Vitamin B2 (Riboflavin)

Riboflavin is the precursor of two prosthetic groups, flavin mononucleotide (FMN) and flavin dinucleotide (FAD) in flavoproteins. The two coenzymes, which are tightly bound to the protein moiety cycle between the reduced and the oxidized forms. FAD is more represented in these reactions than FMN. The chemical structures of riboflavin and the two coenzymes, FMN and FAD are presented below.



The isoalloxazine ring is involved in the transfer of electrons as shown below:


FAD is able to undergo two sequential one-electron transfers or a simultaneous two-electron transfer, thus bypassing the semiquinone state. Because of that the FAD-dependent enzymes are able to take part in a greater variety of redox reactions than the NAD(P)-dependent dehydrogenases.

FADH2 is generated during fatty acids degradation (called b-oxidation) in mitochondria and peroxisomes as well as during the tricarboxylic acids cycle. The main role of FADH2 is to feed reducing powers (electrons) to the electron transport chain with the end result being ATP synthesis.

An example of a well studied flavoenzyme is glutathione reductase, which catalyzes the NADPH-dependent reduction of oxidized glutathione (GSSG). As seen in the section on antioxidant defense systems, the reduced glutathione (GSH) is necessary for the removal of hydrogen peroxide in erythrocytes and metabolically active cells. The GSSG is reduced to GSH according to the reaction:

GSSG + NADPH + H+ ----> 2GSH + NADP+

The electrons flow from NADPH to FAD, which becomes reduced and in turn reduces the oxidized glutathione.

For an indepth discussion on the mechanism of the above reaction the reader should refer to an excellent general biochemistry textbook. (Biochemistry, D. Voet and J.G. Voet, 3rd. Edition Volume 1, John Wiley & Sons, New York, 2003).

Some of the processes in which vitamin B2 is involved are listed below:
  • Involved in energy production and the regeneration of cellular antioxidant glutathione; necessary for carbohydrate, fat and protein metabolism.
  • Plays a crucial role in the formation of antibodies, red blood cells and cell respiration.
  • Promotes healthy skin, nails and hair; alleviates eye fatigue.
  • Mild deficiency may be associated with fatigue, migrane.
  • Interactions: vitamin B2 works together with vitamins B1, B3 and B6. Vitamin B2 is inactivated by light, sulfa drugs, alcohol, estrogens. Riboflavin deficiency may exert some of its effects by reducing the metabolism of other B vitamins, notably folate and vitamin B6.
  • Health benefits: supplementation is useful in conditions of stress and should be taken with other B vitamins and zinc.
  • Best food sources: brewer's yeast, organ meats (liver, kidney, heart), almonds, mushrooms, whole grains.