Crystallographic studies on apocarboxypeptidase A and the complex with glycyl-L-tyrosine.

The crystal structures of zinc-free carboxypeptidase A (apocarboxypeptidase A) and the complex of glycyl-L-tyrosine with apocarboxypeptidase A are described and compared to the corresponding structures of the zinc-containing enzyme. Only small conformational changes in the zinc ligands accompany removal of the metal. Interactions between the tyrosine residue of glycyl-L-tyrosine and apocarboxypeptidase A are similar to those observed in the complex with the holoenzyme. However, in the absence of zinc, the carbonyl oxygen of the glycyl moiety now receives a hydrogen bond from the side chain of arginine-127. Although not as yet observed, a similar shift of the carbonyl oxygen of a susceptible bond from the zinc to arginine-127 could stabilize tetrahedral intermediates generated during the hydrolysis of substrates by carboxypeptidase.