Glycopeptide transpeptidase and D-alanine carboxypeptidase: penicillin-sensitive enzymatic reactions.

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Abstract
A particulate enzyme preparation from Escherichia coli strain Y-10 catalyzes the synthesis of corss-linked glycopeptide strands from UDP-GlcNAc uridene diphosph. qlucosamine acetate and UDPMurNAc [middot] L-ala [middot] D-glu [middot] meso-DAP [middot] D-ala [middot] D-ala (uridine diphosphormuramyl-N-acetyl-L-alanine-D-glucose-meso -diaminopi-meiyl-D-alanine-D-alanine.) The terminal reaction in the sequence is a transpeptidation by means of which 2 peptide units are cross-linked with elimination of D-alanine from the peptide. This transpeptidation can be reversed by addition of D-alanine or several other D-amino acids. A D-alanine carboxypeptidase present in the particulate preparation is also required for the terminal steps. Both of these enzymes are specifically inhibited by penicillin G, ampicillin, and methicillin. A comparison of the sensitivity of the enzymes and of the cells to these penicillins and to several other antibiotics suggests that the relative insensitivity of E. coli to some antibiotics is due to their failure to penetrate to the sensitive site.