Structure of cytochrome c(6) from the red alga Porphyra yezoensis at 1.57 angstrom resolution

The crystal structure of cytochrome c(6) from the red alga Porphyra yezoens is has been determined at 1.57 Angstrom resolution. The crystal is tetragon al and belongs to space group P4(3)2(1)2, with unit-cell parameters a = b = 49.26 (3), c = 83.45 (4) Angstrom and one molecule per asymmetric unit. Th e 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 s tructure of cytochrome c6 follows the topology of class I c-type cytochrome s in which the heme prosthetic group covalently binds to Cys14 and Cys17, a nd the iron has an octahedral coordination with His18 and Met58 as the axia l ligands. The sequence and the structure of the eukaryotic red algal cytoc hrome c6 are very similar to those of a prokaryotic cyanobacterial cytochro me c6 rather than those of eukaryotic green algal c6 cytochromes.