Cytochrome

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Cytochromes are generally membrane-bound hemoproteins that contain heme groups and carry out electron transport.

They are either found as monomeric proteins (i.e. cytochrome c) or as subunits of bigger enzymatic complexes that catalyze redox reactions. They are found in the mitochondrial inner membrane and endoplasmic reticulum of eukaryotes, in the chloroplasts of plants, in photosynthetic microorganisms, and in bacteria.

Structure and function

The heme group is a highly conjugated ring system (which means its electrons are very mobile) surrounding a metal ion, which readily interconverts between the oxidation states. For many cytochromes the metal ion present is that of iron, which interconverts between Fe²⁺ (reduced) and Fe³⁺ (oxidized) states (electron-transfer processes) or between Fe²⁺ (reduced) and Fe³⁺ (formal, oxidized) states (oxidative processes). Cytochromes are thus capable of performing oxidation and reduction. Because the cytochromes (as well as other complexes) are held within membranes in an organized way, the redox reactions are carried out in the proper sequence for maximum efficiency.

In the process of oxidative phosphorylation, which is the principal energy-generating process undertaken by organisms which need oxygen to survive, other membrane-bound and soluble complexes and cofactors are involved in the chain of redox reactions, with the additional net effect that protons (H⁺) are transported across the mitochondrial inner membrane. The resulting transmembrane proton gradient [(protonmotive force)] is used to generate ATP, which is the universal chemical energy currency of life. ATP is consumed to drive cellular processes that require energy (such as synthesis of macromolecules, active transport of molecules across the membrane, and assembly of flagella).

Types

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

Three types of cytochrome are distinguished by their prosthetic groups:

The definition of cytochrome c is not defined in terms of the heme group.^[2] There is no "cytochrome e", but there is a cytochrome f, which is often considered a type of cytochrome c.^[3]

In mitochondria and chloroplasts, these cytochromes are often combined in electron transport and related metabolic pathways:

A completely distinct family of cytochromes are known as the cytochrome P450 oxidases, 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.

References

1. ^ MeSH Cytochrome+d
2. ^ MeSH Cytochrome+c+Group.
3. ^ Cytochrome at eMedicine Dictionary