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 c ompletely conserved within the chymotrypsin-like serine protease famil y, plays a central role in catalysis. Highly conserved Ala55 also like ly plays an important role,in catalysis due to its location just behin d the catalytic triad. The only exception to the conserved Ala55 in ma mmalian serine proteases is Val55 in bovine protein C. Interestingly, it has been demonstrated that the replacement of Ala55 with Thr result s in the reduced activity of plasmin in patients with venous thrombosi s and with retinochoroidal vascular disorders, which indicates the imp ortance of Ala55 in catalysis. In the present study, we constructed a bovine protein C model which shows that Va155 causes no serious rearra ngement of the catalytic site structure. We also constructed an A55T v ariant model of trypsin for comparison. The A55T substitution alters H is57 into an inactive conformation, forming an unusual hydrogen bond b etween Thr55 O gamma 1 and His57 N epsilon 2. The present study shows that the Ala/Val55 residue contributes heavily to the active conformat ion of His57 and enables His57 to accept a proton from Ser195 O gamma effectively.