Synergistic binding of ligands to angiotensin-converting enzyme

We have investigated the interaction of ligands in the active site of the angiotensin-converting enzyme from rabbit lung by monitoring the concurrent effects of two inhibitors on enzyme activity. A strong synergism is found in the binding of N-acetyl-L-proline (an analog of the COOH-terminal dipeptide portion of preferred substrates) and acetohydroxamate (a zinc ligand). Analysis of the inhibition data with the Yone-tani-Theorell plot yields an unusually low value of 0.0063 for the interaction constant (alpha). This result indicates that each of the above ligands stimulates the binding of the other by about 150-fold. Similar but often less pronounced synergism is observed for other zinc ligands and with some other N-acyl amino acids. These specific structural requirements suggest that the above effect is associated with an induced-fit mechanism which brings the important zinc atom into a catalytically optimal state only in the presence of certain preferred substrates.