Activation of bovine factor X (Stuart factor) - analogy with pancreatic zymogen-enzyme systems

Abstract

The activation of bovine coagulation factor X was studied by kinetic and spectrophotometric measurements. The pH dependence of the hydrolysis of specific ester substrates by activated factor Xa can be ascribed to 2 independently ionizing groups with pKa values of 6.9 and 8.8, respectively. The rates of reaction of factor X, before and after activation, with the active-site titrant methanesulfonyl fluoride, suggest that the reactivity of the active-site serine residue in factor X is similar to that in trypsinogen and in factor Xa similar to that in trypsin. Analogous comparisons using diisopropyl phosphofluoridate as the titrant suggest that a hydrophobic binding site is absent in the enzyme and zymogen. This conclusion is consistent with the lack of change in circular dichroism when acyl derivatives of factor X are converted to their acyl enzyme counterparts.