Structure of cytochrome c₆ from the red alga Porphyra yezoensis at 1.57 Å resolution

The crystal structure of cytochrome c₆ from the red alga Porphyra yezoensis has been determined at 1.57 Å resolution. The crystal is tetragonal and belongs to space group P4₃2₁2, with unit-cell parameters a = b = 49.26 (3), c = 83.45 (4) Å and one molecule per asymmetric unit. The structure was solved by the molecular-replacement method and refined with X-­PLOR to an R factor of 19.9% and a free R factor of 25.4%. The overall structure of cytochrome c₆ follows the topology of class I c-type cytochromes in which the heme prosthetic group covalently binds to Cys14 and Cys17, and the iron has an octahedral coordination with His18 and Met58 as the axial ligands. The sequence and the structure of the eukaryotic red algal cytochrome c₆ are very similar to those of a prokaryotic cyanobacterial cytochrome c₆ rather than those of eukaryotic green algal c₆ cytochromes.