The active site of carboxypeptidase Cu. II. Photooxidation of carboxypeptidases Cua and Cub.

Abstract

Carboxypeptidases CUa and CUb were both inactivated by photooxidation in the presence of methylene blue at pH 5.5 and 8°C. The inactivation was partially prevented by a competitive inhibitor, 3-phenylpropionic acid, of the enzymes. It seemed unlikely that the enzymes underwent a change in molecular size during the photooxidation on the basis of their behavior in electrophoresis and gel filtration. When the photooxidation was carried out at various pH value ranging from 4.5 to 7.5, the rate of inactivation was found to be pH-dependent and the pH profiles conformed to theoretical titration curves with an apparent pKa value of 6.5, suggesting that an imidazole group of a histidine residue is essential for the enzymatic activity. The photooxidized enzymes had significantly decreased histidine contents, whereas the contents of other amino acids remained essentially unchanged. It was shown that one histidine residue is involved in each active site of the enzymes.