Electrocatalytic voltammetry of succinate dehydrogenase: Direct quantification of the catalytic properties of a complex electron-transport enzyme

Succinate dehydrogenase (SDH), the membrane-extrinsic component of Complex II, adsorbs at a pyrolytic graphite edge electrode and catalyzes interconversion of succinate and fumarate depending on the electrochemical potential that is applied. The catalytic activity is measured over a continuous potential range, leading to a quantitative description of the interlinked energetics and kinetics of catalyzed electron transport, including the degree to which. the enzyme is intrinsically tuned, at a particular pH, to function either in the direction of succinate oxidation or fumarate reduction. It is revealed that under reversible conditions (i.e. near the reduction potential of the fumarate/ succinate couple) and at the physiological temperature of 38 degrees C, SDH is biased to catalyze fumarate reduction (reversal of the tricarboxylic acid cycle) at pH values below 7.7. Subtle effects which gate electron transport are detected. First, the sharp drop in catalytic activity observed as the potential is made more negative is an intrinsic property that is associated with two-electron/two-proton reduction of the FAD, and second, binding and release of the competitive inhibitor/regulator oxalacetate is observed as the enzyme is cycled between FAD(ox) (tight binding) and FAD(red) (weaker binding) states. It is thereby demonstrated how the electron-transport characteristics of a complex redox enzyme, integrating both kinetic and thermodynamic information, can be derived from voltammetric experiments.