Cytochromes are redox-activeproteins containing aheme, with a centraliron (Fe) atom at its core, as acofactor. They are involved in theelectron transport chain andredoxcatalysis. They are classified according to the type of heme and itsmode of binding. Four varieties are recognized by theInternational Union of Biochemistry and Molecular Biology (IUBMB), cytochromes a,cytochromes b,cytochromes c andcytochrome d.[1]
Cytochrome function is linked to the reversibleredox change fromferrous (Fe(II)) to theferric (Fe(III)) oxidation state of the iron found in the heme core.[2] In addition to the classification by the IUBMB into four cytochrome classes, several additional classifications such ascytochrome o[3] andcytochrome P450 can be found in biochemical literature.
Cytochromes were initially described in 1884 byCharles Alexander MacMunn as respiratory pigments (myohematin or histohematin).[4] In the 1920s,Keilin rediscovered these respiratory pigments and named them the cytochromes, or “cellular pigments”.[5] He classified these heme proteins on the basis of the position of their lowestenergy absorption band in their reduced state, ascytochromesa (605 nm),b (≈565 nm), andc (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, cytochromea,b, orc, early cytochromes are numbered consecutively, e.g. cytc, cytc1, and cytc2, with more recent examples designated by their reduced state R-band maximum, e.g. cytc559.[6]
Theheme group is a highly conjugated ring system (which allows itselectrons 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.[1] Cytochromes are, thus, capable of performingelectron transfer reactions andcatalysis by reduction or oxidation of their heme iron. The cellular location of cytochromes depends on their function. They can be found asglobular proteins andmembrane proteins.
In the process ofoxidative phosphorylation, a globularcytochrome cc protein is involved in the electron transfer from the membrane-boundcomplex III tocomplex 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 ofoxygen to water. Photosystem II, the firstprotein complex in thelight-dependent reactions of oxygenicphotosynthesis, contains a cytochrome b subunit.Cyclooxygenase 2, an enzyme involved ininflammation, is a cytochrome b protein.
In the early 1960s, a linearevolution of cytochromes was suggested byEmanuel Margoliash[7] that led to themolecular 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 acommon ancestor.[8]
Several kinds of cytochrome exist and can be distinguished byspectroscopy, exact structure of the heme group, inhibitor sensitivity, and reduction potential.[9]
Four types of cytochromes are distinguished by their prosthetic groups:
| Type | Prosthetic group |
|---|---|
| Cytochrome a | heme A |
| Cytochrome b | heme B |
| Cytochrome c | heme C (covalently bound heme b)[10] |
| Cytochrome d | heme D (Heme B with γ-spirolactone)[11] |
There is no "cytochrome e," butcytochrome f, found in thecytochrome b6f complex of plants is a c-type cytochrome.[12]
Inmitochondria andchloroplasts, these cytochromes are often combined inelectron transport and related metabolic pathways:[13]
| Cytochromes | Combination |
|---|---|
| a anda3 | Cytochrome c oxidase ("Complex IV") with electrons delivered to complex by solublecytochrome c (hence the name) |
| b andc1 | Coenzyme Q - cytochrome c reductase ("Complex III") |
| b6 andf | Plastoquinol—plastocyanin reductase |
A distinct family of cytochromes is thecytochrome P450 family, so named for the characteristicSoret peak formed by absorbance of light at wavelengths near 450 nm when the heme iron is reduced (withsodium dithionite) and complexed tocarbon monoxide. These enzymes are primarily involved insteroidogenesis anddetoxification.[14][9]
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