Carboxypeptidase (CP) A6 (CPA6, also known as CPAH; EC 3.4.17.1), belongs to the carboxypeptidase A/B subfamily of the M14 family of metallocarboxypeptidases (MCPs). The M14 family are zinc-binding CPs which hydrolyze single, C-terminal amino acids from polypeptide chains, and have a recognition site for the free C-terminal carboxyl group, which is a key determinant of specificity. CPA6 prefers large hydrophobic C-terminal amino acids as well as histidine, while peptides with a penultimate glycine or proline are very poorly cleaved. Several neuropeptides are processed by CPA6, including Met- and Leu-enkephalin, angiotensin I, and neurotensin. CPA6 converts enkephalin and neurotensin into forms known to be inactive toward their receptors, but converts inactive angiotensin I into the biologically active angiotensin II. Thus, CPA6 plays a possible role in the regulation of neuropeptides in the extracellular environment within the olfactory bulb where it is highly expressed. It is also broadly expressed in embryonic tissue, being found in neuronal tissues, bone, skin as well as the lateral rectus eye muscle. A disruption in the CPA6 gene is linked to Duane syndrome, a defect in the abducens nerve/lateral rectus muscle connection.
Comment:Metallocarboxypeptidases share the zinc binding motif HXXE...H, where the zinc ion is penta-coordinated to ND1 atoms of the histidines, OE1 and OE2 atoms of the glutamic acid, and to a water molecule in a slightly distorted tetrahedral manner.
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