title: Classification of fumarate reductases and succinate dehydrogenases based upon their contrasting behaviour in the reduced benzylviologen/fumarate assay.
abstract: Reduction of fumarate by soluble beef heart succinate dehydrogenase has been shown previously by voltammetry to become increasingly retarded as the potential is lowered below a threshold potential of -80 mV at pH 7.5. The behaviour resembles that of a tunnel diode, an electronic device exhibiting the property of negative resistance. The enzyme thus acts to oppose fumarate reduction under conditions of high thermodynamic driving force. We now provide independent evidence for this phenomenon from spectrophotometric kinetic assays. With reduced benzylviologen as electron donor, we have studied the reduction of fumarate catalysed by various enzymes classified either as succinate dehydrogenases or fumarate reductases. For succinate dehydrogenases, the rate increases as the concentration of reduced dye (driving force) decreases during the reaction. In contrast, authentic fumarate reductases of anaerobic cells (and 'succinate dehydrogenase' from Bacillus subtilis) neither exhibit the electrochemical effect nor deviate from simple kinetic behaviour in the cuvette assay. The 'tunnel-diode' effect may thus represent an evolutionary adaptation to aerobic metabolism.
authors.affiliation: Department of Biochemistry and Biophysics, University of California, San Francisco.
authors.forename: B A
authors.initials: BA
authors.lastname: Ackrell
authors.forename: F A
authors.initials: FA
authors.lastname: Armstrong
authors.forename: B
authors.initials: B
authors.lastname: Cochran
authors.forename: A
authors.initials: A
authors.lastname: Sucheta
authors.forename: T
authors.initials: T
authors.lastname: Yu
mesh_terms: D000818:Animals; D017165:Ascaris suum / Q000201:enzymology; D001412:Bacillus subtilis / Q000201:enzymology; D001594:Benzyl Viologen / Q000378:metabolism*; D002417:Cattle; D004563:Electrochemistry; D004926:Escherichia coli / Q000201:enzymology; D005260:Female; D005650:Fumarates / Q000378:metabolism*; D010084:Oxidation-Reduction; D010920:Placenta / Q000201:enzymology; D051381:Rats; D013053:Spectrophotometry; D013385:Succinate Dehydrogenase / Q000145:classification* / Q000378:metabolism; D013386:Succinates / Q000378:metabolism; D019802:Succinic Acid
chemical_list: D005650:Fumarates; D013386:Succinates; D001594:Benzyl Viologen; C032005:fumaric acid; D019802:Succinic Acid; D013385:Succinate Dehydrogenase