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

The activation of bovine coagulation factor X has been studied by kinetic and spectrophotometric measurements. The pH dependence of the hydrolysis of specific ester substrates by activated factor Xa can be ascribed to two 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 both the enzyme and zymogen. This conclusion is consistent with the lack of change in circular dichroism when acyl derivatives of factor V are converted to their acyl enzyme counterparts.