Cytochrome c oxidase, subunit I bacterial type
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Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in
electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...
and redox catalysis. They are classified according to the type of heme and its mode of binding. Four varieties are recognized by the International Union of Biochemistry and Molecular Biology (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 Cytochromes P450 (CYPs) are a Protein superfamily, superfamily of enzymes containing heme as a cofactor (biochemistry), cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are ...
can be found in biochemical literature.


History

Cytochromes were initially described in 1884 by Charles Alexander 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 Oxidative phosphorylation (UK , US ) or electron transport-linked phosphorylation or terminal oxidation is the metabolic pathway in which cells use enzymes to oxidize nutrients, thereby releasing chemical energy in order to produce adenosine tri ...
, a globular cytochrome cc 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 Light-dependent reactions is jargon for certain photochemical reactions that are involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions, the first occurs at photosystem II (PSII) and ...
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 by
Emanuel Margoliash Emanuel Margoliash (February 10, 1920 – April 10, 2008) was a biochemist who spent much of his career studying the protein cytochrome c. He is best known for his work on molecular evolution; with Walter Fitch, he devised Fitch-Margoliash metho ...
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.


Types

Several kinds of cytochrome exist and can be distinguished by
spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
, exact structure of the heme group, inhibitor sensitivity, and reduction potential. Four types of cytochromes are distinguished by their prosthetic groups: There is no "cytochrome e," but cytochrome f, found in the cytochrome b6f complex of plants is a c-type cytochrome. In
mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
and
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
s, these cytochromes are often combined in electron transport and related metabolic pathways: A distinct family of cytochromes is the
cytochrome P450 Cytochromes P450 (CYPs) are a Protein superfamily, superfamily of enzymes containing heme as a cofactor (biochemistry), cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are ...
family, so named for the characteristic
Soret peak In spectroscopy, a Soret peak or Soret band is an intense peak in the blue wavelength region of the visible spectrum. The peak is named after its discoverer, Jacques-Louis Soret. The term is commonly used in absorption spectroscopy, corresponding ...
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


External links


Scripps Database of Metalloproteins
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