Structure of the potato inhibitor complex of carboxypeptidase A at 2.5-A resolution.

The structure of the complex between the proteolytic enzyme carboxypeptidase A (peptidyl-L-amino-acid hydrolase, EC 3.4.17.1) and the 39-amino-acid carboxypeptidase A inhibitor from potatoes has been determined at 2.5-A resolution. A combination of multiple isomorphous replacement, molecular replacement, and noncrystallographic symmetry averaging techniques was used to solve the structure. The chain trace of the inhibitor and details of the binding interactions in the complex are described. A surprising aspect of the complex is that the carboxy-terminal peptide bond of the inhibitor has been hydrolyzed, and the carboxy-terminal glycine is trapped in the binding pocket of carboxypeptidase A. Consequently, the complex resembles a stage in the catalytic mechanism after hydrolysis of the peptide bond. The ring of tyrosine-248, which is known to undergo large conformational changes upon substrate binding, is in the "down" position and interacts with the inhibitor in the complex.