[Dissociation constants of succinate dehydrogenase complexes with succinate, fumarate and malonate]

The rates of the oxidized (Eox) and reduced (Ered) (by NAD . H through the ubiquinone pool) succinate dehydrogenase inhibition by N-ethyl-maleimide are equal and obey pseudo-first order kinetics. The protection of the enzyme against irreversible alkylation was used to quantitate the dissociation constants for Eox and Ered complexes with fumarate, succinate and malonate under conditions when no intramolecular redox reactions might occur. the membrane-bound succinate dehydrogenase catalyzes the succinate : phenazine-methosulphate reductase reaction in the presence of thenoyltrifluoroacetone by a Slater-Bonner mechanism. A comparison of the constants measured by the protection with those derived from the steady-state kinetics shows that succinate affinity for Eox is about 10 times higher than that for Ered; the reverse relations were found for fumarate, whereas the affinity for malonate only slightly depends on the redox state of the enzyme. The data obtained suggest that the dicarboxylate binding at the active site induces changes in the enzyme redox potential. The surface charge does not contribute significantly to the energy of the dicarboxylate binding to the active site of the membrane-bound enzyme.