Crystallographic structure of Rhodospirillum molischianum ferricytochrome c′ at 2.5 Å resolution

Abstract This paper describes the 2.5 A crystallographic structure determination of ferricytochrome c ′ from the photosynthetic bacterium Rhodospirillum molischianum . The molecule is a symmetric dimer, with each 128-residue polypeptide chain incorporating a covalently bound protoheme IX prosthetic group. The monomer is structurally organized as an array of four nearly parallel α-helices, which pack most closely at one end and thereafter spatially diverge to accommodate the heme prosthetic group. Although local features of the heme attachment pattern resemble those seen in cytochrome c , the heme iron in cytochrome c ′ is pentaco-ordinate with a solvent-exposed histidine residue furnishing the single axial ligand to the heme iron. Subunit association in the dimeric molecule is principally stabilized by helix interactions, which are qualitatively similar to those occurring within each monomer. These interactions result in a dimer geometry that situates the exposed regions of both hemes on the same molecular surface. The structural basis for some of the physiochemical properties cytochrome c ′ are examined and compared to those of other heme proteins of known structure.