title: Thermodynamic and electron paramagnetic resonance characterization of flavin in succinate dehydrogenase.
abstract: Thermodynamic parameters of succinate dehydrogenase flavin were determined potentiometrically from the analysis of free radical signal levels as a function of the oxidation-reduction potential. Midpoint redox potentials of consecutive 1-electron transfer steps are -127 and -31 mV at pH 7.0. This corresponds to a stability constant of intermediate stability, 2.5 x 10(-2), which suggests flavin itself may be a converter from n = 2 to n = 1 electron transfer steps. The pK values of the free radical (FlH . in equilibrium Fl . -) and the fully reduced form (FlH2 in equilibrium FlH-) were estimated as 8.0 +/- 0.2 and 7.7 +/- 0.2, respectively. Succinate dehydrogenase flavosemiquinone elicits an EPR spectrum at g = 2.00 with a peak to peak width of 1.2 mT even in the protonated form, suggesting the delocalization in the unpaired electron density. A close proximity of succinate dehydrogenase flavin and iron-sulfur cluster S-1 was demonstrated based on the enhancement of flavin spin relaxation by Center S-1.
mesh_terms: D000693:Anaerobiosis; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D005415:Flavins / Q000032:analysis*; D005609:Free Radicals; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010084:Oxidation-Reduction; D010462:Peptostreptococcus / Q000201:enzymology; D011199:Potentiometry; D013385:Succinate Dehydrogenase / Q000378:metabolism*; D013816:Thermodynamics
chemical_list: D005415:Flavins; D005609:Free Radicals; D013385:Succinate Dehydrogenase