Publisher Summary This chapter describes purification and properties of cytochrome c-555 from phototrophic green sulfur bacteria. The green sulfur bacteria are strictly anaerobic. They utilize reduced sulfur compounds such as sulfide, sulfur, and less commonly, thiosulfate as electron donors and carbon dioxide as the primary carbon source. Acetate is assimilated in the presence of sulfide and carbon dioxide. The green bacteria have a unique pathway for carbon assimilation via a reversed tricarboxylic acid (TCA) cycle. The photosynthetic reaction center is related to plant, algal, and cyanobacterial photosystem. The green sulfur bacteria are a rich source of cytochromes. Cytochrome c-555 is a small (10 kDa), soluble protein containing 86-99 amino-acid residues and a single heme. It has a relatively low redox potential of 80-160 mV at pH 7, which may reflect the anaerobic environment in which the species lives. The cells can be broken by repeated cycles of freezing and thawing, by sonication, or by French press. If it is not desired to purify other cytochromes or ferredoxins in addition to cytochrome c-555, then the extract can be desalted with a buffer change to 1 mM phosphate—pH 7—by dialysis or Sephadex G-25 gel filtration.

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