N-terminal acetyl group is essential for the inhibitory function of carboxypeptidase Y inhibitor (I C)

Abstract

Carboxypeptidase Y (CPY) inhibitor, I C, a yeast cytoplasmic inhibitor in which the N-terminal amino acid is acetylated, was expressed in Escherichia coli and produced as an unacetylated form of I C (unaI C). Circular dichroism and fluorescence measurements showed that unaI C and I C were structurally identical and produce identical complexes with CPY. However, the K i values for unaI C for anilidase and peptidase activity of CPY were much larger, by 700- and 60-fold, respectively, than those of I C. The reactivities of phenylmethylsulfonyl fluoride and p-chloromercuribenzoic acid toward the CPY–unaI C complex were considerably higher than those toward the CPY–I C complex. Thus, the N-terminal acetyl group of I C is essential for achieving a tight interaction with CPY and for its complete inactivation.