Structures of Cytochromec-549 and Cytochromec₆from the CyanobacteriumArthrospira maxima^,

Abstract

Cytochrome c(6) and cytochrome c-549 are small (89 and 130 amino acids, respectively) monoheme cytochromes that function in photosynthesis. They appear to have descended relatively recently from the same ancestral gene but have diverged to carry out very different functional roles, underscored by the large difference between their midpoint potentials of nearly 600 mV. We have determined the X-ray crystal structures of both proteins isolated from the cyanobacterium Arthrospira maxima. The two structures are remarkably similar, superimposing on backbone atoms with an rmsd of 0.7 A. Comparison of the two structures suggests that differences in solvent exposure of the heme and the electrostatic environment of the heme propionates, as well 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 cytochrome c-549 and cytochrome c(6) suggests that the proteins oligomerize. Finally, the cytochrome c-549 dimer we observe can be readily fit into the recently described model of cyanobacterial photosystem II.

Crystal structure of oxygen-evolving photosystem II fromThermosynechococcus vulcanusat 3.7-Å resolution

“…compatible with the model predicted theoretically (38) or that analyzed by single-particle analysis of cryoelectron microscopic images from higher plants (39). The structure of cyt c550 was modeled partially with the help of the recently reported structure of this cyt from two other cyanobacteria (40,41), generating a structure very similar to those from the other cyanobacteria. The newly assigned 12-kDa protein has an all-␣ architecture composed of five or more short ␣-helices, with no homologous structure in the database.…”

Crystallization and Structural Analysis of Cytochromec6from the DiatomPhaeodactylum tricornutumat 1.5 Å Resolution

“…2C. The overall structure of Ptc6 was similar to the structures of algal and cyanobacterial cyt c 6 from red alga P. yezoensis, 12) green alga C. reinharditii, 14) and cyanobacteria A. maxima, 15) with main-chain root mean square deviations of 0.8, 0.6, and 0.6 Å respectively using the DALI program.…”

“…2A). The C trace alignment of cyt c 6 from Ptc6, green alga C. reinharditii, 14) red alga P. yezoensis, 12) and cyanobacteria A. maxima 15) is shown in Fig. 2C.…”

Outer membrane cytochrome c, OmcF, from Geobacter sulfurreducens: High structural similarity to an algal cytochrome c6

“…3,4 The structures of seven cytochromes c 6 have been previously determined. [4][5][6][7][8][9][10] Further, a c 6 -like cytochrome (PetJ2) of unknown function was recently identified in Synechoccus sp. PCC 7002 with a reduction potential of 1148 mV and high pI.…”