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Cytochrome f

Cytochrome f is the largest subunit of cytochrome b₆f complex (plastoquinol—plastocyanin reductase; EC 1.10.99.1). 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.

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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.

References

Bendall, D.S. (2004). "The unfinished story of cytochrome f". Photosynth. Res. 80: 265–276. PMID 16328825.
Cramer, W.A., Martinez, S.E., Huang, D., Tae, G.S., Everly, R.M., Heymann, J.B., Cheng, R.H., Baker, T.S. and Smith, J.L. (1994). "Structural aspects of the cytochrome b₆f complex; structure of the lumen-side domain of cytochrome f". J. Bioenerg. Biomembr. 26: 31–47. PMID 8027021.
Martinez, S.E., Huang, D., Ponomarev, M., Cramer, W.A. and Smith, J.L. (1996). "The heme redox center of chloroplast cytochrome f is linked to a buried five-water chain". Protein Science 5: 1081–1092. PMID 8762139.
Prince, R.C. and George, G.N. (1995). "Cytochrome f revealed". Trends Biochem. Sci. 20: 217–218. PMID 7631417.