Cytochrome

[6] The heme group is a highly conjugated ring system (which allows its electrons to be very mobile) surrounding an iron ion.

[1] Cytochromes are, thus, capable of performing electron transfer reactions and catalysis by reduction or oxidation of their heme iron.

In the early 1960s, a linear evolution of cytochromes was suggested by Emanuel Margoliash[7] that led to the molecular clock hypothesis.

The apparently constant evolution rate of cytochromes can be a helpful tool in trying to determine when various organisms may have diverged from a common ancestor.

[8] Several kinds of cytochrome exist and can be distinguished by spectroscopy, exact structure of the heme group, inhibitor sensitivity, and reduction potential.