cytochrome

(sī'tə-krōm')
n.
Any of a class of iron-containing proteins important in cell respiration as catalysts of oxidation-reduction reactions.

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Any of a group of proteins that carry as prosthetic groups various iron porphyrins called hemes. Hemes also constitute prosthetic groups for other proteins, but the function of prosthetic groups in the cytochromes is largely restricted to oxidation to the ferric heme, with the iron in the 3⁺ valence state, and reduction to ferrous heme with a 2⁺ iron. Thus, by alternate oxidation and reduction the cytochromes can transfer electrons to and from each other and other substances, and can operate in the oxidation of substrates. The energy released in their oxidation reactions is conserved by using it to drive the formation of the energy-rich compound adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate. This process of coupling the oxidation of substrates to phosphorylation of ADP is called oxidative phosphorylation. In cells of eukaryotic organisms, the cytochromes have rather uniform properties; they are part of the respiratory chain and are located in the mitochondria. In contrast, prokaryotes exhibit much more varied cytochromes. Cytochromes are found even in metabolic pathways that employ oxidants other than oxygen. See also Adenosine triphosphate (ATP); Protein.

Respiratory chain

There are four cytochromes in the respiratory chain of eukaryotes, termed respectively aa₃, b, c, and c₁. Cytochrome aa₃, also called cytochrome oxidase, functions by oxidizing reduced cytochrome c (ferrocytochrome c) to the ferric form. It then transfers the reducing equivalents acquired in this reaction to molecular oxygen, reducing it to water. The cytochrome oxidase reaction is probably the most important reaction in biology since it drives the entire respiratory chain and takes up over 95% of the oxygen employed by organisms, thus providing nearly all of the energy needed for living processes.

The energy released during oxidation is utilized to actively pump protons (H⁺) from the matrix of the mitochondrion through the inner membrane into the intermembrane space. This creates a proton gradient across the membrane, with the matrix space having a lower proton concentration and the outside having a higher proton concentration. This chemical and potential gradient can be released by allowing protons to flow down the gradient and back into the mitochondrial matrix, thereby driving the formation of ATP. A pair of electrons flowing down the respiratory chain yields three molecules of ATP, a remarkable feat of energy conservation. This is called the chemiosmotic mechanism of oxidative phosphorylation, which is generally considered a true picture of respiratory chain function.

Cytochrome oxidase

The cytochrome oxidase of eukaryotes is a very complex protein assembly containing from 8 to 13 polypeptide subunits, two hemes, a and a₃, and two atoms of copper. The two hemes are chemically identical but are placed in different protein environments, so that heme a can accept an electron from cytochrome c and heme a₃ can react with oxygen. When cytochrome oxidase has accepted four electrons, one from each of four molecules of reduced cytochrome c, both its hemes and both its copper atoms are in reduced form, and it can transfer the electrons in a series of reactions to a molecule of oxygen to yield two molecules of water.

Cytochrome oxidase straddles the inner membrane of mitochondria, part of it on the matrix side, part within the membrane, and part on the outer surface or cytochrome c side of the inner membrane.

Cytochrome c

Cytochrome c is the only protein member of the respiratory chain that is freely mobile in the mitochondrial intermembrane space. It is a small protein consisting of a single polypeptide chain of 104 to 112 amino acid residues, wrapped around a single heme prosthetic group. The cytochromes c of eukaryotes are all positively charged proteins, with strong dipoles, while the systems from which cytochrome c accepts electrons, cytochrome reductase, and to which cytochrome c delivers electrons, cytochrome oxidase, are negatively charged. There is good evidence that this electrostatic arrangement correctly orients cytochrome c as it approaches the reductase or the oxidase, so that electron transfer can take place very efficiently, even though the surface area at which the reaction occurs is less than 1% of the total surface of the protein.

The amino acid sequences of the cytochromes c of eukaryotes have been determined for well over 100 different species, from yeast to humans, and have provided some very interesting correlations between protein structure and the evolutionary relatedness of different taxonomic groups. The extensive degree of similarity over the entire range of extant organisms has been taken as evidence that this is an ancient structure, developed long before the divergence of plants and animals, which in the course of its evolutionary descent has been adapted to serve a variety of electron transfer functions in different organisms.

Cytochrome reductase

Like cytochrome oxidase, the cytochrome reductase complex is an integral membrane protein system. There are numerous subunits, consisting of two molecules of cytochrome b, one molecule of a nonheme iron protein, and one molecule of cytochrome c₁. As in the case of the oxidase, the two cytochrome b hemes are chemically identical, but are present in somewhat different protein environments. The reductase complex is reduced by reaction with the reduced form of the fat-soluble coenzyme Q, dissolved within the inner mitochondrial membrane, which is itself reduced by the succinate dehydrogenase, the NADH dehydrogenase, and other systems. See also Coenzyme.

Other cytochromes

In addition to the mitochondrial respiratory chain cytochromes, animals have a heme protein, termed cytochrome P450, located in the liver and adrenal gland cortex. In the liver it is part of a mono-oxygenase system that can utilize oxygen and the reduced coenzyme NADPH, to hydroxylate a large variety of foreign substances and drugs and thus detoxify them; in the adrenal it functions in the hydroxylation of steroid precursors in the normal biosynthesis of adrenocortical hormones.

Two varieties of cytochrome b, termed b₅₆₃ and b₅₅₉, and one of cytochrome c, c₅₅₂, are involved in the photosynthetic systems of plants. Other plant cytochromes occur in specialized tissues and certain species.

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Haem-containing proteins present in every type of living cell (except the strictly anaerobic bacteria). Some cytochromes react with oxygen directly; others are intermediates in the oxidation of reduced coenzymes. Unlike haemoglobin, the iron in the haem of cytochromes undergoes oxidation and reduction.

A protein pigment containing a metal involved in redox reactions in the electron transport chain in mitochondria.

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any member of a class of hemoproteins whose characteristic mode of action involves the transfer of electrons in association with a reversible change in oxidation state (between Fe(ii) and Fe(iii), and sometimes Fe(iv)) of the heme iron. The name embraces all intracellular hemoproteins except hemoglobin, myoglobin, peroxidases, catalase, tryptophan 2,3-dioxygenase, nitrite and sulfite reductases, and heme-thiolate proteins. There are four major established subclasses; well-established members of each subclass are generally distinguished by consecutive numerical subscripts; remaining members are designated by a suffixed number based upon the wavelength (in nanometres) of the α-band (e.g. cytochrome c₁, cytochrome c₅₅₅); see cytochrome absorption bands. (1) In cytochromes a the heme prosthetic group contains a formyl side-chain, i.e. heme a. (2) Cytochromes b contain protoheme or a related heme (without a formyl group) as prosthetic group, not covalently bound to the protein. The group includes cytochrome o, cytochrome P-450, and helicorubin, as well as ubiquinol-cytochrome c reductase, cytochrome b, cytochrome b₂, cytochrome b₅, cytochrome b₂₄₅, cytochrome b₅₉₉, cytochrome b₆, and cytochrome b₆ — f complex. (3) In cytochromes c there are covalent linkages, not necessarily thioether linkages, between the heme side-chains and the protein (see also cytochrome c, cytochrome c₁). (4) Cytochromes d contain a tetrapyrrollic chelate of iron as prosthetic group, in which the degree of conjugation of double bonds is less than in porphyrin. See also cytochrome f.

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Any of a class of hemoproteins, widely distributed in animal and plant tissue, whose main function is electron transport; distinguished according to their prosthetic group as a, b, c and d.

• c. b₅ reductase — a flavoprotein involved in the desaturation of fatty acids in the liver.
• c. oxidase — an a type cytochrome which contains copper and receives electrons from another cytochrome, of the c type, and transfers them to oxygen atoms allowing the oxygen to combine with hydrogen atoms to form water. A nutritional deficiency of copper leads to a general reduction in metabolic rate because of the absence of cytochrome oxidase a.
• c. system — the sum of cytochromes which play a part in the body's metabolic processes. Includes the cytochrome oxidases and cytochrome reductases.

n

One of a class of hemoproteins that act as electron transport. Cytochromes are classified as a, b, c, and d.

For a list of words related to cytochrome, see:

• Cell Structure and Function - cytochrome: carrier molecule in mitochondria that participates in transfer of electrons in oxidation reactions

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