Effect of Exceptional Valine Replacement for Highly Conserved Alanine-55 on the Catalytic Site Structure of Chymotrypsin-like Serine Protease

The catalytic triad consisting of His57, Asp102 and Ser195, which is completely conserved within the chymotrypsin-like serine protease family, plays a central role in catalysis. Highly conserved Ala55 also likely plays an important role in catalysis due to its location just behind the catalytic triad. The only exception to the conserved Ala55 in mammalian serine proteases is Val55 in bovine protein C. Interestingly, it has been demonstrated that the replacement of Ala55 with Thr results in the reduced activity of plasmin in patients with venous thrombosis and with retinochoroidal vascular disorders, which indicates the importance of Ala55 in catalysis. In the present study, we constructed a bovine protein C model which shows that Val55 causes no serious rearrangement of the catalytic site structure. We also constructed an A55T variant model of trypsin for comparison. The A55T substitution alters His57 into an inactive conformation, forming an unusual hydrogen bond between Thr55 O gamma 1 and His57 N epsilon 2. The present study shows that the Ala/Val55 residue contributes heavily to the active conformation of His57 and enables His57 to accept a proton from Ser195 O gamma effectively.