Originally described in 1884 by MacMunn as respiratory pigments (myohematin or histohematin), and reclassified as cytochromes or “cellular pigments” by Keilin in 1920, cytochromes are redox-active proteins that contain a heme. These proteins have a central Fe atom at their core as a cofactor. Cytochromes are involved in electron transport chain and redox catalysis. Now, they are classified according to the type of heme and its mode of binding.
Cytochromes are recognized by the International Union of Biochemistry and Molecular Biology (IUBMB) and placed into four categories. These classes are: Cytochromes a, Cytochromes b, Cytochromes c and Cytochromes d.
As well as these four categories, additional categories, such as cytochrome o and cytochrome P450 can be found in biochemical literature.
Cytochrome function has been linked to the reversible redox change from ferrous (Fe(II)) to the ferric (Fe(III)) oxidation state of the iron found in the heme core.
The function of cytochromes will largely influence their cellular location. Most often, cytochromes will be either globular proteins or membrane proteins. During oxidative phosphorylation, a globular cytochrome cc protein will become involved in the electron transfer from the membrane-bound complex III to complex IV.
In regards to structure, the heme group is a highly conjugated ring system surrounding an iron ion. This allows the heme group’s electrons to be mobile. The iron in cytochromes tend to exist in a ferrous (Fe2+) and a ferric (Fe3+) state with a ferroxo (Fe4+) state found in catalytic intermediates. It only makes sense, then, that cytochromes can perform electron transfer reactions and catalysis by reduction or oxidation of their heme iron.