In Rhodobacter sphaeroides, transfer of the first electron in quinol oxidation by the bc(1) complex shows kinetic features (a slow rate (approx. 1.5X10(3)/s), high activation energy (approx. 65 kJ/mol) and reorganization energy, lambda (2.5 V)) that are unexpected from Marcus theory and the distances shown by the structures. Reduction of the oxidized iron-sulfur protein occurs after formation of the enzyme-substrate complex, and involves a I-I-transfer in which the electron transfer occurs through the approx. 7 Angstrom of a bridging histidine forming a H-bond with quinol and a ligand to 2Fe-2S, The anomalous kinetic features can be explained by a mechanism in which the electron transfer is constrained by coupled transfer of the proton. We discuss this in the context of mutant strains with modified E-m.7 and pK for the iron-sulfur protein, and Marcus theory for proton-coupled electron transfer. We suggest that transfer of the second proton and electron involve movement of semiquinone in the Q(o) site, and rotation of the Glu of the conserved -PEWY- sequence. Mutational studies show a key role for the domain proximal to heme b(L). The effects df mutation at Tyr-302 (Tyr-279 in bovine sequence) point to a possible linkage between conformational changes in the proximal domain, and changes leading to closure of the iron-sulfur protein access channel at the distal domain. (C) 2000 Elsevier Science B.V. All rights reserved.