Cytochromes b₅ are ubiquitous electron transport hemoprotein

Hemoprotein

A hemeprotein , or heme protein, is a metalloprotein containing a heme prosthetic group- an organic compound that allows a protein to carry out a function that it cannot do alone....

s found in animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s, plant

Plant

Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s, fungi and purple phototrophic bacteria

Purple bacteria

Purple bacteria or purple photosynthetic bacteria are proteobacteria that are phototrophic, that is capable of producing energy through photosynthesis...

. The microsomal

Microsome

In cell biology, microsomes are vesicle-like artifacts re-formed from pieces of the endoplasmic reticulum when eukaryotic cells are broken-up in the laboratory; by definition, microsomes are not ordinarily present in living cells....

 and mitochondrial variants are membrane-bound, while bacterial and those from erythrocytes and other animal tissues are water-soluble. The family of cytochrome b₅-like proteins includes (besides cytochrome b₅ itself) hemoprotein domains covalently associated with other redox domains in flavocytochrome cytochrome b₂ (L-lactate dehydrogenase; ), sulfite oxidase , plant and fungal nitrate reductases , and plant and fungal cytochrome b₅/acyl lipid desaturase fusion proteins.

Structure

3-D structures of a number of cytochrome b₅ and yeast flavocytochrome b₂ 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

Histidine Histidine, an essential amino acid, has a positively charged imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans...

 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 b₅, 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 b₅ in some biochemical reactions

cytochrome-b₅ reductase

Cytochrome b5 reductase

Cytochrome-b5 reductase is a NADH-dependent enzyme that converts methemoglobin to hemoglobin...

NADH + H⁺ + 2 ferricytochrome b₅ = NAD⁺ + 2 ferrocytochrome b₅

L-ascorbate—cytochrome-b₅ reductase

L-ascorbate-cytochrome-b5 reductase

In enzymology, a L-ascorbate—cytochrome-b5 reductase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are L-ascorbate and ferricytochrome b5, whereas its 3 products are monodehydroascorbate, ferrocytochrome b5, and H+....

L-ascorbate + ferricytochrome b₅ = monodehydroascorbate + ferrocytochrome b₅

CMP-N-acetylneuraminate monooxygenase

CMP-N-acetylneuraminate monooxygenase

In enzymology, a CMP-N-acetylneuraminate monooxygenase is an enzyme that catalyzes the chemical reactionThe 4 substrates of this enzyme are CMP-N-acetylneuraminate, ferrocytochrome b5, O2, and H+, whereas its 3 products are CMP-N-glycoloylneuraminate, ferricytochrome b5, and H2O.This enzyme...

CMP-N-acetylneuraminate + 2 ferrocytochrome b₅ + O₂ + 2 H⁺ = CMP-N-glycoloylneuraminate + 2 ferricytochrome b₅ + H₂O

stearoyl-CoA 9-desaturase

Stearoyl-CoA 9-desaturase

In enzymology, a stearoyl-CoA 9-desaturase is an enzyme that catalyzes the chemical reactionThe 4 substrates of this enzyme are stearoyl-CoA, ferrocytochrome b5, O2, and H+, whereas its 3 products are oleoyl-CoA, ferricytochrome b5, and H2O....

stearoyl-CoA + 2 ferrocytochrome b₅ + O₂ + 2 H⁺ = oleoyl-CoA + 2 ferricytochrome b₅ + H₂O

linoleoyl-CoA 9-desaturase

linoleoyl-CoA + 2 ferrocytochrome b₅ + O₂ + 2 H⁺ = γ-linolenoyl-CoA + 2 ferricytochrome b₅ + H₂O

External links

- Solution structure of rat cytochrome b₅ (form A) - Solution structure of rat cytochrome b₅ (form B) - X-ray structure of cytochrome b₅₅₈ from Ectothiorhodospira vacuolata - Methemoglobinemia due to deficiency of cytochrome b₅