Cytochrome b within both molecular and cell biology, is a protein found in the mitochondria of

eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...

cells. It functions as part of the electron transport chain and is the main subunit of transmembrane

cytochrome bc1 Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of heme and its mode of bi ...

and b6f complexes.

Function

In the mitochondrion of eukaryotes and in aerobic prokaryotes, cytochrome b is a component of

respiratory 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 thi ...

complex III () — also known as the bc1 complex or ubiquinol-cytochrome c reductase. In plant

chloroplasts 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 in ...

and cyanobacteria, there is an analogous protein, cytochrome b6, a component of the plastoquinone-plastocyanin reductase (), also known as the b6f complex. These complexes are involved in electron transport, 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, and th ...

is used for the generation of ATP. These complexes play a vital role in cells.

Structure

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 residues that probably has 8

transmembrane segment A transmembrane domain (TMD) is a membrane-spanning protein domain. TMDs generally adopt an alpha helix topological conformation, although some TMDs such as those in porins can adopt a different conformation. Because the interior of the lipid bi ...

s. In plants and cyanobacteria, cytochrome b6 consists of two protein subunits encoded by the petB and petD genes. Cytochrome b/b6 non-covalently binds two

heme Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver. In biochemical terms, heme is a coordination complex "consis ...

groups, known as b562 and b566. Four conserved histidine residues are postulated to be the ligands of the iron atoms of these two heme groups.

Use in phylogenetics

Cytochrome b is commonly used as a region of mitochondrial DNA for determining

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

relationships between organisms, due to its sequence variability. It is considered to be most useful in determining relationships within

families Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...

and genera. 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.

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 alternative ...

– 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 biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, ...

group 11. The cyt b mutations and provide resistance against the main body of group 11, although G143A does not work against metyltetraprole (11A). G143A is significant in ''Botrytis cinerea'' in California strawberry production.

References

External links

* {{Hemeproteins Protein domains Protein families Transmembrane proteins