TY - JOUR
T1 - The Effect of Opposing Effectors on Activation Level of Succinate Dehydrogenase
T2 - Equilibrium and Kinetic Studies
AU - Gutman, Menachem
PY - 1976/3/1
Y1 - 1976/3/1
N2 - The activation of mitochondrial succinate dehydrogenase by various activators is a result of dissociation of oxaloacetate tightly bound to the nonactive enzyme. But, quantitative correlation between the effector concentrations and the active fraction of the enzyme was not at hand. In this study we measured the level of active succinate dehydrogenase equilibrated with a wide range of opposing effectors: oxaloacetate (1-500 μ) and activator (0.02-1.5 M NaBr). The results are compatible with a model assuming two stable forms of the enzyme: a nonactive enzyme-oxa-loacetate complex and an active enzyme free of oxaloacetate. The active form is stabilized by binding two Br- and one H+. The rate of activation (ka) and exchange between enzyme bound and free oxaloacetate k(ex) were measured. Both ka and ktx are hyperbolically dependent on Br- concentration but differ in magnitude and pH dependence. kex at infinite Br- concentration is pH dependent but ka is not. The two reactions, activation and exchange, also differ in their activation energy being 32 and 21.5 kcal/mol, respectively. It is concluded that, in the course of activation, Br- interacts at two distinct steps. First to produce a ternary, nonactive [enzyme-oxaloacetate-Br-] complex. From this complex, oxaloacetate dissociates and the oxaloacetate-free enzyme assumes its active form. Finally, the active enzyme is stabilized by binding another Br-. The rate-limiting step in deactivation is binding of oxaloacetate to active enzyme. The complex formed undergoes a very rapid transformation to the stable nonactive form. This pathway, under certain conditions, can reverse its direction and contribute to the overall rate of activation. It is suggested that the equilibrium between the two stable forms of the enzyme can be reached by two parallel pathways, each contributing independently to the observed rate of activation, while the final equilibrium is determined by the free energy between the products and the reactants.
AB - The activation of mitochondrial succinate dehydrogenase by various activators is a result of dissociation of oxaloacetate tightly bound to the nonactive enzyme. But, quantitative correlation between the effector concentrations and the active fraction of the enzyme was not at hand. In this study we measured the level of active succinate dehydrogenase equilibrated with a wide range of opposing effectors: oxaloacetate (1-500 μ) and activator (0.02-1.5 M NaBr). The results are compatible with a model assuming two stable forms of the enzyme: a nonactive enzyme-oxa-loacetate complex and an active enzyme free of oxaloacetate. The active form is stabilized by binding two Br- and one H+. The rate of activation (ka) and exchange between enzyme bound and free oxaloacetate k(ex) were measured. Both ka and ktx are hyperbolically dependent on Br- concentration but differ in magnitude and pH dependence. kex at infinite Br- concentration is pH dependent but ka is not. The two reactions, activation and exchange, also differ in their activation energy being 32 and 21.5 kcal/mol, respectively. It is concluded that, in the course of activation, Br- interacts at two distinct steps. First to produce a ternary, nonactive [enzyme-oxaloacetate-Br-] complex. From this complex, oxaloacetate dissociates and the oxaloacetate-free enzyme assumes its active form. Finally, the active enzyme is stabilized by binding another Br-. The rate-limiting step in deactivation is binding of oxaloacetate to active enzyme. The complex formed undergoes a very rapid transformation to the stable nonactive form. This pathway, under certain conditions, can reverse its direction and contribute to the overall rate of activation. It is suggested that the equilibrium between the two stable forms of the enzyme can be reached by two parallel pathways, each contributing independently to the observed rate of activation, while the final equilibrium is determined by the free energy between the products and the reactants.
UR - http://www.scopus.com/inward/record.url?scp=0017252865&partnerID=8YFLogxK
U2 - 10.1021/bi00651a027
DO - 10.1021/bi00651a027
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AN - SCOPUS:0017252865
SN - 0006-2960
VL - 15
SP - 1342
EP - 1348
JO - Biochemistry
JF - Biochemistry
IS - 6
ER -