Tyrosyl interactions at the active site of carboxypeptidase B

The phosphorescence emission spectra of native carboxypeptidase B and of chemically modified carboxypeptidase B (at arginyl residues) was measured in the presence and absence of peptide and ester substrates (acetyl‐L‐arginine and its hydroxy ester analog: acetyl‐L‐argininic acid). Ester binding did not affect the state of the tyrosyl residue as compared with its state in the substrate‐free enzyme. In the modified enzyme, which is devoid of peptidase activity, binding of the peptide pseudosubstrate did not perturb the state of the tyrosyl residue. The luminescence spectra of Zn²⁺‐ and Co²⁺‐carboxypeptidase B in the presence of the metal coordinating ligand cyanide, used to displace the water from the metal coordination sphere, is also described. Cyanide did not affect the luminescence spectra of the active‐site tyrosyl residue in either Zn²⁺‐or Co²⁺‐carboxypeptidase, indicating that the tyrosyl residue was not interacting directly with the metal bound water. Hence, the effect of peptide on tyrosyl phosphorescence is not caused by the displacement of the tyrosyl from the coordination sphere, but rather by direct interaction of the peptide bond. The data are consistent with the proposition that the tyrosyl residue participates as a proton donor in amide but not in ester hydrolysis.