The Cytochrome b₆f Complex

Summary

The plastoquinol-cytochrome c ₅₅₃/plastocyanin oxidoreductase (Cyt b ₆ f complex) catalyzes the rate limiting, quinol-oxidation step in oxygenic photosynthesis. Overall, it transfers electrons between the two photochemical reaction centers (PS II and PS I), is required for cyclic electron flow around PS I, and establishes a transmembrane gradient of protons for ATP synthesis. Four polypeptides (Cyt b ₆, subunit IV, the Rieske Fe-S protein, and Cyt f) encoded by the petBD and petCA operons, respectively, and four prosthetic groups (two b-hemes, one c-type heme, and a 2Fe-2S center) catalyze these activities in vitro. Additional low-molecular-mass subunits may have roles in vivo. The Cyt b ₆ f complex in cyanobacteria provides the only known pathway for plastoquinol oxidation and appears to be indispensable for both photosynthesis and heterotrophy. This has precluded the propagation of inactivating mutations and complicated molecular genetic analysis. The related Cyt bc ₁ complex is found in all mitochondria and many bacteria. The Rieske, Cyt b, and Cyt c ₁ polypeptides of bc ₁ complexes correspond to the Rieske, Cyt b ₆/subunit IV, and Cyt f, respectively, of Cyt b ₆ f complexes. The Cyt b subunit in the former has been split into separate Cyt b ₆ and subunit IV polypeptides in the latter. Both complexes have binding sites for quinol oxidation (Q_ox or Q_o) and quinone reduction (Q_red or Q_r)— each associated with a b-heme on opposite sides of the membrane. Modified Q-cycle models explain most of the experimental data on electron and proton transfer reactions in the Cyt bc ₁ complex but are more controversial in the Cyt b ₆ f complex. Other differences include the biophysical characteristics of prosthetic groups, turnover rates, and inhibitor specificities. The molecular basis for these differences has not been elucidated, although features such as the split Cyt b ₆ and subunit IV proteins may be involved. Additional questions pertain to the pathway for electrons during cyclic flow, possible alternative electron acceptors, the role(s) of monomer or dimer forms in vivo, the role of the b ₆ f complex as a sensor of redox potential and mediator of state transitions, and mechanisms for attachment of hemes and the Rieske Fe-S center and assembly of the complex. Knowledge of the Cyt bc ₁ complex has been advanced greatly in recent years through intensive mutational analyses summarized in this chapter. Comparable studies have not, until very recently, been possible in the Cyt b ₆ f complex but will clearly help to elucidate its unique features and define molecular differences relative to the Cyt bc ₁ complex. Progress has also been impeded by the absence of three-dimensional structural information for either the Cyt b ₆ f or bc ₁ complexes. Very recently the structure of the water soluble, heme-binding domain of turnip Cyt f has been solved by X-ray crystallography at 2.8 Å resolution; the structure reveals several novel features including the use of the amino-terminus as an axial ligand for the heme. Such structures will provide a rational framework for subsequent mutational and biochemical studies and we can expect these combined approaches to begin to unravel the mysteries of membrane Cyt complexes.