FLAVIN ADENINE DINUCLEOTIDE (FAD): CHARACTERISTICS, BIOSYNTHESIS - BIOLOGY - 2025

The FAD (flavin adenine dinucleotide) is an organic molecule, coenzyme in several enzymes of various metabolic pathways. Like other flavin-nucleotide compounds, it acts as a prosthetic group of oxidation-reduction enzymes. These enzymes are known as flavoproteins.

FAD is strongly bound to flavoprotein, in the enzyme succinate dehydrogenase; for example, it is covalently attached to a histidine residue.

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Flavoproteins act in the citric acid cycle, in the electronic transport chain and the oxidative degradation of amino acids and fatty acids, their function being to oxidize alkanes to alkenes.

characteristics

The FAD consists of a heterocyclic ring (isoaloxacin) that gives it a yellow color, attached to an alcohol (ribitol). This compound can be partially reduced generating a stable radical FADH, or totally reduced producing FADH ₂.

When it is covalently bound to enzymes it is considered a prosthetic group, that is, it forms a non-amino acid part of the protein.

Flavoproteins in their oxidized form present important absorption bands in the visible spectrum area, giving them an intense coloration that ranges from yellow to red and green.

When these enzymes are reduced, they suffer a discoloration, due to a change in the absorption spectrum. This characteristic is used to study the activity of these enzymes.

Plants and some microorganisms capable of synthesizing flavins, but in higher animals (such as man), the synthesis of the isoaloxacin ring is not possible, so these compounds are acquired through the diet, such as vitamin B ₂.

In FAD, the simultaneous transfer of two electrons, or sequential transfers of each electron, can be generated to produce the reduced form FADH ₂.

FAD biosynthesis

As mentioned above, the ring that makes up the coenzyme FAD cannot be synthesized by animals, so that to obtain said coenzyme a precursor obtained from the diet is required, which is generally a vitamin. These vitamins are only synthesized by microorganisms and plants.

FAD is generated from vitamin B ₂ (riboflavin) through two reactions. In riboflavin, a ribityl side chain is phosphorylated at the -OH group of the C5 carbon by the enzyme flavokinase.

In this step, the flavin mononucleotide (FMN) is generated which, despite its name, is not a true nucleotide, since the ribityl chain is not a real sugar.

After the formation of the FMN and through a pyrophosphate group (PPi), the coupling with an AMP occurs through the action of the enzyme FAD pyrophosphorylase, finally producing the coenzyme FAD. The enzymes flavokinase and pyrophosphorylase are found abundantly in nature.

Importance

Although many enzymes can carry out their catalytic functions by themselves, there are some that require an external component that gives them the chemical functions that they lack in their polypeptide chains.

The external components are called cofactors, which can be metal ions and organic compounds, in which case they are known as coenzymes, as is the case with FAD.

The catalytic site of the enzyme-coenzyme complex is called a holoenzyme, and the enzyme is known as an apoenzyme when it lacks its cofactor, a state in which it remains catalytically inactive.

The catalytic activity of various enzymes (flavin-dependent) need to be bound to FAD to carry out its catalytic activity. In them, FAD acts as an intermediate transporter of electrons and hydrogen atoms produced in the conversion of substrates to products.

There are various reactions that depend on flavins, such as the oxidation of carbon bonds in the case of the transformation of saturated to unsaturated fatty acids, or the oxidation of succinate to fumarate.

Flavin-dependent dehydrogenases and oxidases

Flavin-dependent enzymes contain a firmly attached FAD as a prosthetic group. The areas of this coenzyme that are involved in the redox of various reactions can be reversibly reduced, that is, the molecule can change reversibly to the FAD, FADH and FADH _{2 states}.

The most important flavoproteins are dehydrogenases linked to electron transport and respiration, and are found in the mitochondria or its membranes.

Some flavin-dependent enzymes are succinate dehydrogenase, which acts in the citric acid cycle, as well as acyl-CoA-dehydrogenase, which intervenes in the first dehydrogenation stage in the oxidation of fatty acids.

Flavoproteins that are dehydrogenases have a low probability that reduced FAD (FADH ₂) can be reoxidized by molecular oxygen. On the other hand, in flavoprotein oxidases, FADH ₂ easily tends to be reoxidized, producing hydrogen peroxide.

In some mammalian cells there is a flavoprotein called NADPH-cytochrome P450 reductase, which contains both FAD and FMN (flavin mononucleotide).

This flavoprotein is a membrane enzyme embedded in the outer membrane of the endoplasmic reticulum. FAD bound to this enzyme is the electron acceptor for NADPH during oxygenation of the substrate.

FAD in metabolic pathways

Succinate dehydrogenase is a membrane flavoprotein located in the inner mitochondrial membrane of cells, containing covalently bound FAD. In the citric acid cycle, this is responsible for oxidizing a saturated bond in the center of the succinate molecule, transforming said bond into a double one, to produce fumarate.

The coenzyme FAD is the receptor of the electrons coming from the oxidation of this bond, reducing it to its FADH ₂ state. These electrons are later transferred to the electronic transport chain.

Complex II of the electron transport chain contains the flavoprotein succinate dehydrogenase. The function of this complex is to pass electrons from succinate to coenzyme Q. FADH ₂ is oxidized to FAD, thus transferring the electrons.

The flavoprotein acyl-CoA-dehydrogenase catalyzes the formation of a trans double bond to form trans-enoyl CoA in the metabolic pathway of fatty acid β-oxidation. This reaction is chemically the same as that carried out by succinate dehydrogenase in the citric acid cycle, with the coenzyme FAD being the receptor for the H product of dehydrogenation.

References

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