Flavin group

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There are 18 key atoms in isoalloxazine that make up its characteristic three-ring structure. The R-group varies and differentiates various flavins. Flavin group.svg
There are 18 key atoms in isoalloxazine that make up its characteristic three-ring structure. The R-group varies and differentiates various flavins.
Riboflavin Riboflavin.svg
Riboflavin

Flavins (from Latin flavus, "yellow") refers generally to the class of organic compounds containing the tricyclic heterocycle isoalloxazine or its isomer alloxazine, and derivatives thereof. The biochemical source of flavin is the vitamin riboflavin. The flavin moiety is often attached with an adenosine diphosphate to form flavin adenine dinucleotide (FAD), and, in other circumstances, is found as flavin mononucleotide (or FMN), a phosphorylated form of riboflavin. It is in one or the other of these forms that flavin is present as a prosthetic group in flavoproteins.

Contents

The flavin group is capable of undergoing oxidation-reduction reactions, and can accept either one electron in a two-step process or two electrons at once. Reduction is made with the addition of hydrogen atoms to specific nitrogen atoms on the isoalloxazine ring system:

Equilibrium between the oxidized (left) and totally reduced (right) forms of flavin. FAD FADH2 equlibrium.png
Equilibrium between the oxidized (left) and totally reduced (right) forms of flavin.

In aqueous solution, flavins are yellow-coloured when oxidized, taking a red colour in the semi-reduced anionic state or blue in the neutral (semiquinone) state, and colourless when totally reduced. [1] The oxidized and reduced forms are in fast equilibrium with the semiquinone (radical) form, shifted against the formation of the radical: [2]

Flox + FlredH2 ⇌ FlH

where Flox is the oxidized flavin, FlredH2 the reduced flavin (upon addition of two hydrogen atoms) and FlH the semiquinone form (addition of one hydrogen atom).

In the form of FADH2, it is one of the cofactors that can transfer electrons to the electron transfer chain.

Photoreduction

Both free and protein-bound flavins are photoreducible, that is, able to be reduced by light, in a mechanism mediated by several organic compounds, such as some amino acids, carboxylic acids and amines. [2] This property of flavins is exploited by various light-sensitive proteins. For example, the LOV domain, found in many species of plant, fungi and bacteria, undergoes a reversible, light-dependent structural change which involves the formation of a bond between a cysteine residue in its peptide sequence and a bound FMN. [3]

FAD

FAD Flavin adenine dinucleotide.png
FAD

Flavin adenine dinucleotide is a group bound to many enzymes including ferredoxin-NADP+ reductase, monoamine oxidase, D-amino acid oxidase, glucose oxidase, xanthine oxidase, and acyl CoA dehydrogenase.

FADH/FADH2

FADH and FADH2 are reduced forms of FAD. FADH2 is produced as a prosthetic group in succinate dehydrogenase, an enzyme involved in the citric acid cycle. In oxidative phosphorylation, two molecules of FADH2 typically yield 1.5 ATP each, or three ATP combined.

FMN

FMN Flavin mononucleotide.png
FMN

Flavin mononucleotide is a prosthetic group found in, among other proteins, NADH dehydrogenase, E.coli nitroreductase and old yellow enzyme.

See also

Related Research Articles

A dehydrogenase is an enzyme belonging to the group of oxidoreductases that oxidizes a substrate by reducing an electron acceptor, usually NAD+/NADP+ or a flavin coenzyme such as FAD or FMN. Like all catalysts, they catalyze reverse as well as forward reactions, and in some cases this has physiological significance: for example, alcohol dehydrogenase catalyzes the oxidation of ethanol to acetaldehyde in animals, but in yeast it catalyzes the production of ethanol from acetaldehyde.

<span class="mw-page-title-main">Electron transport chain</span> Energy-producing metabolic pathway

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 this electron transfer with the transfer of protons (H+ ions) across a membrane. The electrons that are transferred from NADH and FADH2 to the ETC involves four multi-subunit large enzymes complexes and two mobile electron carriers. Many of the enzymes in the electron transport chain are embedded within the membrane.

<span class="mw-page-title-main">Respiratory complex I</span> Protein complex involved in cellular respiration

Respiratory complex I, EC 7.1.1.2 is the first large protein complex of the respiratory chains of many organisms from bacteria to humans. It catalyzes the transfer of electrons from NADH to coenzyme Q10 (CoQ10) and translocates protons across the inner mitochondrial membrane in eukaryotes or the plasma membrane of bacteria.

<span class="mw-page-title-main">Flavin adenine dinucleotide</span> Redox-active coenzyme

In biochemistry, flavin adenine dinucleotide (FAD) is a redox-active coenzyme associated with various proteins, which is involved with several enzymatic reactions in metabolism. A flavoprotein is a protein that contains a flavin group, which may be in the form of FAD or flavin mononucleotide (FMN). Many flavoproteins are known: components of the succinate dehydrogenase complex, α-ketoglutarate dehydrogenase, and a component of the pyruvate dehydrogenase complex.

<span class="mw-page-title-main">Flavin mononucleotide</span> Chemical compound

Flavin mononucleotide (FMN), or riboflavin-5′-phosphate, is a biomolecule produced from riboflavin (vitamin B2) by the enzyme riboflavin kinase and functions as the prosthetic group of various oxidoreductases, including NADH dehydrogenase, as well as cofactor in biological blue-light photo receptors. During the catalytic cycle, a reversible interconversion of the oxidized (FMN), semiquinone (FMNH), and reduced (FMNH2) forms occurs in the various oxidoreductases. FMN is a stronger oxidizing agent than NAD and is particularly useful because it can take part in both one- and two-electron transfers. In its role as blue-light photo receptor, (oxidized) FMN stands out from the 'conventional' photo receptors as the signaling state and not an E/Z isomerization.

<span class="mw-page-title-main">Flavoprotein</span> Protein family

Flavoproteins are proteins that contain a nucleic acid derivative of riboflavin. These proteins are involved in a wide array of biological processes, including removal of radicals contributing to oxidative stress, photosynthesis, and DNA repair. The flavoproteins are some of the most-studied families of enzymes.

Flavodoxins (Fld) are small, soluble electron-transfer proteins. Flavodoxins contains flavin mononucleotide as prosthetic group. The structure of flavodoxin is characterized by a five-stranded parallel beta sheet, surrounded by five alpha helices. They have been isolated from prokaryotes, cyanobacteria, and some eukaryotic algae.

<span class="mw-page-title-main">SDHA</span> Protein-coding gene in the species Homo sapiens

Succinate dehydrogenase complex, subunit A, flavoprotein variant is a protein that in humans is encoded by the SDHA gene. This gene encodes a major catalytic subunit of succinate-ubiquinone oxidoreductase, a complex of the mitochondrial respiratory chain. The complex is composed of four nuclear-encoded subunits and is localized in the mitochondrial inner membrane. SDHA contains the FAD binding site where succinate is deprotonated and converted to fumarate. Mutations in this gene have been associated with a form of mitochondrial respiratory chain deficiency known as Leigh Syndrome. A pseudogene has been identified on chromosome 3q29. Alternatively spliced transcript variants encoding different isoforms have been found for this gene.

Oxidative decarboxylation is a decarboxylation reaction caused by oxidation. Most are accompanied by α- Ketoglutarate α- Decarboxylation caused by dehydrogenation of hydroxyl carboxylic acids such as carbonyl carboxylic acid, malic acid, isocitric acid, etc.

<span class="mw-page-title-main">4-Hydroxyphenylacetate 3-monooxygenase</span> Class of enzymes

4-hydroxyphenylacetate 3-monooxygenase (EC 1.14.14.9) is an enzyme that catalyzes the chemical reaction

<span class="mw-page-title-main">NADH peroxidase</span>

In enzymology, a NADH peroxidase (EC 1.11.1.1) is an enzyme that catalyzes the chemical reaction

Flavin reductase a class of enzymes. There are a variety of flavin reductases, which bind free flavins and through hydrogen bonding, catalyze the reduction of these molecules to a reduced flavin. Riboflavin, or vitamin B, and flavin mononucleotide are two of the most well known flavins in the body and are used in a variety of processes which include metabolism of fat and ketones and the reduction of methemoglobin in erythrocytes. Flavin reductases are similar and often confused for ferric reductases because of their similar catalytic mechanism and structures.

<span class="mw-page-title-main">NDUFB2</span> Protein-coding gene in the species Homo sapiens

NADH dehydrogenase [ubiquinone] 1 beta subcomplex subunit 2, mitochondrial is an enzyme that in humans is encoded by the NDUFB2 gene. NADH dehydrogenase (ubiquinone) 1 beta subcomplex, 2, 8kDa is an accessory subunit of the NADH dehydrogenase (ubiquinone) complex, located in the mitochondrial inner membrane. It is also known as Complex I and is the largest of the five complexes of the electron transport chain.

<span class="mw-page-title-main">NDUFB3</span> Protein-coding gene in the species Homo sapiens

NADH dehydrogenase (ubiquinone) 1 beta subcomplex, 3, 12kDa is a protein that in humans is encoded by the NDUFB3 gene. NADH dehydrogenase (ubiquinone) 1 beta subcomplex, 3, 12kDa is an accessory subunit of the NADH dehydrogenase (ubiquinone) complex, located in the mitochondrial inner membrane. It is also known as Complex I and is the largest of the five complexes of the electron transport chain. Mutations in this gene contribute to mitochondrial complex I deficiency.

<span class="mw-page-title-main">NDUFA3</span> Protein-coding gene in the species Homo sapiens

NADH dehydrogenase [ubiquinone] 1 alpha subcomplex subunit 3 is a protein that in humans is encoded by the NDUFA3 gene. The NDUFA3 protein is a subunit of NADH dehydrogenase (ubiquinone), which is located in the mitochondrial inner membrane and is the largest of the five complexes of the electron transport chain.

<span class="mw-page-title-main">NDUFA7</span> Protein-coding gene in the species Homo sapiens

NADH dehydrogenase [ubiquinone] 1 alpha subcomplex subunit 7 is an enzyme that in humans is encoded by the NDUFA7 gene. The NDUFA7 protein is a subunit of NADH dehydrogenase (ubiquinone), which is located in the mitochondrial inner membrane and is the largest of the five complexes of the electron transport chain.

<span class="mw-page-title-main">NDUFA4L2</span> Protein-coding gene in the species Homo sapiens

NADH dehydrogenase (ubiquinone) 1 alpha subcomplex, 4-like 2 is a protein that in humans is encoded by the NDUFA4L2 gene. The NDUFA4L2 protein is a subunit of NADH dehydrogenase (ubiquinone), which is located in the mitochondrial inner membrane and is the largest of the five complexes of the electron transport chain.

<span class="mw-page-title-main">NDUFB4</span> Protein-coding gene in the species Homo sapiens

NADH dehydrogenase (ubiquinone) 1 beta subcomplex, 4, 15kDa is a protein that in humans is encoded by the NDUFB4 gene. The NDUFB4 protein is a subunit of NADH dehydrogenase (ubiquinone), which is located in the mitochondrial inner membrane and is the largest of the five complexes of the electron transport chain.

<span class="mw-page-title-main">NDUFB5</span> Protein-coding gene in the species Homo sapiens

NADH dehydrogenase (ubiquinone) 1 beta subcomplex, 5, 16kDa is a protein that in humans is encoded by the NDUFB5 gene. The NDUFB5 protein is a subunit of NADH dehydrogenase (ubiquinone), which is located in the mitochondrial inner membrane and is the largest of the five complexes of the electron transport chain.

<span class="mw-page-title-main">NDUFAB1</span> Protein-coding gene in the species Homo sapiens

NADH dehydrogenase (ubiquinone) 1 beta subcomplex, 5, 16kDa is a protein that in humans is encoded by the NDUFAB1 gene. The NDUFAB1 protein is a subunit of NADH dehydrogenase (ubiquinone), which is located in the mitochondrial inner membrane and is the largest of the five complexes of the electron transport chain.

References

  1. Michaelis L, Schubert MP, Smythe CV (1936). "Potentiometric study of the flavins". Journal of Biological Chemistry. 116 (2): 587–607. doi: 10.1016/S0021-9258(18)74634-6 .
  2. 1 2 Massey V, Stankovich M, Hemmerich P (January 1978). "Light-mediated reduction of flavoproteins with flavins as catalysts". Biochemistry. 17 (1): 1–8. doi:10.1021/bi00594a001. PMID   618535.
  3. Alexandre MT, Domratcheva T, Bonetti C, van Wilderen LJ, van Grondelle R, Groot ML, Hellingwerf KJ, Kennis JT (July 2009). "Primary reactions of the LOV2 domain of phototropin studied with ultrafast mid-infrared spectroscopy and quantum chemistry". Biophysical Journal. 97 (1): 227–37. Bibcode:2009BpJ....97..227A. doi:10.1016/j.bpj.2009.01.066. PMC   2711383 . PMID   19580760.

Further reading