Inter- and intra-molecular electron transfer in the cytochrome bc1 complex

In this review, we compare the intra-molecular and inter-molecular electron transfer rate constants of the high-potential branch of the cytochrome bc(1) complex. Several methods such as the conventional stopped-flow spectroscopy, pH-induced electron transfer, photoactivated ruthenium complex induced electron transfer and photoreleaseable caged quinol, have been used to determine reaction rates between redox centers in an attempt to elucidate the reaction mechanism of this vital energy conserving complex. Since the most active pure cytochrome bc(1) complex has a turnover number of 800 s(-1), any step with a rate constant much larger than this will not be rate-limiting. The most likely rate-limiting step is the cytochrome b redox state governed movement of the head domain of iron-sulfur protein from its electron-accepting site ("fixed" or "b-state" position) to its electron donating site ("c(1)-state" position). (C) 2002 Published by Elsevier Science B.V.