Peptidoglycan Kevin D Young, University of North Dakota School of Medicine and Health Sciences, Grand Forks, North Dakota, USA Peptidoglycan is the rigid macromolecule that surrounds and protects individual bacterial cells. It defines cell shape and anchors extracellular organelles while remaining porous enough to admit essential nutrients and large compounds. Because it is unique to bacteria, peptidoglycan is one of the most valuable targets to which antibiotics may be directed. Introduction When a membrane-enclosed cell is dropped into a dilute solution,waterrushesintoequalizetheosmoticdifference betweenthecytoplasmandtheexternalenvironment.Ifthe osmotic difference is large enough, the cell will swell until the membrane explodes like an overfilled balloon (lysis). Nonetheless, many microorganisms live fully independent lives in dilute solutions (soil, rivers, lakes and oceans) becausetheyareprotectedfromlysisbyarigidexoskeleton (cell wall) that is strong enough to withstand the internal osmoticpressure.Inbacteria,thisexoskeletoniscomposed ofasinglemacromoleculecalledpeptidoglycan(synonym, murein). Peptidoglycan is assembled from relatively simple subunits that are polymerized and covalently crosslinked in such a way that the final structure represents a set of variations on a basic theme. The subunits are synthesized in the cytoplasm, transported across the inner membrane, andincorporatedintoanextracytoplasmicmacromolecule by penicillin-binding proteins (PBPs) and associated enzymes. This structure forms the rigid backbone of the wall and influences biochemical pathways that involve steps that enter or exit the cell. In addition, because it is unique to bacteria, peptidoglycan synthesis is the target of many antibiotics. Structure Peptidoglycan composition The backbone of peptidoglycan is a chain of two alternating sugars, N-acetylglucosamine (NAG, GlnNAc) and N-acetylmuramic acid (NAM, MurNAc), which form a carbohydrate polymer (the glycan chain) (Figure 1). Extending from NAM is an oligopeptide of five amino acids,severalofwhichareintheunusualDconformation. The combination of NAG–NAM disaccharide and its peptide side-chain is called a muropeptide. Peptides from neighbouringglycanchainscanbelinkedcovalentlytoone another(Figure 1)sothatpeptidoglycancanbevisualizedas a lattice of carbohydrate chains connected by multiple peptide crosslinks. Thisdeceptivelysimplestructurecreatesamolecularnet, or sieve, and is more complex than is usually appreciated. There are over 100 peptidoglycan types, distinguished by different amino acids in the side-chains and different types ofcrosslinking(SchleiferandKandler,1972).MostNAM- linkedpeptidescontainadiaminoacidinthethirdposition of the chain. One amino group forms a peptide bond with theaminoacidinthefourthposition,whiletheotheramino group forms a covalent bond with an amino acid in the third,fourthorfifthpositionofaneighbouringpeptide.In some organisms, an additional oligopeptide of variable composition bridges these primary peptides. For example, in Staphylococcus aureus, five glycine residues are inter- posed between the diamino acid (lysine) of one chain and the fourth residue (d-alanine) of the second. Although each species has one basic type of peptidogly- can, it is not a simple repeating polymer. Instead, the peptidoglycan of any one bacterium is a composite of multiple subcomponents (muropeptides). The Gram- negative bacterium Escherichiacoli contains 40–50 differ- ent muropeptides. The side-chains differ in length (two to five amino acids), in composition (lipoprotein attached or glycine in the fourth or fifth position) and in crosslinking (three different combinations) (Glauner, 1988). In other organisms, peptidoglycan can be modified further by O- acetylation of the glycan, by failure to N-acetylate the sugarresidues,orbyattachingsecondarypolymerssuchas teichoic acid or lipoproteins. No-one knows why this structuralintricacyexists.Somemuropeptidesmayservea basic structural purpose, others may be embellishments, and others may serve specific biological functions. Determination of peptidoglycan composition The presence and nature of the different muropeptides are measured by digesting intact peptidoglycan with murami- dase, an enzyme that cleaves the glycan chains to release individual NAG–NAM disaccharides (Figure 1). The disaccharides are released as monomers (single NAG– NAMunitswithanattachedpeptideside-chain),asdimers (two disaccharides crosslinked to one another by their peptide side-chains), or as trimers or tetramers (three or Article Contents Secondary article . Introduction . Structure . Physiological Functions . Assembly of the Repeating Unit . Polymer Formation . Models of Peptidoglycan Growth . Inhibition of Peptidoglycan Synthesis by Antibiotics 1 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net