Structure and function of lytic transglycosylases from Pseudomonas aeruginosa

摘要

As with plants, algae, and fungi, bacteria produce a rigid polymer of beta,1 -> 4 linked glycosides that comprises the bulk of their cell walls. These polymers withstand the internal turgor pressure of the cytoplasm thereby maintaining the integrityof the cytoplasmic membranes. However, unlike cellulose, xylan, and chitin, which are homopolymers of glucose, xylose, and N-acetylglucosamine (GlcNAc), respectively, the peptidoglycan (or murein) of bacteria is a heteropolymer of two amino sugars, GlcNAc and N-acetylmuramic acid (MurNAc), the latter of which is modified with a peptide (Figure 1). This stem peptide is comprised of alternating L- and D- amino acids which may vary with genera and species. Chains of these alternating aminosugars are thought to form "hoops" around the circumference of the cell, which in the Gram-negative bacteria, may only be one layer thick. For this reason, the carbohydrate polymers need to be crosslinked together in order to provide the required rigidity of the wall polymer. This crosslinking is achieved through neighbouring stem peptides where, typically, the fourth amino acid residue (usually D-alanine) of one is covalently attached to the third residue (usually a diamino acid such as L-lysine or diaminopimelic acid) of the other. Thus, the peptidoglycan of a bacterium is one large macromolecule, or sacculus, which maintains the integrity and shape of the cell.