Cytochromes are redox-active
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s 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 ...
and
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
catalysis
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
. 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
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
change from
ferrous (Fe(II)) to the
ferric (Fe(III)) oxidation state of the
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
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 superfamily of enzymes containing heme as a cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various co ...
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 ''c
1'', and cyt ''c
2'', with more recent examples designated by their reduced state R-band maximum, e.g. cyt ''c
559''.
Structure and function
The
heme group is a highly conjugated ring system (which allows its
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have n ...
s to be very mobile) surrounding an iron ion. The iron in cytochromes usually exists in a ferrous (Fe
2+) and a ferric (Fe
3+) state with a ferroxo (Fe
4+) state found in catalytic intermediates.
Cytochromes are, thus, capable of performing
electron transfer reactions and
catalysis
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
by reduction or oxidation of their heme iron. The cellular location of cytochromes depends on their function. They can be found as
globular protein
In biochemistry, globular proteins or spheroproteins are spherical ("globe-like") proteins and are one of the common protein types (the others being fibrous, disordered and membrane proteins). Globular proteins are somewhat water-soluble (for ...
s 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 t ...
, 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
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...
to water. Photosystem II, the first
protein complex
A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multienzyme complexes, in which multiple catalytic domains are found in a single polypeptide chain.
Protein ...
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
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
, contains a cytochrome b subunit.
Cyclooxygenase 2, an enzyme involved in
inflammation
Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...
, is a cytochrome b protein.
In the early 1960s, a linear
evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
of cytochromes was suggested by
Emanuel Margoliash that led to the
molecular clock
The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleo ...
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 ...
, 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 cells of most Eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is used ...
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 i ...
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 superfamily of enzymes containing heme as a cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various co ...
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 to ...
formed by absorbance of light at wavelengths near 450 nm when the heme iron is reduced (with
sodium dithionite) and complexed to
carbon monoxide
Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simpl ...
. These enzymes are primarily involved in
steroidogenesis and
detoxification
Detoxification or detoxication (detox for short) is the physiological or medicinal removal of toxic substances from a living organism, including the human body, which is mainly carried out by the liver. Additionally, it can refer to the period of ...
.
References
External links
Scripps Database of Metalloproteins*
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