Structure of the Soluble Domain of Cytochrome f from the Cyanobacterium Phormidium laminosum,

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Abstract
Cytochrome f from the photosynthetic cytochrome b6f complex is unique among c-type cytochromes in its fold and heme ligation. The 1.9-Å crystal structure of the functional, extrinsic portion of cytochrome f from the thermophilic cyanobacterium Phormidium laminosum demonstrates that an unusual buried chain of five water molecules is remarkably conserved throughout the biological range of cytochrome f from cyanobacteria to plants [Martinez et al. (1994) Structure2, 95−105]. Structure and sequence conservation of the cytochrome f extrinsic portion is concentrated at the heme, in the buried water chain, and in the vicinity of the transmembrane helix anchor. The electrostatic surface potential is variable, so that the surface of P. laminosum cytochrome f is much more acidic than that from turnip. Cytochrome f is unrelated to cytochrome c1, its functional analogue in the mitochondrial respiratory cytochrome bc1 complex, although other components of the b6f and bc1 complexes are homologous. Identical function of the two complexes is inferred for events taking place at sites of strong sequence conservation. Conserved sites throughout the entire cytochrome b6f/bc1 family include the cluster-binding domain of the Rieske protein and the heme b and quinone-binding sites on the electrochemically positive side of the membrane within the b cytochrome, but not the putative quinone-binding site on the electrochemically negative side.