Cytochrome f

Cytochrome f is the largest subunit of cytochrome b6f complex (plastoquinol&mdash;plastocyanin reductase; ). The crystal structures of soluble (lumen-side) domain of cytochrome f reveal two structural domains: a small one above a larger one which, in turn, is on top of the attachment to the membrane domain. The large domain consists of an antiparallel β-sandwich and a short heme-binding peptide, which form a three-layer structure. The small domain is inserted between two β-strands of the large domain and is an all-β domain.

The heme is bound between two short helices at the N-terminus of cytochrome f. Within the second helix is the sequence motif for the c-type cytochromes, Cys-Xaa-Xaa-Cys-His, which is covalently attached to the heme through thioether bonds of the two cysteine residues. The fifth heme iron ligand is histidine and the sixth heme iron ligand is the α-amino group of N-terminal tyrosine residue (Tyr-1).

Cytochrome f has an internal network of water molecules. It has been hypothesized that this water chain functions as a proton wire. The water chain appears to be a conserved feature of cytochromes f.