LD-Carboxypeptidase, a serine protease, includes microcin C7 self immunity protein.
LD-carboxypeptidase (Muramoyltetrapeptide carboxypeptidase; EC 18.104.22.168; Merops family S66; initially described as Carboxypeptidase II) family also includes the microcin c7 self-immunity protein (MccF) as well as uncharacterized proteins including hypothetical proteins. LD-carboxypeptidase hydrolyzes the amide bond that links the dibasic amino acids to C-terminal D-amino acids. The physiological substrates of LD-carboxypeptidase are tetrapeptide fragments (such as UDP-MurNAc-tetrapeptides) that are produced when bacterial cell walls are degraded; they contain an L-configured residue (L-lysine or meso-diaminopimelic acid residue) as the penultimate residue and D-alanine as the ultimate residue. A possible role of LD-carboxypeptidase is in peptidoglycan recycling whereby the resulting tripeptide (precursor for murein synthesis) can be reconverted into peptidoglycan by attachment of preformed D-Ala-D-Ala dipeptides. Some enzymes possessing LD-carboxypeptidase activity also act as LD-transpeptidase by replacing the terminal D-Ala with another D-amino acid. MccF contributes to self-immunity towards microcin C7 (MccC7), a ribosomally encoded peptide antibiotic that contains a phosphoramidate linkage to adenosine monophosphate at its C-terminus. Its possible biological role is to defend producer cells against exogenous microcin from re-entering after having been exported. It is suggested that MccF is involved in microcin degradation or sequestration in the periplasm.