Carboxypeptidase A catalyses the release of the C-terminal amino acid from a polypeptide chain. The initial image shows carboxypeptidase with a dipeptide (green) in the active site. Natural substrates would usually be longer.
The enzyme has . A zinc ion (violet) is present. The ion is involved in binding and catalysis.
Substrate binding involves three interactions between the substrate and the active site[1].
1. .
2. .
3. .
[1] involves electron withdrawal from the carbonyl of the penultimate residue (in this case, Gly) by the zinc ion and an arginyl residue, and acid-base catalysis by a glutamyl residue of a nucleophilic attack on that same group.
(X-ray crystallography with intact substrate was possible because Gly-Tyr is hydrolyzed very slowly and because X-ray crystallography was carried out at -9°C.)
