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(sī`təkrōm'), protein containing heme (see coenzymecoenzyme
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) that participates in the phase of biochemical respiration called oxidative phosphorylationphosphorylation,
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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 Adenosine diphosphate (ADP), Adenosine triphosphate (ATP), Mitochondria, 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. See Respiration

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. See Cell membranes

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. See Proteins, evolution of

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 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. See Adrenal gland, Liver

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. See Photosynthesis



a complex iron-containing protein whose prosthetic (nonprotein) group is represented by heme (hemoprotein). The group was first described in 1886 by C. A. MacMunn (Scotland) under the name histohematins, but its role in living cells remained unclear until 1925, when the group was rediscovered by D. Keilin.

Cytochromes are widely distributed in plant and animal cells and in some microorganisms, such as yeasts and some facultative anaerobes. Bound to the membranes of the mitochondria, endoplasmic reticulum, chloroplasts, and chromatophores, they play an important part in many processes occurring in living organisms, such as cell respiration, photosynthesis, and microsomal oxidation.

All cytochromes are capable of donating and accepting electrons by a reversible change in the valence of the iron atoms in the heme. Combined into short or long chains, depending on the potential of the final electron acceptor, cytochromes transport electrons from dehydrogenases to the final acceptors. The transport of electrons from cytochrome to cytochrome enables the cell to utilize the energy of chemical compounds or sunlight for energy or repair purposes. For example, as part of the chain of mitochondrial respiratory enzymes, cytochromes, together with cytochrome oxidase, carry out the final stages of oxidation of the substrates. The energy released in the process is utilized to form adenosine triphosphate (ATP) or is utilized as a membrane potential. Cytochromes of the endoplasmic reticulum form short nonphosphorylating chains that are part of a system responsible for the metabolism and neutralization of aromatic compounds.

Cytochromes are divided into four types— a, b, c, and d —according to spectral characteristics, the chemical structure of the heme side chains, and the nature of the bond between heme and the protein molecule. Each type, in turn, is subdivided further into several types. The cytochromes whose individuality has been established are designated by an italic lowercase Latin letter, which indicates the group to which the cytochrome belongs, and by a subscript number, for example, the cytochrome c1. In reduced state, cytochromes form a distinct spectrum with three pronounced absorption bands, characteristic of each type of cytochrome and useful in identifying cytochromes by spectrophotometric methods.

About 30 cytochromes are known, but only a few of them have been obtained in the form of individual proteins. It is difficult to obtain highly purified cytochromes inasmuch as they are strongly bound to membranes and can be separated only by treatment with surfactants or proteolytic enzymes. The cytochromes b3 and c are exceptions in that they can be easily extracted with saline solutions.

A comparison of the sequence of amino acids in the protein part of the cytochrome c molecule obtained from different organisms reveals that sequences of 35 and 11 amino-acid residues in different parts of the chain remain unchanged. The number of exchanges in other parts of the protein chain of this cytochrome obtained from different species of organisms is directly related to the phylogenetic differences between the species. For example, the molecules of the cytochrome c in horses and yeast differ in 48 amino-acid residues, while those in ducks and chickens differ only in two; those in swine, cows, and sheep are identical.


Archakov, A. I. Mikrosomal’noe okislenie. Moscow, 1975.
Lehninger, A. Biokhimiia: Molekuliarnye osnovy struktury i funktsii kletki. Moscow, 1976. (Translated from English.)



Any of the complex protein respiratory pigments occurring within plant and animal cells, usually in mitochondria, that function as electron carriers in biological oxidation.
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