Regulation of Mitochondrial Succinate Dehydrogenase by Substrate Type Activators

The activation of succinate dehydrogenase in submitochondrial particles was measured in the presence of oxaloacetate (a negative modulator) and substrate type activators (succinate, malonate, and fumarate). Quantitative analysis of equilibrium and kinetic experiments led to the following model for activation [formula omitted] The only stable forms of the enzyme are the nonactive complex with oxaloacetate (E_NAOAA) and the active complex with the activator (E_AAct). The rate-limiting step in activation is the spontaneous conformation change from the nonactive to the active complex with oxaloacetate (E_AOAA). This reaction is shown to precede both the dissociation of oxaloacetate and the binding of activator. The only stable form of active enzyme is the active 1:1 complex with the activator (E_A Act). The free active enzyme (E_A) is unstable in the sense that both activators and oxaloacetate rapidly react with it forming tight complexes. The reaction between oxaloacetate and the free active enzyme is extremely fast (k = 1.24 × 106 M^-1 s^-1 at 30 °C and pH 7.4). The role of activator is to stabilize E_A in a complex which cannot react directly with oxaloacetate. Because of the low concentration of E_A, the rate-limiting step in deactivation is the bimolecular reaction between E_A and OAA. The kinetic and thermodynamic parameters were combined in equations which predict for any combination of negative (oxaloacetate) and positive (substrate type activators) modulators the equilibrium level of active enzyme and the rate at which this state is approached. The dissociation constants of the activators from the regulatory site are markedly different from the respective K_s or K_i values. This indicates that the substrate binding site and the regulatory site are not the same. The thiohemiacetal model for deactivation of oxaloacetate is discussed.