The heme redox center of chloroplast cytochrome f is linked to a buried five‐water chain

The crystal structure of the 252‐residue lumen‐side domain of reduced cytochrome f, a subunit of the proton‐pumping integral cytochrome b6f complex of oxygenic photosynthetic membranes, was determined to a resolution of 1.96 Å from crystals cooled to —35°. The model was refined to an R‐factor of 15.8% with a 0.013‐Å RMS deviation of bond lengths from ideality. Compared to the structure of cytochrome f at 20°, the structure at —35° has a small change in relative orientation of the two folding domains and significantly lower isotropic temperature factors for protein atoms. The structure revealed an L‐shaped array of five buried water molecules that extend in two directions from the NΔ1 of the heme ligand His 25. The longer branch extends 11 Å within the large domain, toward Lys 66 in the prominent basic patch at the top of the large domain, which has been implicated in the interaction with the electron acceptor, plastocyanin. The water sites are highly occupied, and their temperature factors are comparable to those of protein atoms. Virtually all residues that form hydrogen bonds with the water chain are invariant among 13 known cytochrome f sequences. The water chain has many features that optimize it as a proton wire, including insulation from the protein medium. It is suggested that this chain may function as the lumen‐side exit port for proton translocation by the cytochrome b6f complex.