A proposed pathway of proton translocation through thebc complexes of mitochondria and chloroplasts

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Abstract
The cytochromebc complexes of the electron transport chain from a wide variety of organisms generate an electrochemical proton gradient which is used for the synthesis of ATP. Proton translocation studies with radiolabeled N,N′-dicyclohexylcarbodiimide (DCCD), the well-established carboxyl-modifying reagent, inhibited proton-translocation 50–70% with minimal effect on electron transfer in the cytochromebc 1 and cytochromebf complexes reconstituted into liposomes. Subsequent binding studies with cytochromebc 1 and cytochromebf complexes indicate that DCCD specifically binds to the subunitb and subunitb 6, respectively, in a time and concentration dependent manner. Further analyses of the results with cyanogen bromide and protease digestion suggest that the probable site of DCCD binding is aspartate 160 of yeast cytochromeb and aspartate 155 or glutamate 166 of spinach cytochromeb 6. Moreover, similar inhibition of proton translocating activity and binding to cytochromeb and cytochromeb 6 were noticed with N-cyclo-N-(4-dimethylamino-napthyl)carbodiimide (NCD-4), a fluorescent analogue of DCCD. The spin-label quenching experiments provide further evidence that the binding site for NCD-4 on helix cd of both cytochromeb and cytochromeb 6 is localized near the surface of the membrane but shielded from the external medium.

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