The similarity of the amino acid sequences, and particularly the distribution of hydrophobic residues and histidine heme ligands of the cyt b6 and subunit IV core of the b6f complex to the N - and C - terminal domains of the cytochrome b polypeptide in the cytochrome bc1 complex of the mitochondrial respiratory chain and purple photosynthetic bacteria , implies a common evolutionary origin of this core . Moving away from the intramembrane core of the structure in a direction either parallel or perpendicular to the plane of the membrane, the conservation of sequence and structure diminishes. The rubredoxin-like core of the Rieske [2Fe-2S] protein in the different bc complexes is more similar than the remainder of its soluble domain that is distal from the membrane -23]. It is striking that cytochromes f and c1 have absolutely no significant sequence similarity beyond the signature Cys-X-Y-Cys-His sequence for the covalent binding of heme through two thioether cysteinyl bonds -24, 25]. Another unique feature of the b6f complex is the presence of the heme c^ (see Section 7.3.4- , which is also part of a b6 f-like complex in the gram- positive bacteria  .
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