Surface - independent Acceleration of Factor XI 1 Activation by Zinc Ions

The effect of zinc ions (Zn(I1)) on the activation of factor XI1 in the absence of a procoagulant surface was investigated by initial velocity kinetic studies at I = 0.16, pH 7.4, and 26 “C. Zinc ions at concentrations greater than 160 p M potentiated 99-fold the kcat/KM for the activation of factor XI1 by kallikrein and, at an optimum concentration of 110 pM, accelerated 140fold the apparent koaJKM for factor XI1 autoactivation. High molecular weight kininogen had no effect on either metal-potentiated reaction. Analysis of the factor XI1 concentration dependence of initial activation rates revealed that Zn(II), at levels that saturate the effect, accelerates kallikrein activation of factor XI1 by lowering KM (from 52 to 7.3 p ~ ) and raising kcat (from 2.6 to 31 min”). For the autocatalytic activation reaction of factor XI1 in the presence of optimal Zn(II), apparent KM and kcat values of 2.4 @M and 0.041 min-’, respectively, were determined, but these parameters were not resolvable in the absence of the metal ion. Zinc ions minimally affected kallikrein enzymatic activity and inhibited factor XIIa enzymatic activity with KI values of 20-40 p ~ , suggesting that the rate-enhancing effects of the metal ion are due to interactions with the substrate (factor XII) rather than with the enzyme. The Zn(I1) inhibition of factor XIIa enzymatic activity accounted for a decreased Zn(I1) enhancement of factor XI1 autoactivation at high metal ion concentrations (>110 p ~ ) . The Zn(I1) concentration dependence of the acceleration of factor XI1 activation reactions were sigmoid and characterized by Hill coefficients of 3.3-4.3, suggesting that cooperative binding of at least four zinc ions to factor XI1 was responsible for the Zn(I1) potentiating effect. The Zn(I1) enhancement of the rates of factor XI1 activation decreased both above and below pH 7.4 with midpoint pH values of 6.6-7.0 and 8.0, consistent with histidine and possibly water ligands mediating Zn(I1) binding to the protein. Despite an apparent weaker binding of Zn(I1) to factor XI1 at pH 6.6, indistinguishable maximum accelerating effects of the metal ion were observed at saturation at this pH, indicating that the increased positive charge of factor XI1 resulting from protonation at the lower pH did not mimic the effect of Zn(I1) binding. These results imply that zinc ions induce a conformational change in factor XI1 that makes it a better substrate for its enzyme activators.