Dimethylbiguanide inhibits cell respiration via an indirect effect targeted on the respiratory chain complex I.

▪ Abstract The cytochrome bc complexes represent a phylogenetically diverse group of complexes of electron-transferring membrane proteins, most familiarly represented by the mitochondrial and bacterial bc 1 complexes and the chloroplast and cyanobacterial b 6 f complex. All these complexes couple electron transfer to proton translocation across a closed lipid bilayer membrane, conserving the free energy released by the oxidation-reduction process in the form of an electrochemical proton gradient across the membrane. Recent exciting developments include the application of site-directed mutagenesis to define the role of conserved residues, and the emergence over the past five years of X-ray structures for several mitochondrial complexes, and for two important domains of the b 6 f complex.

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Structure and Function of Cytochrome bc Complexes

Cardiolipin requirement for electron transfer in complex I and III of the mitochondrial respiratory chain.

Comparative aspects of quinol-cytochrome c/plastocyanin oxidoreductases.

Electron and proton transfers through quinones and cytochrome bc complexes

Publisher Summary This chapter describes the preparation and properties of reduced coenzyme Q-cytochrome c reductase (complex III of the respiratory chain) The rate of reduction of cytochrome c by reduced coenzyme Q 2 is estimated from the amount of cytochrome c that is reduced (absorbency change at 550 mμ) in a sample of the assay mixture that has been allowed to react for 10 seconds The enzyme-catalyzed reaction is stopped by the addition of an appropriate stop reagent Three procedures for the preparation of (reduced coenzyme Q) cytochrome c reductase are described All three procedures utilize the S-1 fraction of the procedure described for the preparation of DPNH cytochrome c reductase The purification procedure described in the chapter utilizes the final supernatant solution obtained in the preparation of DPNH-cytochrome c reductase (Reduced eoenzyme Q)-eytoehrome e reduetase can be stored as a solution in the Tris-suerose-histidine buffer for weeks at –20° with little loss in activity The reduced forms of coenzyme Q 1 , eoenzyme Q 2 , and eoenzyme Q 10 all serve as electron-donor substrates with complex III (Reduced coenzyme Q)-cytochrome c reductase is inhibited almost completely by antimycin A at concentrations approaching the concentration of cytochrome c l in the solution of enzyme (15 μg per milligram of protein of complex III) In the assay system described, (reduced coenzyme Q)-cytochrome c reductase is active in the pH range from 60 to 85, with an optimal activity at pH 74

J.S. Rieske

Papers

[44] Preparation and properties of reduced coenzyme Q-cytochrome c reductase (complex III of the respiratory chain)