The Structure of Oxidized Cytochrome C=2= of Rhodospirillum Rubrum

Abstract The structure of ferricytochrome c2 from the non-sulfur purple photosynthetic bacterium Rhodospirillum rubrum has been determined at 2 A resolution by x-ray crystallographic methods. The 112-residue polypeptide chain encloses a single covalently bound heme in a predominantly hydrophobic environment, leaving only one edge exposed to the solvent at the front of the molecule. Distributed around the exterior perimeter of the heme crevice are 11 lysine residues which form an essentially uninterrupted positively charged patch. The molecule contains four segments of α-helix, with the conformation of the remainder of the chain being primarily governed by interactions with the heme. A hypothetical scheme for folding of the molecule is presented, based primarily upon the assumption that the helical regions are formed first and are subsequently distorted by hydrophobic interactions with the heme. The heme iron is coordinated to nitrogen Ne2 of His 18 and to the sulfur atom of Met 91 in the fifth and sixth positions. A notable feature of the structure is an apparently off axis or bent Met 91 sulfur-iron bond. The non-axial nature of this bond is attributed to a charge-pair interaction between the Tyr 70 hydroxyl oxygen, bearing a partial negative charge, and the Met 91 sulfur atom, bearing a partial positive charge delocalized from the ferriheme iron. This interaction serves to stabilize the oxidized heme. The additional participation of Tyr 70 in a hydrogen bond network via Tyr 52 to Ser 89 on the front surface of the molecule suggests that the physiological mechanism for reduction of cytochrome c2 involves concerted protonation of Ser 89 concomitant with electron donation to the heme. The process is reversed for oxidation. This proposed mechanism has several chemical and structural implications, the most important being that the site of interaction with physiological oxido-reductants is at the front of the cytochrome c2 molecule. These conclusions appear to be substantiated by a variety of published findings, including pH dependence of the midpoint potential of cytochrome c2 and polylysine inhibition of cytochrome c2 reactivity in physiological oxidation-reduction systems.