| Abstract
| - A new approach for studying intramolecular electron transfer in multicenter enzymes is described.Two fumarate reductases, adsorbed on an electrode in a fully active state, have been studied using square-wave voltammetry as a kinetic method to probe the mechanism of the long-range electron transfer to andfrom the buried active site. Flavocytochrome c3 (Fcc3), the globular fumarate reductase from Shewanellafrigidimarina, and the soluble subcomplex of the membrane-bound fumarate reductase of Escherichia coli(FrdAB) each contain an active site FAD that is redox-connected to the surface by a chain of hemes orFe−S clusters, respectively. Using square-wave voltammetry with large amplitudes, we have measuredthe electron-transfer kinetics of the FAD cofactor as a function of overpotential. The results were modeledin terms of the FAD group receiving or donating electrons either via a direct mechanism or one involvinghopping via the redox chain. The FrdAB kinetics could be described by both models, while the Fcc3 datacould only be fit on the basis of a direct electron-transfer mechanism. This raises the likelihood that electrontransfer can occur via a superexchange mechanism utilizing the heme groups to enhance electronic coupling.Finally, the FrdAB data show, in contrast to Fcc3, that the maximum ET rate at high overpotential is relatedto the turnover number for FrdAB measured previously so that electron transfer is the limiting step duringcatalysis.
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