cytochrome

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.

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.

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

For more information on cytochrome, visit Britannica.com.

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

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.

Cytochrome c with heme c.

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

They are found either as monomeric proteins (e.g., 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.

Contents 1 History 2 Structure and function 3 Types 4 References 5 External links

History

Cytochromes were initially described in 1884 by MacMunn as respiratory pigments (myohematin or histohematin). In the 1920s, Keilin rediscovered these respiratory pigments and named them the cytochromes, or “cellular pigments”, and classified these heme proteins, on the basis of the position of their lowest energy absorption band in the reduced state, as cytochromes a (605 nm), b (~565 nm), and c (550 nm). The UV-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 ^[1].

Structure and function

The heme group is a highly-conjugated ring system (which allows its electrons to be 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⁺ (oxidized) and Fe³⁺ (reduced) states (electron-transfer processes) or between Fe²⁺ (oxidized) and Fe³⁺ (formal, reduced) 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:

Type	prosthetic group
Cytochrome a	heme a
Cytochrome b	heme b
Cytochrome d	tetrapyrrolic chelate of iron[2]

The definition of cytochrome c is not defined in terms of the heme group.^[3] There is no "cytochrome e," but there is a cytochrome f, which is often considered a type of cytochrome c.^[4]

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

Cytochromes	Combination
a and a3	Cytochrome c oxidase ("Complex IV") with electrons delivered to complex by soluble cytochrome c (hence the name)
b and c1	Coenzyme Q - cytochrome c reductase ("Complex III")
b6 and f	Plastoquinol—plastocyanin reductase

A completely distinct family of cytochromes is 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. ^ Reedy, C. J. & Gibney, B. R. (February 2004). "Heme protein assemblies". Chem Rev 104 (2): 617–49. doi:10.1021/cr0206115. PMID 14871137. 
2. ^ MeSH Cytochrome+d
3. ^ MeSH Cytochrome+c+Group.
4. ^ Cytochrome at eMedicine Dictionary

External links

• Scripps Database of Metalloproteins
• MeSH Cytochromes

v • d • e Proteins: hemeproteins Globins Hemoglobin Subunits Alpha locus on 16: α (HBA1, HBA2) · ζ (HBZ) · θ (HBQ1) · μ (HBM) Beta locus on 11: β (HBB) · δ (HBD) · γ (HBG1, HBG2) · ε (HBE1) Tetramers stages of development: HbA (α2β2) · HbA2 (α2δ2) · HbF/Fetal (α2γ2) · HbE Gower 2 (α2ε2) · HbE Gower 1 (ζ2ε2) pathology: HbH (β4) · HbS (α2βS2) · HbC (α2βC2) · HbE (α2βE2) Compounds Carboxyhemoglobin · Carbaminohemoglobin · Oxyhemoglobin/Deoxyhemoglobin · Sulfhemoglobin Other human Glycosylated hemoglobin · Methemoglobin Nonhuman Chlorocruorin · Erythrocruorin Other human: Myoglobin (Metmyoglobin) · Neuroglobin · Cytoglobin plant: Leghemoglobin Other Cytochrome (Cytochrome b, Cytochrome P450) · Hemocyanin · Methemalbumin

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