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chapter 16

Carbohydrate Metabolism III: Glycoproteins, Glycolipids, GPI Anchors, Proteoglycans, and Peptidoglycans

FIGURE 16-19

Polymerization of MurNAc-pentapeptide units to form linear peptidoglycans: stage 2 of peptidoglycan biosynthesis.

The lipid in these structures is undecaprenol; lipid-P-P is undecaprenyl pyrophosphate. The use of tRNA as a carrier and

activator of glycine is unusual, since ribosomes and mRNA are not involved in the reaction.

reuse. Bacitracin may exert its bactericidal activity partly

by inhibiting this hydrolysis. During assembly, the lin-

ear peptidoglycan ends up outside the cell membrane

at the site of cell wall assembly. The carrier lipid may

participate in translocation of the disaccharide-peptide

unit across the membrane. Polysaccharide cleavage from

the lipid carrier, which must occur before final cell

wall assembly (third stage), is inhibited by the antibiotic

vancomycin.

The third stage is insertion and attachment by cross-

linking of the linear peptidoglycan into the growing three-

dimensional peptidoglycan shell. The cross-linking in-

volves a transpeptidation in which the terminal glycine

residue of a peptidoglycan displaces the carboxy-terminal

D-alanine on an adjacent peptidoglycan, releasing a D-

alanine and forming a pentaglycine cross-linkage. Cross-

linking ranges from 20-50% of the available side chains

in gram-positive and gram-negative bacilli to over 90%

in some cocci. In staphylococci, cross-linking of the pep-

tide moiety of peptidoglycan is mediated via an additional

spacer, the interpeptide bridge, consisting of five glycine

residues. Transpeptidation is the key reaction inhibited by

penicillins and cephalosporins.

The energy for cross-link formation is supplied by

hydrolysis of the peptide bond between the terminal

D-alanine residues of the pentapeptide (Figure 16-20).

D-Alanine carboxypeptidases (CPases) catalyze re-

moval of the terminal D-alanyl residue from uncross-

linked pentapeptides, and the resultant tetrapeptide can-

not serve as substrate for transpeptidation. In this way,

CPases may regulate the degree of cross-linking. Some

CPases catalyze transpeptidation reactions between side