Structure of Ptrotozoan Cytochromes

Abstract

C-type cytochromes from two protozoa, Crithidia oncopelti and Kuglena gracilis. were purified and partly characterised. Crithidia cytochrome C557 and Euglena cytochrome C558 had certain atypical spectral characteristics. The sequence of amino acids in both proteins was determined and compared with the large number of eukaryotic cytochrome c sequences already known. One of the two amino acids which are involved in the covalent attachment of the prosthetic group to the protein in c-type cytochromes is replaced by a non-binding amino acid in both protozoan cytochromes. It is proposed that this change results in the observed spectral peculiarities of these two cytochromes. A second c—type cytochrome, which is implicated in photosynthesis, was isolated from Euglena and the sequence of amino acids was determined. This protein is not closely related in structure to the mitochondrial cytochromes c although certain features are similar. Homology with a prokaryotic c—type cytochrome is proposed, with possible implications for the evolution of photosynthesis.