Surface-independent acceleration of factor XII activation by zinc ions. II. Direct binding and fluorescence studies

To determine the role of Zn(II)-factor XII interactions in the rate-enhancing effect of Zn(II) on factor XII activation demonstrated in the preceding paper, equilibrium binding of zinc ions to factor XII, and the spectroscopic changes accompanying this binding were investigated. Equilibrium dialysis provided direct evidence for the binding of Zn(II) to factor XII. The binding data were consistent with 7.8 +/- 0.3 zinc ions binding with an indistinguishable Kd of 91 +/- 6 microM. Binding of Zn(II) was accompanied by a 10% quenching of the intrinsic protein fluorescence and a 2-nm red shift of the wavelength of maximum emission. These spectroscopic changes were specific for factor XII and were not observed with factor XIIa. The Zn(II) concentration dependence of factor XII fluorescence quenching was sigmoid and paralleled the Zn(II)-accelerating effect of factor XII activation by kallikrein and factor XIIa, indicating that the spectral change was reporting Zn(II)-factor XII interactions responsible for the enhanced activation rate. The apparent cooperativity of Zn(II) effects on factor XII fluorescence quenching and activation kinetics, and the apparent noncooperativity in Zn(II) binding to factor XII measured by equilibrium dialysis could be explained by a two-state model in which Zn(II) binding is linked to a conformational change in the protein. The Zn(II)-induced quenching of factor XII fluorescence exhibited a pH dependence consistent with the involvement of histidine residues in the binding of Zn(II). Dynamic quenching of factor XII protein fluorescence by iodide or acrylamide, in the absence and presence of Zn(II), revealed heterogeneity in the environment of the 13 tryptophan residues of factor XII that was markedly reduced by metal ion binding. Together, these results indicate that cooperative interactions of Zn(II) with factor XII induce structural changes in the zymogen that facilitate its proteolytic cleavage and activation.