducing end (2 in Figure 3). In the first case, the growing glycan chain attached to undecaprenyl pyrophosphate is the glycosyl donor substrate and is transferred to the 4-OH group of the GlcNAc unit of lipid II, which is the glycosyl acceptor substrate. This mechanism has in fact been established for transglycosylation in a few Gram-positive organisms (Ward and Perkins, 1973; Fuchs-Cleveland and Gilvarg, 1976; Weston et al., 1977). It is noteworthy that lipopolysaccharide O-antigene polymerization proceeds in a similar manner (Raetz, 1996). The other possibility is that lipid II is functioning as the donor and its disaccharide-peptide is transferred to the 4-OH group of the GlcNAc of the growing chain, which is the acceptor and is not necessarily bound to undecaprenyl pyrophosphate. In the in vitro polymerization reaction with lipid II and purified PBP1b, chain elongation is initiated in the absence of any growing chain (Suzuki et al., 1980; Nakagawa et al., 1984), suggesting that lipid II can be recognized by both the donor and acceptor sites of the enzyme and that even with an elongation at the nonreducing end the growing chain must be initially bound to undecaprenyl pyrophosphate. However, both isolated mature peptidoglycan and various forms of nascent peptidoglycan are generally devoid of any linkage to undecaprenyl pyrophosphate. Only in a few cases was such a linkage found (Ward and Perkins, 1973; Fuchs-Cleveland and Gilvarg, 1976; Thorpe and Perkins, 1979). Therefore, some enzymatic process leading to the release of the glycan chain from undecaprenyl pyrophosphate must take place.

Very little is known about the structure of the catalytic center of peptidoglycan glycosyltransferases. Unfortunately, crystallization of a purified form of E. coli PBP1b has not led to the elucidation of its three-dimensional structure (Ishino et al., 1988). However, the 3D structures of four N-acetylmuramidases (three lysozymes and transglycosylase Slt70) involved in the breakdown of the peptidoglycan glycan chains are now known (Thunnissen et al., 1995). They have the same catalytic machinery and the overall packing of the secondary structure elements of their catalytic centers is similar. By the cleavage of β-1,4-N-acetylmuramyl-N-acetylglucosamine bonds and the release of disaccharide peptide units, they each catalyze a reaction that is the reverse of the polymerization reaction. Thus, they were helpful for shedding light on the organization of the catalytic center and mechanism of E. coli PBP 1b (Terrak et al., 1999). In the transglycosylation reaction it is likely that the sugar units of both lipid II and of a large piece of the growing chain are recognized by multiple sites of the polymerase, as already established in a number of glycosyltransferases that recognize both a glycosyl donor and a glycosyl acceptor (Kosmol et al., 1997 and references therein). These sites could be related to the binding sites of the peptidoglycan N-acetylmuramidases, which have several subsites each for one sugar unit of peptidoglycan (Thunnissen et al., 1995). The peptide moiety of the monomer unit plays a minor role in the recognition process as exemplified with shorter or modified peptide subunits (see references in van Heijenoort and Gutmann, 2000).

A speculative transition state involving the formation of an oxicarbenium cation was proposed for the peptidoglycan glycosyltransferases (Kosmol et al., 1997; Ritzeler et al., 1997; Terrak et al., 1999). The transglycosylation reaction is an SN2 displacement leading to an inversion of configuration at C1, from the α-configuration in the precurseur to the β-configuration in the glycosylated nucleophile acceptor (Figure 4). From the analysis of residues that are central to catalysis in the N-acetylmuramidases, the role of possible homologous residues of PBP1b was investgated (Terrak et al., 1999). Conserved dicarboxylic amino acid residues are probably elements of the glycosyltransferase catalytic center. It was proposed that in E. coli PBP1b the Glu-233 γCOOH donates its proton to the oxygen atom of the scissile phosphoester bond of the growing chain, leading to the formation of an oxicarbenium cation that undergoes attack by the 4-OH group of a nucleophile N-acetylglucosamine. Asp-234 and Glu-290 could be involved in the stabilization of the oxicarbenium cation and the activation of the 4-OH group.

In vivo functioning of the peptidoglycan glycosyltransferases

In a given organism there are several peptidoglycan glycosyltransferases (Goffin and Ghuysen, 1998). For instance, there are four in E. coli (PBP1a, PBP1b, PBP1c, and Mgt) and four in S. pneumoniae (PBP1a, PBP1b, PBP2a, and Mgt). Presumably, this multiplicity corresponds to a variety of functions. However, not all peptidoglycan glycosyltransferases are essential. In E. coli PBP1c and Mgt can be deleted, whereas either PBP1a or PBP1b must be present (Suzuki et al., 1978; Spratt, 1983; Di Berardino et al., 1996; Denome et al., 1999). In S. pneumoniae class A PBPs 1a, 1b, and 2a are not individually essential but PBPs 1a and 2a together are (Hoskins et al., 1999; Paik et al., 1999). It has been proposed that nonessential peptidoglycan glycosyltransferases could functionally contribute to the permeability characteristics of the cell envelope (Dijkstra and Keck, 1996). They could be involved in localized peptidoglycan rearrangements accompanying the assembly of envelope-spanning complexes and various transfer processes through the envelope.

Considering that peptidoglycan is a continuous structure intricately associated with other macromolecules of the cell envelope, its assembly must proceed in coordination with other cell syntheses and also with the different steps of the cell cycle, such as elongation and septation. It is thus a highly complex process that is not readily accessible to biochemical analysis, and it involves both spatial and temporal controls. The topological localization of the incorporation of nascent peptidoglycan has been analyzed by various labeling experiments, mostly in E. coli (see leading references in Höltje, 1998) and by electron microscopy in cocci (Shockman et al., 1974). Bacteria have at least two peptidoglycan-synthesizing systems. In rod-shaped organisms one is responsible for cell elongation, whereas the other one is involved in cell septation. In cocci most material is incorporated during the septation process and a second system is involved in cell wall thickening. It is noteworthy that there are a number of similarities between the polymerization of bacterial peptidoglycan and the formation of cell wall chitin in yeast (references in Osmond et al., 1999; Valdivieso et al., 1999). Chitin consists mainly of unbranched chains of β,1→4 linked N-acetylglucosamine residues and thus resembles the glycan chains of peptidoglycan. Although making up only 1–2% of the cell wall of Saccharomyces cerevisiae,like peptidoglycan, it is important for cell integrity. It is found in a ring at the base of the emerging bud, at the septum and in the lateral wall. Furthermore, S. cerevisiae has a multiplicity of functionally distinct chitin synthases.

In E. coli it was established that single glycan chains are inserted into the peptidoglycan during cell elongation (see references in Höltje, 1998). This process requires the accompanying hydrolytic cleavage of preexisting peptide cross-bridges or glycan chains and the cross-linking by transpeptidation to old material. In one model it was proposed that the bifunctional polymerase moves around the sacculus and inserts a single new strand between preexisting ones (Park and Burman, 1985). In another model it was postulated that new material is attached to the existing peptidoglycan before covalent bonds are cleaved to promote its insertion into the stress-bearing layer (Höltje, 1998). To take into account the complexity of the spatial and temporal constraints, as well as the multiple functional interactions, it was hypothesized that there are two peptidoglycan-synthesizing multienzyme complexes: one for elongation, the other one for septation (Höltje, 1998). Polymerases PBP1a and/or PBP1b would be present in both complexes, and their roles would be to provide a common peptidoglycan primer that could be processed for elongation or septation by proteins specific of each complex (Wientjes and Nanninga, 1991). Recently, possible in vivo protein–protein interactions between PBP1b and other putative proteins of these complexes were substantiated by in vitro binding experiments (Vollmer et al., 1999).

Two other compounds are involved in the transglycosylation reaction, namely, its substrate lipid II and released undecaprenyl pyrophosphate. The rate of transport of lipid II through the membrane and that of its input in the reaction must match the rate of peptidoglycan synthesis, which is about 1000 units per second in E. coli (van Heijenoort et al., 1992). Because the lipid II pool is low (1000–2000 copies per cell) in this organism, it has been discussed (van Heijenoort, 1994) whether it freely diffuses over the whole membrane or whether it is tunneled directly to the periplasmic peptidoglycan polymerases from transferases MraY and MurG, which catalyze its synthesis. This would imply a highly integrated coordinated process between the synthesis, transport, and use of the lipid substrate, as illustrated by the possible association of MraY and MurG in a peptidoglycan-synthesizing complex (Nanninga, 1998). In S. aureus there is evidence for lipid intermediate-protein interactions, which could indicate an association with MraY in a peptidoglycan-synthesizing complex (Weppner and Neuhaus, 1978). The undecaprenyl pyrophosphate released in the course of the transglycosylation reaction, both from the donor and the acceptor, is dephosphorylated to undecaprenyl phosphate, which is again available for the formation of lipid intermediates (Rogers et al., 1980; Ward, 1984). The phosphatase catalyzing the dephosporylation was studied in one case (Goldman and Strominger, 1972).

Inhibitors of the transglycosylation reaction

Glycopeptides and moenomycins are the two best studied groups of antibiotics known to interfere with the transglycosylation reaction (see references in Huber, 1979 and Reynolds, 1989). Although their modes of action differ, they possess certain common features: large molecular masses, no penetration into cells, preferential in vivo action on Gram-positive organisms, and an accumulation of peptidoglycan precursors in intact cells and in cell free systems. The nonpenetration of such inhibitors further substantiates the localization of the polymerization reactions on the outside of the cytoplasmic membrane.

Vancomycin (Figure 5) and teicoplanin are the main two members of the glycopeptide group clinically used against Gram-positive pathogens. The different aspects of the work on these antibiotics has been extensively reviewed (see referecnes in Reynolds, 1989 and Williams and Bardsley, 1999). Moreover, the emergence of vancomycin resistance in enterococci has led to the characterization of a modified peptidoglycan metabolism and to an important literature on the subject (see references in Arthur et al., 1996). The mode of action of glycopeptides involves the specific noncovalent binding to the C-terminal D-alanyl-D-alanine motif of the peptidoglycan monomer unit. The structures of various glycopeptides, including vancomycin, have been derived from nuclear magnetic resonance and X-ray studies and have led to a model of the binding involving specific hydrogen bonds and other noncovalent interactions (Williams, 1996). Because glycopeptides do not cross the membrane, complexes are formed only with the D-alanyl-D-alanine termini of exported lipid II or with those of nascent peptidoglycan. In the first case, the resulting segregation of the lipid substrate will lead to an arrest of glycan chain elongation; in the second case the binding to the peptide subunits of growing chains can by steric hindrance have a shielding effect on both transglycosylation and transpeptidation. Presently, it is difficult to assess whether both mechanisms are functioning in vivo or whether one is predominant. Recently, it has been established that other factors are involved in the antibacterial activity of glycopeptides (Williams and Bardsley, 1999 and references therein). In particular, it was found that glycopeptides (other than teicoplanin) form dimers in aqueous solution and that this dimerization is cooperative with ligand binding. It was proposed that dimerization facilitates the binding between the antibiotic dimer and the membrane-anchored peptidoglycan precursors, thereby leading to the formation of a complex. The structure–activity relationships of the vanomycin-type glycopeptide antibiotics have been evaluated in detail (Nagarajan, 1993).

Moenomycins are phosphoglycolipid compounds produced as complexes of very similar components by various Streptomyces species and are used as growth promoter in animal nutrition(Huber, 1979). The structure (Figure 5) of the main constituent, moenomycin A, was determined by Welzel and co-workers (Welzel et al., 1981, 1983). A very similar structure was reported for the related antibiotic, pholipomycin (Takahashi et al., 1983; Scherkenbeck et al., 1993). More recently, a 3-D structure of moenomycin A in aqueous solution was presented (Kurz et al., 1998). Moenomycins interfere with the transglycosylation reaction as substantiated by their effect on the polymerizing activity of cell-free systems (Huber, 1979 and references therein; van Heijenoort and van Heijenoort, 1980) and that of E. coli PBP1b assayed with lipid II (Suzuki et al., 1980; Nakagawa et al., 1984). Moreover, moenomycin was shown to bind reversibly to E. coli PBP1b (van Heijenoort et al., 1987). Owing to structural similarities between moenomycin and lipid II it has been proposed that the drug might compete with the substrate for a binding site on the enzyme (Linnett and Strominger, 1973; van Heijenoort et al., 1987). However, from the study of simplified models of peptidoglycan and moenomycin A, it was found they could be superimposed on the basis of the sugar chains with a perfect overlapping of four sugar rings (Ritzeler et al., 1997). This clearly suggests that moenomycin can compete with the growing chain for binding to the polymerase and is in agreement with the possible presence on the polymerase of binding sites for several sugar units. It is noteworthy that not all class A HMM-PBPs are sensitive to moenomycin (Paik et al., 1999; Hoskins et al., 1999). Similarly, the S. aureus and S. pneumoniae Mgts are sensitive to moenomycin (Park and Matsuhashi, 1984; Park and Burman , 1985) but not the E. coli and M. luteus Mgts (Hara and Suzuki, 1984; Park and Matsuhashi, 1984).Furthermore, moenomycin can interact with proteins devoid of peptidoglycan glycosyltransferase activity at concentrations 100- to 1000-fold higher than that inhibiting PBP1b (Graves-Woodward and Pratt, 1999, and references therein).

The structure–activity relationships of moenomycin A were evaluated by degradation studies and synthesis of analogues (El-Abadla et al., 1999 and references therein). Minimum structural requirements for biological activity were determined with in vitro and in vivo assays and led to a core structure that retained glycosyltransferase inhibitory activity. The oligosaccharide part of moenomycin is recognized with high selectivity. There are two recognition sites for moenomycin-type transglycosylation inhibitors on the enzyme: one at the donor site and one at the acceptor site. Like moenomycin A itself, structural analogues with at least three sugars bind presumably to the donor site. They are active against Gram-positive bacteria as well as in the in vitro enzyme assay. Moenomycin analogues with two sugars are more or less inactive in vivo but do inhibit the enzyme in vitro provided they have the right substitution pattern. Their binding to the donor or acceptor site is not yet clearly established. The C25 lipid moiety (Figure 5) appears to be essential because its removal (Marzian et al., 1994) or the introduction of one or several OH groups into the lipid chain (Kempin et al., 1995) caused complete loss of inhibitory activity. However, its role in the anchoring of the antibiotic to the cytoplasmic membrane has remained elusive. An experimental system allowing the investigation of the interaction of moenomycins with artificial membranes has recently been developed (Anikin et al., 1999).

Yet other compounds have been found to inhibit glycan chain formation. This is the case of mersacidin which is a lantibiotic (Brötz et al., 1997). Recently, vancomycin derivatives that are active against vancomycin-resistant organisms and that inhibit peptidoglycan synthesis without binding to D-Ala-D-Ala were described (Allen et al., 1996; Ge et al., 1999; Goldman et al., 2000). They interfere with transglycosylation by an as yet unknown mechanism. Based on the conserved inhibitory activity of certain moenomycin-derived saccharides and on structural comparisons between the lipid substrate and the peptidoglycan monomer unit, attempts have recently been made to synthesize specific inhibitors (Hecker et al., 1990; Qiao and Vederas, 1993; Moss and Southgate, 1993; Brooks et al., 1995). None of the compounds that were tested were active because they lacked important structural features of moenomycins. More recently, the solid phase synthesis of a library of moenomycin disaccharide analogues has led to the identification of novel compounds in vitro (Sofia et al., 1999). Although far less potent than moenomycin A, they possessed antibacterial activity and were inhibitors of in vitro peptidoglycan synthesis. Assays for the identification of compounds that interfere with the binding of moenomycin to the glycosyltransferase site of E. coli PBP 1a and PBP 1b (Vollmer and Höltje, 2000) or that inhibit the transglycosylation step (Branstrom et al., 2000) have recently been proposed.

Concluding remarks

Most of our current knowledge about peptidoglycan glycosyltransferases concerns E. coli PBP1b. With the development of genomics many other ones should now be accessible. However, the study of their enzymatic properties and mechanism of catalysis is dependent on the ready availability of their lipid substrate and the development of efficient assays. The determination of the crystal structure of soluble forms of a peptidoglycan glycosyltransferase will be a crucial step for understanding the mechanism of the transglycosylation reaction and it is essential that efforts be made on this topic. These different approaches will also undoubtedly be helpful for the design of new specific inhibitors. By its location on the outside of the cytoplasmic membrane and its specificity, the transglycosylation step remains an attractive potential target for the search for novel antibacterials. The variable inhibitory effect of moenomycin suggests a possible variability in the binding sites of the sugar units. Because the substrate (lipid II) and the product (uncross-linked glycan chains) are very similar in all peptidoglycan transglycosylation reactions, the structure of the catalytic site could be less variable, in which case transition state analogs would be more suitable general inhibitors. Among the many unanswered questions with respect to the in vivo functioning of the transglycosylation step a few are of particular interest. The mechanism of translocation of the lipid intermediate substrate and that of the undecaprenyl pyrophosphate product are yet completely unexplored. A possible approach could be to study the effects of conditional mutations in putative translocases on peptidoglycan synthesis. As for the monofunctional glycosyltransferases, further studies need to focus on their location in the cytoplasmic membrane, their essentiality, and their exact in vivo function. Finally, it should be stressed that unraveling the precise organization of the hypothetical peptidoglycan-synthesizing complexes will undoubtedly remain a highly challenging problem.

Acknowledgments

This work was supported by grant UMR8619 from the Centre National de la Recherche Scientifique and grant PRFMMIP from the Ministère de l’Education Nationale, de la Recherche et de la Technologie. I thank Didier Blanot (Université Paris-Sud, France), Martine Nguyen-Distèche (Université de Liège, Belgium), and Peter Welzel (Universität Leipzig, Germany) for their comments and suggestions.

Abbreviations

A2pm, diaminopimelic acid; GlcNAc, N-acetylglucosamine; HMM, high-molecular mass; Mgt, monofunctional glycosyltransferase; MurNAc, N-acetylmuramic acid; PBP, penicillin-binding protein.

Formation of the glycan chains in the synthesis of bacterial peptidoglycan (3)

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Fig. 1. Scheme of the primary structure of bacterial peptidoglycan. Abbreviations: GlcNAc: N-acetylglucosamine; MurNAc: N-acetylmuramic acid; DA: diamino acid (generally diaminopimelic acid or L-lysine); n: number of amino acids in the cross-bridge (n = 0 to 5 depending on the organism). D-Ala: often missing in the peptidoglycan of many organisms.

Formation of the glycan chains in the synthesis of bacterial peptidoglycan (4)

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Fig. 2. Peptidoglycan synthesis and antibiotics interfering with polymerization.

Formation of the glycan chains in the synthesis of bacterial peptidoglycan (5)

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Fig. 3. Extension of a glycan chain by transglycosylation. 1: elongation at the reducing end of the glycan chain; 2: elongation at the nonreducing end of the glycan chain; G: GlcNAc; M: MurNAC; P : undecaprenyl phosphate; P-P : undecaprenyl pyrophosphate; pep: pentapeptide.

Formation of the glycan chains in the synthesis of bacterial peptidoglycan (6)

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Fig. 4. Mechanism of transglycosylation with chain elongation at the reducing end. R = D-lactoyl-pentapeptide. The indicated dicarboxylic amino acid residues are those of the glycosyltransferase module of E. coli PBP1b presumably involved in the catalytic process.

Formation of the glycan chains in the synthesis of bacterial peptidoglycan (7)

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Fig. 5. Antibiotics interfering with the transglycosylation reaction.

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FAQs

What process assembles the glycan chains together during peptidoglycan synthesis? ›

Peptidoglycan synthases

Peptidoglycan synthesis requires glycosyltransferases (GTases) to polymerize the glycan chains and DD-transpeptidases (DD-TPases) to crosslink the peptides7 (FIG.

What forms glycan strands of peptidoglycan? ›

The glycan strands in peptidoglycan. The glycan strands are formed by oligomerization of monomeric disaccharide peptide units (lipid II) by transglycosylation reactions.

What are the steps of peptidoglycan synthesis? ›

1) can be divided into four sets of reactions: (1) formation of UDP-GlcNAc from fructose-6-phosphate, (2) formation of UDP-MurNAc from UDP-GlcNAc, (3) assembly of the peptide stem leading to UDP-MurNAc-pentapeptide and (4) 'side' or 'annex' pathways of synthesis of d-glutamic acid and the dipeptide d-alanyl-d-alanine.

What connects the glycan chains in peptidoglycan? ›

Peptidoglycan is a polymeric macromolecule made up of linear glycan strands attached to each other by peptide bridges.

What is glycan in peptidoglycan? ›

The main structural features of bacterial peptidoglycan are linear glycan chains interlinked by short peptides. The glycan chains are composed of alternating units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc), all linkages between sugars being beta,1-->4.

What is peptidoglycan also known as? ›

Medical Definition of peptidoglycan

: a polymer that is composed of polysaccharide and peptide chains and is found especially in bacterial cell walls. — called also mucopeptide, murein.

How are two strands of glycan attached to one another during peptidoglycan synthesis? ›

Neighboring glycan chains are interlinked either by a direct peptide linkage between a peptide subunit of a chain with one of another chain or by a short peptide bridge between two peptide subunits.

Which of the following is the glycan portion of peptidoglycan? ›

The glycan component of all peptidoglycans consists of a β-1,4-linked chain of alternating N-acetylglucosamine and N-acetylmuramic acid residues. The monosaccharide d-N-acetylglucosamine (GlcNAc) is a common amino sugar which makes up the polysaccharide chitin found in plants and arthropods.

What is the structure of bacterial peptidoglycan? ›

Peptidoglycan is the major structural polymer in most bacterial cell walls and consists of glycan chains of repeating N -acetylglucosamine and N -acetylmuramic acid residues cross-linked via peptide side chains. Peptidoglycan hydrolases are produced by many bacteria, bacteriophages and eukaryotes.

What are the steps involved in cell wall synthesis? ›

These steps involve the synthesis of the peptidoglycan precursors lipid I and lipid II, the flipping of lipid II across the cytoplasmic membrane, and the polymerization of glycan chains from lipid II and their incorporation into the existing cell wall by peptidoglycan synthases.

Where does peptidoglycan synthesis occur? ›

Peptidoglycan synthesis occurs in three distinctive compartments of bacteria, namely the cytoplasm, the cytoplasmic membrane and the periplasmic space [3].

What is the function of peptidoglycan in bacterial cells? ›

Peptidoglycan is an essential component of the bacterial cell envelope and protects the cell from bursting due to turgor and maintains cell shape.

Which of the following two sugars are present in peptidoglycan chains? ›

The peptidoglycan layer in the bacterial cell wall is a crystal lattice structure formed from linear chains of two alternating amino sugars, namely N-acetylglucosamine (GlcNAc or NAGA) and N-acetylmuramic acid (MurNAc or NAMA).

What is the meaning of glycan? ›

(GLY-kan) A large carbohydrate molecule. It contains many small sugar molecules that are joined chemically. Also called polysaccharide.

What is the chemical structure of peptidoglycan? ›

The peptidoglycan is a heteropolymer composed of polysaccharide chains which are cross-linked through short peptides. The polysaccharide moiety (glycan) is made up of beta-1,4 glycosidically linked N-acetylglucosamine and N-acylmuramic acid residues.

What is a glycan chain? ›

Glycans are chains of monosaccharides linked together by glycosidic bonds. In glycoproteins and glycolipids, the reducing end of a glycan is covalently linked to amino acids or lipids, respectively.

What is the last step in synthesis of peptidoglycan? ›

The last step in synthesis of peptidoglycan is
1)attaching two amino acids to form a cross-link
2)attachment of a peptide to muramic acid
3)binding of penicillin to a membrane protein
4)attachment of a portion of peptidoglycan to a membrane lipid
5)NULL

How do molecules pass through peptidoglycan to get into the cell? ›

Once across the outer membrane and in the periplasm, molecules work their way through the porous peptidoglycan layers before being transported by integral proteins across the cell membrane. The peptidoglycan layers are linked to the outer membrane by the use of a lipoprotein known as Braun's lipoprotein (good ol' Dr.

What is the purpose of the peptidoglycan layer? ›

Function of Peptidoglycan

Cross-linking between amino acids in the layer of peptidoglycan forms a strong mesh-like structure that provides structure to the cell. Peptidoglycan provides a very important role in bacteria because bacteria are unicellular; it gives strength to the outer structure of the organism.

Why is it called a peptidoglycan? ›

The term peptidoglycan was derived from the peptides and the sugars (glycan) that make a molecule; it is also called 'murein' or 'mucopeptide. ' This complex interwoven network of sugar polymer and amino acids surrounds the entire bacterial cell.

Which of the following are components of peptidoglycan? ›

Peptidoglycan, also called murein, is a vast polymer consisting of interlocking chains of identical peptidoglycan monomers (Figure 2.3. 1). A peptidoglycan monomer consists of two joined amino sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), with a pentapeptide coming off of the NAM (Figure 2.3. 2).

How are peptidoglycan chains crosslinked? ›

The glycan chains in peptidoglycan or murein are cross-linked by transpeptidation of the peptide side chains.

What is crosslinking in peptidoglycan? ›

The bacterial cell wall, a strong, rigid, highly cross-linked polymer composed of polysaccharide chains with peptide cross- links, hence known as peptidoglycan or murein, is essential for bacteria to maintain their shape and to enable them to withstand large changes in osmotic pressure.

How are NAG and NAM connected? ›

Peptidoglycan is a giant molecule that forms the cell wall that surrounds bacterial cells. It is composed of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) residues connected by β-(1,4)-glycosidic bonds and cross-linked with short polypeptide chains.

What is the composition of the peptidoglycan layer is found in the cell wall of bacteria quizlet? ›

Bacterial Cell Wall: Peptidoglycan is a polymer of millions of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) sugars based on glucose molecules linked together in long chains cross-braced with four amino acids that link individual polymer chains together in a chain-link fence pattern.

What type of bacterial cell wall is composed of a thin layer of peptidoglycan and an outer membrane? ›

Gram-negative bacteria are surrounded by a thin peptidoglycan cell wall, which itself is surrounded by an outer membrane containing lipopolysaccharide. Gram-positive bacteria lack an outer membrane but are surrounded by layers of peptidoglycan many times thicker than is found in the Gram-negatives.

Which of the following correctly describes peptidoglycan? ›

Which of the following correctly describe(s) peptidoglycan? It degrades misfolded proteins and invading pathogens and processes organic food particles for nutrition. Pseudomonas aeruginosa concentrates iron inside of its cells.

How many layers of peptidoglycan are in Gram-positive bacteria? ›

Peptidoglycan provides rigidity to the cell wall; the cell walls of Gram-positive bacteria may contain up to 40 layers of peptidoglycan, conferring significant mechanical strength.

Which of the following structure in bacterial cell is responsible for cell wall formation? ›

Cell Wall Peptidoglycans: Both Gram-positive and Gram-negative bacteria possess cell wall peptidoglycans, which confer the characteristic cell shape and provide the cell with mechanical protection.

What is the structure of bacterial cell wall? ›

The cell wall consists mainly of peptidoglycan (PG), a mesh of polysaccharide strands (composed of a poly-[N-acetylglucosamine (GlcNAc)-N-acetylmuramic acid (MurNAc)] backbone) cross-linked via short peptide bridges attached to the MurNAc residues (Vollmer et al., 2008a).

What are the 3 main types of bacterial cell walls? ›

There are two main types of bacterial cell walls, those of gram-positive bacteria and those of gram-negative bacteria, which are differentiated by their Gram staining characteristics. For both these types of bacteria, particles of approximately 2 nm can pass through the peptidoglycan.

What is the function of cell wall in bacterial cell? ›

The cell wall has multiple functions during bacterial growth, including maintaining bacterial cell integrity and shape as well as resisting internal turgor pressure. Furthermore, it must remain flexible to accommodate the remodeling that is required for cell division and growth.

How are bacterial cell walls synthesized? ›

Rod-shaped bacteria such as B. subtilis or E. coli have two modes of cell wall synthesis: new peptidoglycan is inserted along a helical path (A), leading to elongation of the lateral wall, and is inserted in a closing ring around the future division site, leading to the formation of the division septum (B).

What happens when peptidoglycan synthesis is disrupted? ›

By inhibiting peptidoglycan synthesis, the growth of bacteria is prevented. These bacteria will be subjected to osmotic lysis.

Where will you find peptidoglycan layer in cell? ›

Peptidoglycan, also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria (but not Archaea; []), forming the cell wall.

What is the main function of peptidoglycan quizlet? ›

What is the main function of peptidoglycan? Protecting against osmotic stress.

What is the composition of peptidoglycan quizlet? ›

Peptidoglycan is a polymer of millions of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) sugars based on glucose molecules linked together in long chains cross-braced with four amino acids that link individual polymer chains together in a chain-link fence pattern.

What is the importance of peptidoglycan in Gram staining? ›

The Gram stain procedure distinguishes between Gram positive and Gram negative groups by coloring these cells red or violet. Gram positive bacteria stain violet due to the presence of a thick layer of peptidoglycan in their cell walls, which retains the crystal violet these cells are stained with.

What process assembles the glycan chains together during peptidoglycan synthesis? ›

Peptidoglycan synthases

Peptidoglycan synthesis requires glycosyltransferases (GTases) to polymerize the glycan chains and DD-transpeptidases (DD-TPases) to crosslink the peptides7 (FIG.

What is peptidoglycan synthesis? ›

The biosynthesis of bacterial cell wall peptidoglycan is a complex process that involves enzyme reactions that take place in the cytoplasm (synthesis of the nucleotide precursors) and on the inner side (synthesis of lipid-linked intermediates) and outer side (polymerization reactions) of the cytoplasmic membrane.

Which of the following amino sugar are present in the bacterial cell wall? ›

Thus, the correct answer is 'Diaminopimellic acid.

What is glycan in peptidoglycan? ›

The main structural features of bacterial peptidoglycan are linear glycan chains interlinked by short peptides. The glycan chains are composed of alternating units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc), all linkages between sugars being beta,1-->4.

How is glycan formed? ›

These are formed by the repetitive addition of galactose and N-acetyl-glucosamine units. Polylactosamine chains on O-linked glycans are often capped by the addition of a sialic acid residue (similar to neuraminic acid).

What are glycans how are they formed? ›

Glycans are formed of several monosaccharides linked by chemical bonds. These long, chain-like structures with a variety of forms, from long, straight chains to incredibly complex branched ones. The term can also be applied to the carbohydrate portion of organic molecules.

Which of the following is the glycan portion of peptidoglycan? ›

The glycan component of all peptidoglycans consists of a β-1,4-linked chain of alternating N-acetylglucosamine and N-acetylmuramic acid residues. The monosaccharide d-N-acetylglucosamine (GlcNAc) is a common amino sugar which makes up the polysaccharide chitin found in plants and arthropods.

How are two strands of glycan attached to one another during peptidoglycan synthesis? ›

Neighboring glycan chains are interlinked either by a direct peptide linkage between a peptide subunit of a chain with one of another chain or by a short peptide bridge between two peptide subunits.

How are two strands of glycan attached to one another during peptidoglycan synthesis? ›

Neighboring glycan chains are interlinked either by a direct peptide linkage between a peptide subunit of a chain with one of another chain or by a short peptide bridge between two peptide subunits.

What is the last step in synthesis of peptidoglycan? ›

The last step in synthesis of peptidoglycan is
1)attaching two amino acids to form a cross-link
2)attachment of a peptide to muramic acid
3)binding of penicillin to a membrane protein
4)attachment of a portion of peptidoglycan to a membrane lipid
5)NULL

What is a glycan chain? ›

Glycans are chains of monosaccharides linked together by glycosidic bonds. In glycoproteins and glycolipids, the reducing end of a glycan is covalently linked to amino acids or lipids, respectively.

What is the function of Transglycosylase? ›

Overview. Transglycosylases are a class of GH enzymes that can catalyze the transformation of one glycoside to another. That is, these enzymes catalyze the intra- or intermolecular substitution of the anomeric position of a glycoside.

What two subunits are required to make peptidoglycan? ›

Peptidoglycan, also called murein, is a vast polymer consisting of interlocking chains of identical peptidoglycan monomers (Figure 2.3. 1). A peptidoglycan monomer consists of two joined amino sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), with a pentapeptide coming off of the NAM (Figure 2.3.

How are peptidoglycan chains crosslinked? ›

The glycan chains in peptidoglycan or murein are cross-linked by transpeptidation of the peptide side chains.

What is the chemical structure of peptidoglycan? ›

The peptidoglycan is a heteropolymer composed of polysaccharide chains which are cross-linked through short peptides. The polysaccharide moiety (glycan) is made up of beta-1,4 glycosidically linked N-acetylglucosamine and N-acylmuramic acid residues.

Where does peptidoglycan synthesis occur? ›

Peptidoglycan synthesis occurs in three distinctive compartments of bacteria, namely the cytoplasm, the cytoplasmic membrane and the periplasmic space [3].

What are the steps involved in cell wall synthesis? ›

These steps involve the synthesis of the peptidoglycan precursors lipid I and lipid II, the flipping of lipid II across the cytoplasmic membrane, and the polymerization of glycan chains from lipid II and their incorporation into the existing cell wall by peptidoglycan synthases.

What is the function of peptidoglycan in bacterial cells? ›

Peptidoglycan is an essential component of the bacterial cell envelope and protects the cell from bursting due to turgor and maintains cell shape.

What are glycans how are they formed? ›

Glycans are formed of several monosaccharides linked by chemical bonds. These long, chain-like structures with a variety of forms, from long, straight chains to incredibly complex branched ones. The term can also be applied to the carbohydrate portion of organic molecules.

What process assembles the glycan chains together? ›

Peptidoglycan glycan chains are assembled by the polymerization of the N-acetyl glucosaminyl-β-(1→4)-N-acetylmuramyl disaccharide unit of lipid II (or its modified forms) with the formation of β-(1→4) linkages (Fig.

What are glycans used for? ›

Glycans attached to matrix molecules, such as proteoglycans, are important for the maintenance of tissue structure, porosity, and integrity. Such molecules can also contain binding sites for other specific types of glycans that in turn aid the overall organization of the matrix.

Is Transglycosylase a penicillin binding protein? ›

Both the transglycosylase and transpeptidase functions in plastid penicillin-binding protein are essential for plastid division in Physcomitrella patens - PMC. The . gov means it's official.

How many PBPs are there? ›

Most of its 16 PBPs have been extensively studied for their role in vegetative peptidoglycan synthesis and in sporulation. Bacillus subtilis has four class A PBPs.

Where is Autolysin located? ›

Autolytic activity is found within the C-terminal region with catalytic domain homologous to the glucosaminidase domain. LytD is found in the cell wall.

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