Cytochrome b is a protein found in the membranes of aerobic cells. In
eukaryotic
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
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 us ...
(inner membrane) and in aerobic
prokaryotes
A prokaryote (; less commonly spelled procaryote) is a single-celled organism whose cell lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Ancient Greek (), meaning 'before', and (), meaning 'nut' ...
, cytochrome b is a component of
respiratory chain
An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples this ...
complex III
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
() — also known as the bc1 complex or ubiquinol-cytochrome c reductase. In plant
chloroplasts
A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
and
cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
, there is a homologous protein, cytochrome b6, a component of the
plastoquinone-plastocyanin reductase (), also known as the b6f complex. These complexes are involved in
electron transport
An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples this ...
, the pumping of protons to create a proton-motive force (
PMF). This
proton gradient
An electrochemical gradient is a gradient of electrochemical potential, usually for an ion that can move across a membrane. The gradient consists of two parts:
* The chemical gradient, or difference in solute concentration across a membrane.
...
is used for the generation of
ATP. These complexes play a vital role in cells.
Structure and function
Cytochrome b/b6 is an integral
membrane protein
Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane proteins are a permanent part of a cell membrane ...
of approximately 400
amino acid residue
In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...
s that probably has 8
transmembrane segments. In plants and cyanobacteria, cytochrome b6 consists of two
protein subunit
In structural biology, a protein subunit is a polypeptide chain or single protein molecule that assembles (or "''coassembles''") with others to form a protein complex.
Large assemblies of proteins such as viruses often use a small number of t ...
s encoded by the petB and petD genes. Cytochrome b/b6 non-covalently binds two
heme
Heme (American English), or haem (Commonwealth English, both pronounced /Help:IPA/English, hi:m/ ), is a ring-shaped iron-containing molecule that commonly serves as a Ligand (biochemistry), ligand of various proteins, more notably as a Prostheti ...
groups, known as b562 and b566. Four conserved histidine residues are postulated to be the
ligands
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ...
of the iron atoms of these two heme groups.
The heme groups are key parts of the internal electron transfer pathway and indispensable to the functioning of the two quinol oxidizing complexes. Two units of b/b6 also form a quinol entry pathway.
Use in phylogenetics
Cytochrome b is commonly used as a region of
mitochondrial DNA
Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondrion, mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the D ...
for determining
phylogenetic
In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
relationships between organisms, due to its sequence variability. It is considered to be most useful in determining relationships within
families
Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...
and
genera
Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...
. Comparative studies involving cytochrome b have resulted in new classification schemes and have been used to assign newly described species to a genus as well as to deepen the understanding of evolutionary relationships.
Clinical significance
Mutations in cytochrome b primarily result in
exercise intolerance
Exercise intolerance is a condition of inability or decreased ability to perform physical exercise at the normally expected level or duration for people of that age, size, sex, and muscle mass. It also includes experiences of unusually severe pos ...
in human patients; though more rare, severe multi-system pathologies have also been reported.
Single-point mutations in cytochrome b of ''Plasmodium falciparum'' and ''P. berghei'' are associated with resistance to the anti-malarial drug
atovaquone
Atovaquone, sold under the brand name Mepron, is an antimicrobial medication for the prevention and treatment of ''Pneumocystis jirovecii'' pneumonia (PCP).
Atovaquone is a chemical compound that belongs to the class of naphthoquinones. Atova ...
.
Human genes
Human genes encoding cytochrome b proteins include:
*
CYB5A
Cytochrome b5, form A (gene name CYB5A), is a human microsomal cytochrome b5.
Cytochrome b5 is a membrane bound hemoprotein which functions as an electron carrier for several membrane bound oxygenases. It has two isoforms produced by alternativ ...
– cytochrome b5 type A (microsomal)
*
CYB5B
Cytochrome b is a protein that in humans is encoded by the ''MT-CYB'' gene. Its gene product is a subunit of the respiratory chain protein ubiquinol–cytochrome ''c'' reductase (UQCR, complex III or cytochrome ''bc''1 complex), which consists ...
– cytochrome b5 type B (outer mitochondrial membrane)
*
CYBASC3 – cytochrome b, ascorbate dependent 3
*
MT-CYB
Cytochrome b is a protein that in humans is encoded by the ''MT-CYB'' gene. Its gene product is a subunit of the respiratory chain protein ubiquinol–cytochrome ''c'' reductase (UQCR, complex III or cytochrome ''bc''1 complex), which consists ...
– mitochondrially encoded cytochrome b
Fungicide target
Cyt b is targeted by the
QI class of fungicides,
Fungicide Resistance Action Committee
Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, in ...
group 11. The cyt b mutations and provide resistance against the main body of group 11, although G143A does not work against
metyltetraprole
Metyltetraprole is a quinone outside inhibitor fungicide sold under the brand name Pavecto by its inventor, Sumitomo Chemical. ISSN-Lbr>0385-1559/ref> It is the only tetrazolinone fungicide and the only one in the Fungicide Resistance Action C ...
(11A).
G143A is significant
in ''Botrytis cinerea'' in California strawberry production.
[
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References
External links
*
{{Hemeproteins
Electron-transfer proteins
Protein domains
Protein families
Transmembrane proteins