SERPIN-PROTEASE COMPLEXES ARE TRAPPED AS STABLE ACYL-ENZYME INTERMEDIATES

The serine protease inhibitors of the serpin family are an unusual gro up of proteins thought to have metastable native structures. Functiona lly, they are unique among polypeptide protease inhibitors, although t heir precise mechanism of action remains controversial, Conflicting re sults from previous studies have suggested that the stable serpin-prot ease complex is trapped in either a tight Michaelis-like structure, a tetrahedral intermediate, or an acyl-enzyme, In this report we show th at, upon association with a target protease, the serpin reactive-cente r loop (RCL) is cleaved resulting in formation of an acyl-enzyme inter mediate. This cleavage is coupled to rapid movement of the RCL into th e body of the protein bringing the inhibitor closer to its lowest free energy state. From these data we suggest a model for serpin action in which the drive toward the lowest free energy state results in trappi ng of the protease-inhibitor complex as an acyl-enzyme intermediate.