Residue Met(156) contributes to the labile enzyme conformation of coagulation factor VIIa

Serine protease activation is typically controlled by proteolytic cleavage of the scissile bond, resulting in spontaneous formation of the activating Ile(16)-Asp(194) salt bridge. The initiating coagulation protease factor VI Ia (VIIa) differs by remaining in a zymogen-like conformation that confers the control of catalytic activity to the obligatory cofactor and receptor t issue factor (TF). This study demonstrates that the unusual hydrophobic Met (156) residue contributes to the propensity of the VIIa protease domain to remain in a zymogen-like conformation. Mutation of Met(156) to Gln, which i s found in the same position of the highly homologous factor IX, had no inf luence on the amidolytic and proteolytic activity of TF-bound VIIa. Further more, the mutation did not appreciably stabilize the labile Ile(16)-Asp(194 ) salt bridge in the absence of cofactor. VIIa(Gln156) had increased affini ty for TF, consistent with a long range conformational effect that stabiliz ed the cofactor binding site in the VIIa protease domain. Notably, in the a bsence of cofactor, amidolytic and proteolytic function of VIIa(Gln156) wer e enhanced 3 and 9-fold, respectively, compared with wild-type VIIa. The mu tation thus selectively influenced the catalytic activity of free VIIa, ide ntifying the Met(156) residue position as a determinant for the zymogen-lik e properties of free VIIa.