Spectral changes in bovine factor X associated with activation by the venom coagulant protein of Vipera russelli

Bovine Factor X is a zymogen involved in blood coagulation that is converted to activated Factor X in the presence of Ca(II) by the coagulant protein of Russell's viper venom. To monitor structural transitions in Factor X during conversion to activated Factor X, the ultraviolet absorption, fluorescence emission, and circular dichroism spectra of activated Factor X and Factor X were compared. The ultraviolet absorption difference spectrum in the aromatic region comparing activated Factor X and Factor X indicates minima at 292.5, 285, and 278 nm and a small maximum at 305 nm; these differences are due to tryptophan and tyrosine perturbations. The activation of Factor X at 25 degrees in the presence of 8.3 mM CaCl2 with the use of Factor X:venom coagulant protein in molar rations of 1500:1 yielded a time-dependent increase in this spectrum which was linear for about 60 min and which temporally paralleled the development of activated Factor X activity. The binding of Ca(II) to factor X or activated Factor X is associated with a red-shifted tryptophan difference spectrum; however, this perturbation appears to make only a small contribution to the total perturbation observed during Factor X activation. Solvent perturbation studies in 20% glycerol suggest that an average of 3.1 tryptophan residues and 9.0 tyrosine residues are exposed to solvent in Factor X in 8.3 mM CaCl2 at pH 7.4; an additional 0.5 tryptophan residue and tyrosine reside become exposed to solvent during activation of Factor X in 8.3 mM CaCl2. The activation of Factor X by the venom coagulant protein is associated with a small red shift in the intrinsic tryptophan fluorescence emission spectrum. Far- and near-ultraviolet circular dichroism spectroscopy detected no difference between Factor X and activated Factor X. In summary, the activation of Factor X to activated Factor X appears associated with exposure of tryptophan and tyrosine side chains previously buried within the protein and with minimal changes in the secondary structur. These results suggest that conversion of Factor X to activated Factor X involves functionally important, but structurally subtle, changes in the three-dimentional structure.