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  • Title: The primary structure of soluble cytochrome c-551 from the phototrophic green sulfur bacterium Chlorobium limicola, strain Tassajara, reveals a novel c-type cytochrome.
    Author: Klarskov K, Verté F, Van Driessche G, Meyer TE, Cusanovich MA, Van Beeumen J.
    Journal: Biochemistry; 1998 Jul 28; 37(30):10555-62. PubMed ID: 9692944.
    Abstract:
    Chlorobium limicola, strain Tassajara, cytochrome c-551 is a soluble dimeric protein containing identical subunits of about 30 kDa. The amino acid sequence was determined by a combination of automated Edman degradation and mass analysis. There are 258 residues with a single heme binding site located at cysteine positions 172 and 175. In addition, there is a disulfide bridge between Cys78 and Cys109, and a free cysteine at position 219 which was found to occur as cysteic acid. The only homologue of soluble cytochrome c-551 is the soxA protein which is part of the thiosulfate utilization operon of Paracoccus denitrificans. They are 32% identical with three small gaps. This is consistent with the observation that cytochrome c-551 is the electron acceptor for a thiosulfate-oxidizing enzyme. On the basis of the redox potential of 135 mV, the sixth heme ligand should be a methionine. Among the seven methionine residues that are present in c-551, only one is conserved, two residues ahead of the heme-binding site. The far-UV circular dichroism spectrum indicates 40% alpha helix and 25% beta secondary structure. No other known cytochrome c has such a mixed structure; they are either all helical or all beta. Thus, Chlorobium soluble cytochrome c-551 and soxA are likely to be representative of a new class of c-type cytochromes.
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