Cytochrome b within both molecular and

cell biology Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living and ...

, is a protein found in the

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

eukaryotic Eukaryotes () are organisms whose cells have a 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 three domains of life. Bacte ...

cells. It functions as part of the

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 th ...

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 Eukaryotes () are organisms whose cells have a 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 three domains of life. Bacte ...

and in aerobic

prokaryotes A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...

, 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 th ...

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 cell, plant and algae, algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, ...

and

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

, 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 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 ty ...

s 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 "consisti ...

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 (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 electro ...

of the iron atoms of these two heme groups.

Use in phylogenetics

Cytochrome b is commonly used as a region of

mitochondrial DNA Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial D ...

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 Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

. 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