AN ELECTRON SPIN RESONANCE ANALYSIS OF THE MECHANISM OF SUCCINIC DEHYDROGENASE ACTIVITY

Abstract

It was shown that one of the events that follows the binding of substrate to SDH is the uncoupling of a pair of electrons as a consequence of electron transfer between enzyme and substrate. As a result of this process, the complex possesses one, or possibly two, unpaired electrons capable of inducing the observed ESR signals and is therefore a free radical. While the ESR data do not distinguish among possible members of the class of free radical enzyme-substrate complexes, the data do show that there must exist at least one member of this class from which substrate does not measurably dissociate. This last effect appears to result from major structural changes, which are probably covalent in character and which appear to be localized in or near the substrate part of the complex. These con-siderations indicate that free radical complexes play an essential role in the chemical change in the substrate that is catalyzed by SDH. This conclusion, in keeping with the ESR observations generally, is evidence that free radicals are decisive intermediates in the reactions catalyzed by SDH. It is expected that further ESR and biochemical studies will provide a more precise description of the relationships which exist among the electronic states of the complexes, their ther-modynamic constants, and the configurations of possible complexes in which strong binding of substrate to enzyme may occur.