Structures of cytochrome c-549 and cytochrome c(6) from the cyanobacteriumArthrospira maxima

Cytochrome c(6) and cytochrome c-549 are small (89 and 130 amino acids, res pectively) monoheme cytochromes that function in photosynthesis. They appea r to have descended relatively recently from the same ancestral gene but ha ve diverged to carry out very different functional roles, underscored by th e large difference between their midpoint potentials of nearly 600 mV. We h ave determined the X-ray crystal structures of both proteins isolated from the cyanobacterium Arthrospira maxima. The two structures are remarkably si milar, superimposing on backbone atoms with an rmsd of 0.7 Angstrom. Compar ison of the two structures suggests that differences in solvent exposure of the heme and the electrostatic environment of the heme propionates, as wel l as in heme iron ligation, are the main determinants of midpoint potential in the two proteins. In addition, the crystal packing of both A. maxima cy tochrome c-549 and cytochrome c6 suggests that the proteins oligomerize. Fi nally, the cytochrome c-549 dimer we observe can be readily fit into the re cently described model of cyanobacterial photosystem II.