Succinate dehydrogenase (SDH) is a membrane-bound complex of two main components: a membrane-extrinsic component composed of an FAD-binding flavoprotein and an iron-sulfur protein, and a hydrophobic component composed of a cytochrome B and a membrane anchor protein. The cytochrome b component is a mono heme transmembrane protein [1,2,3] belonging to a family that groups:

• Cytochrome b-556 from bacterial SDH (gene sdhC).
• Cytochrome b560 from the mammalian mitochondrial SDH complex.
• Cytochrome b560 subunit encoded in the mitochondrial genome of some algae and in the plant Marchantia polymorpha.
• Cytochrome b from yeast mitochondrial SDH complex (gene SDH3 or CYB3).
• Protein cyt-1 from Caenorhabditis.

These cytochromes are proteins of about 130 residues that comprise three transmembrane regions. There are two conserved histidines which may be involved in binding the heme group. We developed two signature patterns that include these histidine residues.