Structure of the soluble domain of cytochrome f from the cyanobacterium Phormidium laminosum

Cytochrome f from the photosynthetic cytochrome bd complex is unique among c-type cytochromes in its fold and heme ligation. The 1.9-Angstrom crystal structure of the functional, extrinsic portion of cytochrome f from the the rmophilic cyanobacterium Phormidium laminosum demonstrates that an unusual buried chain of five water molecules is remarkably conserved throughout, th e biological range of cytochrome f from cyanobacteria to plants [Martinez e t al. (1994) Structure 2, 95-105], Structure and sequence conservation of t he cytochrome f extrinsic portion is concentrated at the heme, in the burie d water chain, and in the vicinity of the transmembrane helix anchor. The e lectrostatic surface potential is variable, so that the surface of P. lamin osum cytochrome fis much more acidic than that from turnip. Cytochrome f is unrelated to cytochrome c(1), its functional analogue in the mitochondrial respiratory cytochrome bc(1) complex, although other components of the b(6 )f and bc(1) complexes are homologous. identical function of the two comple xes is inferred for events taking place at sites of strong sequence conserv ation. Conserved sites throughout the entire cytochrome b(6)f/bc(1) 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 membra ne within the b cytochrome, but nor the putative quinone-binding site on th e electrochemically negative side.