The role of a tyrosyl residue in the mechanism of action of carboxypeptidase B: luminescence studies

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.

The reactivity of a functional tyrosyl residue in carboxypeptidase B. Nitration of the cadmium enzyme

Cadmium-carboxypeptidase B was nitrated with tetranitromethane. The enzyme polymerized extensively during nitration. In the monomer nitrated Cd-carboxypeptidase B, 70% of the activity of Cd-carboxypeptidase B was retained. In order to identify the tyrosyl residues nitrated, the enzyme was digested with chymotrypsin and subtilisin and the nitrotyrosyl peptides were purified by affinity chromatography on antityrosyl-antibody-Sepharose conjugate followed by two-dimensional thin-layer chromatography. The major nitropeptides, representing 65% of the nitrotyrosyl label, were compatible with the segment of the sequence containing Tyr-240 and Tyr-259. Only 10% of the nitrotyrosyl label was found in the segment of Tyr-248. This indicates that the state of Tyr-248 in Cd-carboxypeptidase B differs from that in zinc-carboxypeptidase B where it shows chemical hyperreactivity due to its proximity to the metal ion.

On the interaction of esters and peptides with carboxypeptidase B

The specificity of porcine carboxypeptidase B towards basic and non-basic substrates was studied by employing several esters of phenyllactate. The structure of these depsipeptides complement exactly those of the corresponding phenylalanyl oligopeptide substrates. These non-basic ester-peptide pairs as well as the basic ester-peptide pair of arginyl derivatives, permits the direct comparison of the pH dependencies of the kinetic constants for the hydrolysis of those substrates by carboxypeptidase B. The data is interpreted in terms of three specific ionizing groups located at the active site of the enzyme. The mode and extent of inhibition of the hydrolysis of a specific substrate by another substrate was characterized kinetically. These results are discussed in relation to a proposed model for esterolytic and proteolytic action of carboxypeptidase B.

Nuclear-magnetic-resonance studies of carboxypeptidase B. Binding of inhibitors to the manganese enzyme

Longitudinal and transverse proton relaxation rates of water in solutions of porcine manganese carboxypeptidase B have been measured in the presence of various competitive inhibitors by pulse nuclear magnetic resonance (NMR) spectrometry. The inhibition constant of Mn-carboxypeptidase activity by L-argininic acid and acetyl-L-arginine was in agreement with the equilibrium constant obtained by the NMR method, indicating similar and specific binding of the inhibitors to the active site of the manganese enzyme. Titration of the water boound to the metal ion revealed the presence of one water molecular which could be displaced from the sphere of the managenese ion by various inhibitors. The structural features of the inhibitors required for this displacement as well as the mode of interaction is described.

The physical state dependence of carboxypeptidase Aalpha and Agamma kinetics

Spectrochemical probes have demonstrated that the conformations of carboxypeptidase A (EC 3.4.12.2) differ in solution and in the crystalline state. Detailed kinetic studies of carboxypeptidase A(alpha) and A(gamma) crystals and solutions now show that the physical state of the enzyme is also a critical parameter that affects the function of the A(alpha) and A(gamma) enzymes in the same manner. The kinetic profiles and the corresponding kinetic constants of substrate hydrolysis are, therefore, important functional indices of the known conformational differences of the enzyme in these two physical states. The complex kinetic behavior of this enzyme, however, precludes meaningful comparisons of activity measurements for crystals and solutions obtained at only one substrate concentration. Underlying differences in varying substrate-inhibiting or -activating binding modes can result in either high or low activity ratios, concealing the true, functional consequences of the change in physical state. Thus, for all substrates examined, crystallization of the enzyme markedly reduces catalytic efficiency, k(cat), from 20- to 1000-fold. Equally as important, the substrate inhibition, apparent in solution for some di- and depsipeptides, is abolished with crystals, while for longer substrates the normal solution kinetics may acquire activation with the crystals. Hypothetical modes of substrate-enzyme interaction, generated by superimposing substrate models on the crystal structure of carboxypeptidase to simulate kinetics in solution, have failed to detect both of these changes, which affect inhibitory or activating binding modes. The only structure of carboxypeptidase yet published and that of its functionally inert complex with the pseudosubstrate, glycyl-L-tyrosine, derive from a unique form of carboxypeptidase A(alpha) crystals. These crystals differ from all others with regard both to their spectral properties and activity toward carbobenzoxy-glycyl-L-phenylalanine, which is 30% of that in solution, though the significance of this value cannot be gauged without knowledge of the relevant kinetic constants. The rapidly accumulating evidence for functional and conformational differences between crystals and solutions and the recent stress on the nonproductive aspects of the carboxypeptidase A(alpha)-glycyl-L-tyrosine complex, based on 30% site occupancy, suggest that the functional implications of its structural features require reevaluation.