The Cytochrome bc 1 and Related bc Complexes: The Rieske/Cytochrome b Complex as the Functional Core of a Central Electron/Proton Transfer Complex

Abstract

The cytochrome (Cyt) bc 1 and related complexes play a central role in purple bacterial photosynthesis, transferring electrons between electron carriers reduced and oxidized by the photochemical reaction centers, oxidizing quinol (QH2) and reducing Cyt c while translocating protons via some variation of the Q-cycle mechanism. In this chapter, we discuss recent advances in the biochemical, biophysical and evolutionary understanding of these complexes. The mechanistic core of these complexes, conserved over billions of years, contains the Cyt b protein (with its two associated b-type hemes) and the Rieske iron-sulfur center. Together, this central core performs the central (and well-conserved) reaction of the Q-cycle, that is the ‘bifurcated’ oxidation of QH2 at the quinol oxidation (Qo) site, with two electrons sent to different acceptors, one to the Rieske iron-sulfur center and the other to the Cyt b chain. The subsequent reactions of the Q-cycle, involving the reduction of secondary carriers (a high potential Cyt c in the case of purple bacteria) and quinone at the quinone reduction (Qi) site are less well conserved both in terms of structure and mechanism. We thus use the term Rieske/Cytochrome b (RB) complexes for these enzymes. Key issues surrounding the mechanisms of the RB complexes are discussed, including a series of currently debated models for the avoidance of deleterious side reactions within the Qo site, the mechanism of stabilization of semiquinone intermediates within the Qi site, and the role of the pivoting iron-sulfur protein subunit.