Recent studies suggested that modification of the membrane contact site of vitamin K-dependent proteins may enhance the membrane affinity and function of members of this protein family. The properties of a factor VII mutant, factor VII-Q10E32, relative to wild-type factor VII (VII, containing P10K32), have been compared. Membrane affinity of VII-Q10E32 was about 20-fold higher than that of wild-type factor VII. The rate of autoactivation VII-Q10E32 with soluble tissue factor was 100-fold faster than wild-type VII and its rate of activation by factor Xa was 30 times greater than that of wild-type factor VII. When combined with soluble tissue factor and phospholipid, activated factor VII-Q10E32 displayed increased activation of factor X. Its coagulant activity was enhanced in all types of plasma and with all sources of tissue factor tested. This difference in activity (maximum 50-fold) was greatest when coagulation conditions were minimal, such as limiting levels of tissue factor and͞or phospholipid. Because of its enhanced activity, factor VII-Q10E32 and its derivatives may provide important reagents for research and may be more effective in treatment of bleeding and͞or clotting disorders.Factor VII, a protein that requires vitamin K for biosynthesis, is a central component in the initiation of coagulation. In complex with tissue factor (TF), the active form of factor VII, factor VII(a), can activate blood clotting factors IX and X (1). Involvement in the initial steps of blood coagulation makes factor VII an attractive target for study. In addition, direct activation of factor X by factor VII(a) effectively bypasses the coagulation step that involves factors VIII(a) and IX(a). Deficiency in factors VIII and IX form the basis of hemophilia A (2) and B (3), respectively. These bleeding disorders are major health concerns in the United States (4). Factor VII deficiency, a rare autosomal recessive disorder, also can cause serious bleeding complications. Wild-type recombinant factor VII(a) can effectively treat patients with factors VIII, IX, and VII deficiencies (5-7). Thus, the possibility of creating a factor VII protein with enhanced biological activity may be useful for therapeutic application as well as for research.An attractive target for enhanced function of vitamin Kdependent plasma proteins is the membrane contact site. This family of proteins displays a broad range of affinities for acidic phospholipid membranes, despite a high degree of sequence homology in the ␥-carboxyglutamic acid (Gla) domain (8, 9). Although the purpose(s) of such diverse affinities are unclear, they may serve to balance pro-and anti-coagulation arms of hemostasis (10) and͞or alter the kinetic mechanism of individual steps of these reactions. Recent studies revealed a correlation between the amino acids at positions 11, 33, and 34 (bovine prothrombin numbering) and overall membrane affinity (8). Mutation of these amino acids may produce proteins with higher membrane affinity and increased function. Indeed, substitution of Hi...