The primary structure of soluble cytochrome c-551 from the phototrophic green sulfur bacterium Chlorobium limicola, strain Tassajara, reveals a novel c-type cytochrome.
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.
Department of Biochemistry, Physiology and Microbiology, University of Gent, Belgium.
K
K
Klarskov
F
F
Verté
G
G
Van Driessche
T E
TE
Meyer
M A
MA
Cusanovich
J
J
Van Beeumen
D000595:Amino Acid Sequence; D001426:Bacterial Proteins; D019414:Chlorobi / Q000201:enzymology* / Q000378:metabolism; D003574:Cytochrome c Group / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D004579:Electron Transport; D006418:Heme / Q000737:chemistry / Q000378:metabolism; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D017421:Sequence Analysis; D012995:Solubility
D001426:Bacterial Proteins; D003574:Cytochrome c Group; D006418:Heme; C017167:cytochrome C(551)