Silver ion inhibition of serine proteases: Crystallographic study of silver-trypsin

Abstract

Silver ion is a potent inhibitor of trypsin and chymotrypsin, with KI's of 4 × 10−5 M. and 3 × 10−5, respectively. A crystallographic study shows that the primary silver ion binding site on trypsin is at the active center between the carboxyl group of Asp 102 and the δ-nitrogen of His 57. This result is correlated with the fact that Ag+ interferes primarily with the acylation rate constant, k2, and does not significantly affect the binding constant, Ks. The location of this site explains the potent inhibitory effect of silver (I) ions on trypsin activity: The imidazole ring of His 57 is repositioned 1.8 Å further out into the solvent to accommodate the silver ion, preventing its normal interaction with the hydroxyl group of Ser 195. Consequently, His 57 cannot directly assist the proton transfer in the catalyzed reaction. Since silver ion binds to the catalytic site in this highly specific manner, silver may be used as a specific probe of the active site of serine proteases.