Contribution of A1 subunit residue Q316 in thrombin-activated factor VIII to A2 subunit dissociation

Blood coagulation factor VIII (fVIII) is activated by thrombin to form an A1/A2/A3-C1-C2 heterotrimer, which functions as a cofactor for factor IXa during intrinsic pathway factor X activation. Human thrombin-activated fVIII (fVIIIa) decays rapidly because of first-order dissociation of the A2 subunit, which may function to regulate the coagulation mechanism. The three fVIII A domains each consist of two cupredoxin-like subdomains. Substitution of the COOH-terminal A1 subdomain of porcine fVIIIa, which decays more slowly than human fVIIIa, reduces the dissociation rate constant for fVIIIa decay. Examination of a human fVIII A1-A2-A3 homology model [Pemberton, S., et al. (1997) Blood 89, 2413-2421) revealed a possible interaction between Q316 in the FG helix of the COOH-terminal A1 subdomain and M539 in the FG helix of the NH2-terminal A2 subdomain, which are sites where human and porcine fVIII differ. Decays of purified recombinant human and porcine fVIIIa and the human fVIIIa mutants Q316H, M539L and Q316H/M539L were compared at 23 and 37 degrees C. The decay rates of the Q316H and Q316H/M539L mutants, but not the M539L mutant, were significantly slower than human fVIIIa. These results indicate that the FG helix of the COOH-terminal A1 cupredoxin-like subdomain of fVIII may be under selective pressure by the requirements of hemostatic balance.