Functional tyrosyl residues of carboxypeptidase A. The effect of protein structure on the reactivity of tyrosine-198.

Coupling of bovine carboxypeptidase A with diazotized 5-amino-1H-tetrazole increases esterase activity, decreases peptidase activity slightly, and modifies one tyrosyl residue. Subsequent nitration of the azoenzyme has no further effect on esterase activity, decreases peptidase activity markedly, and modifies a second tyrosyl residue. Analysis of the azopeptides isolated from a chymotrypsin digest of the doubly modified enzyme by affinity, ion exchange, and high pressure liquid chromatography indicates that the principal residue modified by diazo-1H-tetrazole is Tyr-248. Analysis of the nitropeptides isolated by similar procedures indicates that nitration occurs mainly at Tyr-198. This residue becomes susceptible to modification only as a consequence of a conformational change that accompanies azo coupling of Tyr-248. These results describe a unique example of the influence of protein structure on the reactivity of functional amino acid residues and illustrate an important aspect of chemical modification of enzymes.