A Peptide Region from Factor Va Increases Factor Xa Efficiency for Cleavage of Prothrombin.

The procoagulant enzymatic complex, prothrombinase, which is required for normal hemostasis, is composed of the enzyme, factor Xa, the protein cofactor, factor Va, associated on a cell surface in the presence of divalent metal ions. Incorporation of factor Va into prothrombinase and its interaction with factor Xa increases the catalytic efficiency of the enzyme by five orders of magnitude as compared to factor Xa alone. While the importance of the contribution of factor Va to the activity of factor Xa for rapid thrombin formation by prothrombinase at the place of vascular injury has been long established, the consequence of the interaction of the cofactor with the members of prothrombinase and the molecular mechanism by which factor Va accelerates prothrombin activation remains an enigma. Prothrombin is activated following two cleavages (Arg²⁷¹/Arg³²⁰). Depending on the order of peptide bond cleavage different intermediates are formed. Factor Xa alone cleaves prothrombin sequentially, first at Arg²⁷¹ to produce fragment 1•2 and prethrombin-2, followed by cleavage at Arg³²⁰ to produce fragment 1•2 and thrombin. The prothrombinase complex catalyzes the activation of prothrombin following the opposite pathway (Arg³²⁰ followed by Arg²⁷¹), resulting in a formation of an active intermediate (meizothrombin) and a 300,000-fold increase in the rate of the overall reaction compared with the rate of prothrombin activation observed with factor Xa alone. We have shown that amino acid region 307–348 of factor Va heavy chain is critical for cofactor activity. A peptide containing this amino acid sequence (42 amino acids, N42R) was found to interact with fluorescently labeled factor Xa and to inhibit prothrombinase activity. Our present data show that N42R can be cross-linked to the heavy chain of membrane-bound factor Xa in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). We have also demonstrated that amino acid region 323–331 from N42R (AP4′) contains a binding site for factor Xa of factor Va heavy chain. Our present data show that a peptide containing amino acid residues 317–326 (AP3) inhibited both prothrombinase activity and the high affinity interaction of factor Va with factor Xa on the membrane surface. Moreover, we have found using site directed mutagenesis and recombinant factor Va that amino acids at the NH₂-terminal end of AP4′ (i.e. residues 323–325, Glu-Tyr-Phe) are responsible for the inhibitory effect of AP3 and AP4′ and are crucial for the interaction of factor Va with factor Xa. A tripeptide with this sequence inhibited prothrombinase activity in an assay using a fluorescent thrombin inhibitor. To identify the effect of these peptides on factor Xa’s ability to cleave and activate prothrombin, we studied prothrombin activation by gel electrophoresis. The data demonstrated that several peptides that inhibited both the factor Va-factor Xa interaction on the membrane surface and prothrombinase activity, had the ability to accelerate cleavage of prothrombin by factor Xa alone, in the absence of factor Va. Specifically, N42R and AP3 were found to increase the rate of prothrombin consumption by factor Xa by approximately four-fold when compared to factor Xa acting alone. Both peptides induced acceleration in prethrombin-2 formation suggesting an increased in the rate of cleavage of prothrombin at Arg²⁷¹. These data suggest that the binding of factor Va to factor Xa through amino acid region 323–331 alone produces an effect on factor Xa that increases its potency for cleavage at Arg²⁷¹.