Key Concepts:

Figure 1. The structure of peptidoglycan, the stabilising macromolecule of the bacterial cell wall and structural variations within bacteria. As the name indicates, the peptidoglycan is constructed of peptides (amino acids are boxed) and glycans (polysaccharide chains) that are interconnected. Glycan chains of variable length are composed of alternating, β‐1‐4 glycosidically linked amino sugars, N‐acetyl glucosamine (GlcNAc) and N‐acetylmuramic acid (MurNAc). These may be modified by partial O‐acetylation of the C6 hydroxyl groups (s. upper strand) or N‐deacetylation of the C2 acetamido groups (s. lower strand). The unique bacterial amino sugar MurNAc serves as the branching point within the peptidoglycan network, interconnecting the glycan chains via short peptide bridges that attached to the ether‐linked d‐lactic acid substituent. There is a great variability in the composition and structure of the stem peptides of the peptidoglycan but they usually contain amino acids of the d‐enantiomeric form (d‐alanine, d‐Ala and d‐glutamate, d‐Glu) and di‐basic amino acids (meso‐diaminopimelic acid, DAP, in E. coli and other Gram‐type negative bacteria as well as in Bacilli and Clostridiae; l‐lysin, l‐Lys, in most other Gram‐type positive bacteria, and in some cases l,l‐Dap or l‐ornithine). d‐ and l‐amino acids in the peptidoglycan network are connected in such a way that l‐d‐l‐d‐d‐peptides are formed, which are insensitive to ‘normal’ l‐l‐peptidases/proteases. Notably, the d‐Glu in these stem peptides is linked through an isopeptide bond via its γ‐carboxylic acid. Both, d‐Glu and DAP, may be amidated at their d‐α‐carboxylic acid. Most Gram‐positive bacteria connect two stem peptides via a linker unit, such as a pentaglycine bridge (Gly₅) in Staphylococcus aureus, an l‐Ala‐l‐Ala bridge in Enterococcus faecalis, or a d‐aspartate, d‐Asp (eventually amidated at the α‐carboxylic acid) in Lactococcus lactis and Enterococcus faecium. Gram‐negative bacteria as well as Bacillus and Clostridium sp. use direct crosslinks, connecting the amino group of the d‐centre of DAP³ with the carboxylic acid of the forth amino acid (d‐Ala⁴) of another strand, generating a d‐d‐peptide bond (d,d‐crosslink). These crosslinks are formed concomitantly with the cleavage of a terminal d‐Ala⁴‐d‐Ala⁵ bond of a donor stem peptide by d,d‐transpeptidases and the release of d‐Ala⁵. Since these enzymes generally are susceptible to penicillin and other β‐lactamases, they are also named penicillin‐binding proteins (PBP) (cf. Table 1). In the presence of sub‐lethal concentrations of penicillin, strains can be selected for that contain mainly l,d‐(DAP‐DAP) crosslinks. These are formed by penicillin‐insensitive l‐d‐transpeptidases, which connect the d‐amino group of DAP³ of one stem peptide with the l‐carboxylic acid group of DAP³ of a second (the donor). Enzymes of the latter specificity are also required to attach the peptidoglycan to the outer membrane in Gram‐negative bacteria via Braun's lipoprotein, linking the side‐chain amino group of the protein's C‐terminal lysine with DAP in the peptidoglycan. In Gram‐positive bacteria, the peptidoglycan network is modified by long‐chain polyol‐phosphate polymers (teichoic acids) that are either covalently attached to the C6 hydroxyl groups of MurNAc through phosphodiester bonds (wall teichoic acids, WTAs) or anchored in the cytoplasmic membrane and connected to peptidoglycan via ionic interactions (lipoteichoic acids, LTAs). Thus, cell wall synthesis involves the coordinated assembly – and in turn cell wall degradation the coordinated disassembly – of a thick peptidoglycan–teichoic acid complex in Gram‐positive bacteria as well as a thin peptidoglycan layer linked to an outer membrane in Gram‐negative bacteria.

Figure 2. Schematic structure of the peptidoglycan network of E. coli and sites of cleavage by cell wall hydrolases, targeting either the glycan (blue) or the peptide part (red) of the peptidoglycan. The boxed part of the peptidoglycan network is the same shown in Figure 1 in more chemical detail. For members of the respective enzyme groups see Table 1.

Figure 3. Cleavage of the peptidoglycan by two distinct types of muramidases. Lysozyme‐like muramidases catalyse a hydrolysis reaction yielding products that carry MurNAc at their reducing ends, whereas the lytic transglycosylases catalyse an intramolecular transglycosylation reaction yielding anhMurNAc termini. GlcNAc‐anhMurNAc‐peptides are the major cell wall turnover products of E. coli and other Gram‐negative bacteria, whereas GlcNAc‐MurNAc‐peptides presumably are generated mostly in Gram‐positive bacteria.

Figure 4. Peptidoglycan turnover and recycling in E. coli. In red, enzymes that act on the peptide, in blue, enzymes that act on the glycan (sugar) portion of the peptidoglycan. Enzymes shown in black are involved in peptidoglycan synthesis or general catabolic pathways. For details see text.