Molecular structure of a high potential cytochrome c2 isolated from Rhodopila globiformis

Unlike their mitochondrial counterparts, the c-type cytochromes typically isolated from photosynthetic nonsulfur purple bacteria display a wide range of oxidation-reduction potentials. Here we describe the X-ray crystallographic analysis of the cytochrome c2 isolated from Rhodopila globiformis. This particular c-type cytochrome was selected for study because of its anomalously high redox potential of +450 mV. Crystals employed in the investigation belonged to the space group I4(1) with unit cell dimensions of a = b = 79.2 A, c = 75.2 A, and two molecules in the asymmetric unit. The structure was solved by the techniques of multiple isomorphous replacement with two heavy-atom derivatives and electron density modification procedures. Least-squares refinement of the model reduced the R-factor to 18.7% for all measured X-ray data from 30.0 to 2.2 A. The overall structural motif of the protein is composed of five alpha-helices, one type I turn, and six type II turns. As in other cytochromes c, there are two conserved water molecules located in the heme-binding pocket. Overall, the three-dimensional structure of the R. globiformis molecule is more similar to the eukaryotic c-type cytochromes than to other bacterial proteins.