Energy Transfer and Energy-Coupling Processes

Cytochrome bc complexes acting as quinol-cytochrome c (or plastocyanin) oxi-doreductases are central constituents of many electron transport chains (Ort, Chap. 4; Dutton, Chap. 5, this Vol.). Although mitochondrial respiration is the net reversal of chloroplast photosynthesis, they both oxidize quinol via a cytochrome bc complex. In chloroplasts the cytochrome b6f complex functions between the two photosystems, and it reduces plastocyanin with plastoquinol. In mitochondria the cytochrome bc1 complex reduces cytochrome c with ubi-quinol. Both the b6f type and the bc1 type are also found in prokaryotes. Cyanobacteria contain a cytochrome b6f complex for oxygenic photosynthesis, like chloroplasts (Bricker et al., Chap. 11.4, this Vol.). A cytochrome bc1 complex is found in various facultative aerobic bacteria, like Paracoccus denitrificans and the Rhodospirillaceae (Prince, Chap. 10.4, this Vol.). Cyanobacteria are also able to grow by respiration, and there is good evidence that photosynthetic and respiratory electron transport are linked by the cytochrome b6f complex in the same membrane (Binder 1982). Similarly, photosynthetic and respiratory electron transport in Rhodospirillaceae are connected by a cytochrome bc1 complex (Baccarini-Melandri and Zannoni 1978). These “zwitter”-systems sharing the cytochrome be complex are interesting with respect to the phylogenetic relationship of photosynthesis and respiration.