Cytochromes are proteins containing heme as a cofactor. They are classified according to the type of heme and its mode of binding. Four varieties are recognized by the IUBMB, cytochromes a, cytochromes b, cytochromes c and cytochrome d. Cytochrome function is 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. In addition to the classification by the IUBMB into four cytochrome classes, several additional classifications such as cytochrome o and cytochrome P450 can be found in biochemical literature.
Cytochromes were initially described in 1884 by MacMunn as respiratory pigments (myohematin or histohematin). In the 1920s, Keilin rediscovered these respiratory pigments and named them the cytochromes, or “cellular pigments”. He classified these heme proteins on the basis of the position of their lowest energy absorption band in their reduced state, as cytochromes a (605 nm), b (~565 nm), and c (550 nm). The ultra-violet (UV) to visible spectroscopic signatures of hemes are still used to identify heme type from the reduced bis-pyridine-ligated state, i.e., the pyridine hemochrome method. Within each class, cytochrome a, b, or c, early cytochromes are numbered consecutively, e.g. cyt c, cyt c1, and cyt c2, with more recent examples designated by their reduced state R-band maximum, e.g. cyt c559.
Structure and function
The heme group is a highly conjugated ring system (which allows its electrons to be very mobile) surrounding an iron ion. The iron in cytochromes usually exists in a ferrous (Fe2+) and a ferric (Fe3+) state with a ferroxo (Fe4+) state found in catalytic intermediates. Cytochromes are, thus, capable of performing electron transfer reactions and catalysis by reduction or oxidation of their heme iron. The cellular location of cytochromes depends on their function. They can be found as globular proteins and membrane proteins.
In the process of oxidative phosphorylation, a globular cytochrome c protein is involved in the electron transfer from the membrane-bound complex III to complex IV. Complex III itself is composed of several subunits, one of which is a b-type cytochrome while another one is a c-type cytochrome. Both domains are involved in electron transfer within the complex. Complex IV contains a cytochrome a/a3-domain that transfers electrons and catalyzes the reaction of oxygen to water. Photosystem II, the first protein complex in the light-dependent reactions of oxygenic photosynthesis, contains a cytochrome b subunit. Cyclooxygenase 2, an enzyme involved in inflammation, is a cytochrome b protein.
In the early 1960s, a linear evolution of cytochromes was suggested E. Margoliash which 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.
Four types of cytochromes are distinguished by their prosthetic groups:
|Cytochrome a||heme a|
|Cytochrome b||heme b|
|Cytochrome c||heme c (covalently bound heme b)|
|Cytochrome d||tetrapyrrolic chelate of iron|
|a and a3||Cytochrome c oxidase ("Complex IV") with electrons delivered to complex by soluble cytochrome c (hence the name)|
|b and c1||Coenzyme Q - cytochrome c reductase ("Complex III")|
|b6 and f||Plastoquinol—plastocyanin reductase|
A distinct family of cytochromes is the cytochrome P450 family, so named for the characteristic Soret peak formed by absorbance of light at wavelengths near 450 nm when the heme iron is reduced (with sodium dithionite) and complexed to carbon monoxide. These enzymes are primarily involved in steroidogenesis and detoxification.
- "Nomenclature Committee of the International Union of Biochemistry (NC-IUB). Nomenclature of electron-transfer proteins. Recommendations 1989". Journal of Biological Chemistry. 267 (1): 665–677. 1992-01-05. ISSN 0021-9258. PMID 1309757.
- L., Lehninger, Albert (2000). Lehninger Principles of Biochemistry (3rd ed.). New York: Worth Publishers. ISBN 978-1572591530. OCLC 42619569.
- Puustinen, A.; Wikström, M. (1991-07-15). "The heme groups of cytochrome o from Escherichia coli". Proceedings of the National Academy of Sciences. 88 (14): 6122–6126. doi:10.1073/pnas.88.14.6122. ISSN 0027-8424. PMID 2068092.
- Mac Munn, C. A (1886). "Researches on Myohaematin and the Histohaematins". Philosophical Transactions of the Royal Society of London. 177: 267–298. doi:10.1098/rstl.1886.0007. JSTOR 109482.
- Keilin, D. (1925-08-01). "On cytochrome, a respiratory pigment, common to animals, yeast, and higher plants". Proc. R. Soc. Lond. B. 98 (690): 312–339. doi:10.1098/rspb.1925.0039. ISSN 0950-1193.
- Reedy, C. J.; Gibney, B. R. (February 2004). "Heme protein assemblies". Chem Rev. 104 (2): 617–49. doi:10.1021/cr0206115. PMID 14871137.
- Margoliash, E. (1963). "PRIMARY STRUCTURE AND EVOLUTION OF CYTOCHROME C". Proceedings of the National Academy of Sciences of the United States of America. 50: 672–679. ISSN 0027-8424. PMC 221244. PMID 14077496.
- Kumar, Sudhir (2005). "Molecular clocks: four decades of evolution". Nature Reviews. Genetics. 6 (8): 654–662. doi:10.1038/nrg1659. ISSN 1471-0056. PMID 16136655.
- Cytochrome+c+Group at the US National Library of Medicine Medical Subject Headings (MeSH).
- Cytochrome+d at the US National Library of Medicine Medical Subject Headings (MeSH)
- Bendall, Derek S. (2004). "The Unfinished Story of Cytochrome f". Photosynthesis Research. 80 (1–3): 265–276. doi:10.1023/b:pres.0000030454.23940.f9. ISSN 0166-8595. PMID 16328825.
- Doidge, Norman (2015). The brain’s way of healing : remarkable discoveries and recoveries from the frontiers of neuroplasticity. Penguin Group. p. 173. ISBN 978-0-698-19143-3.