Incorporation of factor Va into prothrombinase is required for coordinated cleavage of prothrombin by factor Xa.

Prothrombin is activated to thrombin by two sequential factor Xa-catalyzed cleavages, at Arg271 followed by cleavage at Arg320. Factor Va, along with phospholipid and Ca2+, enhances the rate of the process by 300,000-fold, reverses the order of cleavages, and directs the process through the meizothrombin pathway, characterized by initial cleavage at Arg320. Previous work indicated reduced rates of prothrombin activation with recombinant mutant factor Va defective in factor Xa binding (E323F/Y324F and E330M/V331I, designated factor VaFF/MI). The present studies were undertaken to determine whether loss of activity can be attributed to selective loss of efficiency at one or both of the two prothrombin-activating cleavage sites. Kinetic constants for the overall activation of prothrombin by prothrombinase assembled with saturating concentrations of recombinant mutant factor Va were calculated, prothrombin activation was assessed by SDS-PAGE, and rate constants for both cleavages were analyzed from the time course of the concentration of meizothrombin. Prothrombinase assembled with factor VaFF/MI had decreased k(cat) for prothrombin activation with Km remaining unaffected. Prothrombinase assembled with saturating concentrations of factor VaFF/MI showed significantly lower rate for cleavage of plasma-derived prothrombin at Arg320 than prothrombinase assembled with saturating concentrations of wild type factor Va. These results were corroborated by analysis of cleavage of recombinant prothrombin mutants rMz-II (R155A/R284A/R271A) and rP2-II (R155A/R284A/R320A), which can be cleaved only at Arg320 or Arg271, respectively. Time courses of these mutants indicated that mutations in the factor Xa binding site of factor Va reduce rates for both bonds. These data indicate that the interaction of factor Xa with the heavy chain of factor Va strongly influences the catalytic activity of the enzyme resulting in increased rates for both prothrombin-activating cleavages.