STRUCTURE REFINEMENT OF CYTOCHROME C555 (CHLOROBIUM, THIOSULFATOPHILUM).

Abstract

The structure of cytochrome c₅₅₅ from the green sulfur bacterium Chlorobium thiosulfatophilum was determined by using a single isomorphous derivative, K₂HgI₄, in combination with its anomalous signal. The initial 2.25 angstrom map was modified by the technique of Fourier inversion. The smoothing function for the electron density map addressed three different features in the map, the solvent density, the protein density and the volume surrounding the heavy atom binding sites known to contain spurious peaks. This structure determination was undertaken for three reasons. First, Chlorobium thiosulfatophilum is a very primitive sulfur metabolizing bacterium and so its cytochrome c₅₅₅ structure is important for its evolutionary implications. Second, the oxidation-reduction potential of cytochrome c₅₅₅ is significantly different from the oxidation-reduction potential of other cytochromes whose structures have been determined. Comparisons with the other structures would provide information concerning the factors that are important in regulating oxidation-reduction potentials. Finally, the three dimensional structure may aid in explaining the pattern of reactivity cytochrome c₅₅₅ displays with mitochondrial cytochrome c oxidase and reductase, which is reversed when compared to other bacterial c-type cytochromes. The resulting structure contains three alpha helices. These features are consistent with other c-type cytochrome molecules previously determined. Two regions of the map appear to be disordered and are difficult to interpret. Possible causes of this observation are discussed and related to the significance of the structure.