title: Kinetic studies on the activation of human factor X. The role of metal ions on the reaction catalyzed by the venom coagulant protein of Viper russelli.
abstract: The effect of Ca2+, Mg2+, and Mn2+ on the initial rate of activation of human Factor X by the venom coagulant protein of Vipera russelli has been investigated. Neither Mg2+ nor Mn2+ alone support the reaction. Ca2+ is an essential activator and exhibits cooperative kinetics. Both Mg2+ and Mn2+ enhance the reaction cooperatively when Ca2+ is present at suboptimal concentrations. Similarly, Ca2+ quenches the intrinsic fluorescence of human Factor X in a cooperative manner. While neither Mg2+ nor Mn2+ by themselves affect the fluorescence of human Factor X, they decrease the cooperativity of the Ca2+ binding to the protein as judged by Hill plots of the Ca2+ -induced fluoresence quenching. EPR measurements indicate that there are three high affinity Mn2+ binding sites on human Factor X which can also bind Ca2+. Positive cooperativity was not observed for Mn2+ binding. These data indicate that Ca2+ can cause a conformational change of the Factor X molecule which allows the activation reaction to proceed. We propose that Mn2+ does not support the activation of human Factor X because it cannot induce a necessary conformational change in the absence of Ca2+.
mesh_terms: D002118:Calcium / Q000494:pharmacology*; D004789:Enzyme Activation / Q000187:drug effects; D005170:Factor X / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D008345:Manganese / Q000494:pharmacology*; D013050:Spectrometry, Fluorescence; D014757:Viper Venoms / Q000494:pharmacology*
chemical_list: D014757:Viper Venoms; D008345:Manganese; D005170:Factor X; D008274:Magnesium; D002118:Calcium