Cytochromes ''b''
5 are ubiquitous
electron transport
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 ...
hemoprotein
A hemeprotein (or haemprotein; also hemoprotein or haemoprotein), or heme protein, is a protein that contains a heme prosthetic group. They are a very large class of metalloproteins. The heme group confers functionality, which can include oxygen ...
s found in
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s,
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
s,
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
and
purple phototrophic bacteria. The
microsomal
In cell biology, microsomes are heterogeneous vesicle-like artifacts (~20-200 nm diameter) re-formed from pieces of the endoplasmic reticulum (ER) when eukaryotic cells are broken-up in the laboratory; microsomes are not present in healthy, liv ...
and
mitochondrial variants are membrane-bound, while bacterial and those from
erythrocyte
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
s and other
animal tissues are water-soluble. The family of cytochrome ''b''
5-like proteins includes (besides cytochrome ''b''
5 itself) hemoprotein domains covalently associated with other redox domains in flavocytochrome cytochrome ''b''
2 (
L-lactate dehydrogenase; ), sulfite oxidase (), plant and fungal nitrate reductases (, , ), and plant and fungal cytochrome ''b''
5/acyl lipid desaturase fusion proteins.
Structure
3-D structures of a number of cytochrome ''b''
5 and yeast flavocytochrome ''b''
2 are known. The fold belongs to the α+β class, with two hydrophobic cores on each side of a β-sheet. The larger hydrophobic core constitutes the heme-binding pocket, closed off on each side by a pair of helices connected by a turn. The smaller hydrophobic core may have only a structural role and is formed by spatially close N-terminal and C-terminal segments. The two
histidine
Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the d ...
residues provide the fifth and sixth heme ligands, and the propionate edge of the heme group lies at the opening of the heme crevice. Two isomers of cytochrome ''b''
5, referred to as the A (major) and B (minor) forms, differ by a 180° rotation of the heme about an axis defined by the α- and γ-meso carbons.
Cytochrome ''b''5 in some biochemical reactions
cytochrome-''b''5 reductase
: NADH + H
+ + 2 ferricytochrome ''b''
5 → NAD
+ + 2 ferrocytochrome ''b''
5
L-
ascorbate—cytochrome-''b''5 reductase
:
L-ascorbate + ferricytochrome ''b''
5 → monodehydroascorbate + ferrocytochrome ''b''
5
CMP-''N''-acetylneuraminate monooxygenase
: CMP-''N''-acetylneuraminate + 2 ferrocytochrome ''b''
5 + O
2 + 2 H
+ → CMP-''N''-glycoloylneuraminate + 2 ferricytochrome ''b''
5 + H
2O
stearoyl-CoA 9-desaturase
Stearoyl-CoA desaturase (Δ-9-desaturase) is an endoplasmic reticulum enzyme that catalyzes the rate-limiting step in the formation of monounsaturated fatty acids (MUFAs), specifically oleate and palmitoleate from stearoyl-CoA and palmitoyl-CoA ...
: stearoyl-CoA + 2 ferrocytochrome ''b''
5 + O
2 + 2 H
+ → oleoyl-CoA + 2 ferricytochrome ''b''
5 + H
2O
linoleoyl-CoA 9-desaturase
: linoleoyl-CoA + 2 ferrocytochrome ''b''
5 + O
2 + 2 H
+ → γ-linolenoyl-CoA + 2 ferricytochrome ''b''
5 + H
2O
See also
*
Cytochrome b
Cytochrome b within both molecular and cell biology, is a protein found in the mitochondria of eukaryotic cells. It functions as part of the electron transport chain and is the main subunit of transmembrane cytochrome bc1 and b6f complexes.
F ...
*
Cytochrome b5 deficiency
*
P450-containing systems Any enzyme system that includes cytochrome P450 protein or domain can be called a P450-containing system.
P450 enzymes usually function as a terminal oxidase in multicomponent electron-transfer chains, called P450-containing monooxygenase systems, ...
*
Cytochrome b5, type A
References
*
*
*
*
External links
* – Solution structure of rat cytochrome ''b''
5 (form A)
* – Solution structure of rat cytochrome ''b''
5 (form B)
* – X-ray structure of cytochrome ''b''
558 from ''Ectothiorhodospira vacuolata''
* {{OMIM, 250790 – Methemoglobinemia due to deficiency of cytochrome ''b''
5
Cytochromes