Role of phosphatidylethanolamine in assembly and function of the factor IXa-Factor VIIIa complex on membrane surfaces

Phospholipid membranes play a significant role during the proteolytic activ ation of blood coagulation proteins. This investigation identifies a role f or phosphatidylethanolamine (PE) during the activation of factor X by the t enase complex, an enzymatic complex composed of the serine protease, factor IXa, a protein cofactor, factor VIIIa, a phospholipid membrane, and Ca2+. Phospholipid vesicles composed of PE, phosphatidylserine (PS), and phosphat idylcholine support factor Xa generation The K-m and k(cat) for factor X ac tivation by the tenase complex are independent of PE in the presence of 20% PS. At lower PS concentrations, the presence of 20 or 35% PE lowers the K- m and increases the k(cat) as compared to those in vesicles without PE. The effect of PE on the k(cat) of the tenase complex is mediated through facto r VIIIa. PE also enhances factor Xa generation by facilitating tenase compl ex formation; PE lowers the K-d(app) of factor IXa for both phospholipid/Ca 2+ and phospholipid/Ca2+/factor VIIIa complexes in the presence of suboptim al PS, In addition, the K(d)s of factor IXa and factor X were lower for pho spholipid vesicles containing PE. N-Methyl-PE increased the k(cat) and decr eased the K-d(app), whereas N,N-dimethyl-PE had no effect on either paramet er, indicating the importance of headgroup size. Lyso-PE had no effect on k inetic parameters, indicating the sn-2 acyl chain dependence of the PE effe ct. Together, these results demonstrate a role for PE in the assembly and a ctivity of the tenase complex and further extend the understanding of the i mportance of PE-containing membranes in hemostasis.