Riboflavin Vitamin B2

The first hint that McCollum's vitamin B was in reality a multifactor complex came when yeast void of antineuritis activity still retained growth-stimulating activity. Originally called vitamin G, riboflavin was renamed vitamin B2 when it was recognized to be part of the yeast B complex. The name riboflavin followed the discovery in 1935 of its association with green fluorescent pigment of whey. Today, we regard riboflavin and niacin as the two principal vitamins that give rise to coen-zymes that function with enzymes known as oxidor-eductases. Both coenzymes transport electrons to and from substrates and in so doing form oxidized or reduced products. The two are referred to as 'redox' (an abbreviation for oxidation-reduction) coenzymes for that reason. Riboflavin was identified as the biochemical compound that gave the color to Warburg's 'yellow enzyme,' glucose-6-phosphate dehydrogenase (G6PDH). G6PDH was observed to catalyze the transfer of electrons from nicotinamide adenine dinucleotide phosphate (NADPH) to methylene blue, a redox sensitive dye that lost color upon reduction, suggesting that riboflavin probably mediated the electron exchange. G6PDH is a key entrance point for glucose into the pentose pathway and a major contributor of NADPH for the biosynthesis of fatty acids and other fats. The reaction is:

Glucose-6-phosphate + 2NADP+ + H2O

Riboflavin (Figure 2C) is associated with two coenzymes, FMN and FAD. FMN is formed by phosphorylating the primary alcohol on the sugar moiety of riboflavin, an ATP-dependent reaction. FAD results from a further condensation of FMN with the 5' AMP moiety of ATP (Figure 2D). What may be considered the active site is the isoalloxazine ring, which can exist in both oxidized and reduced states depending on whether electron pairs are absent or present, respectively. Enzymes that contain FAD or FMN are referred to as flavoproteins. FMN is limited to the membrane proteins of the mitochondria electron transport system whereas FAD is found in both membrane-bound and soluble

W / Thiazolium O-

NH2 h-c^s ring

^ Active site

Thiamine pyrophosphate (TPP)


Riboflavin (vitamin B2)

Flavin adenine dinucleotide (FAD)


Niacin (E) (nicotinamide)




Nicotinamide adenine (F) dinucleotide (NAD+)

Figure 2 Structural relationship of vitamin-coenzyme for (A) thiamine (B^, (C) riboflavin (B2), and (E) niacin. In the left column is the structure of the vitamin, in the right column the coenzyme derived from the vitamin. Note the pyrophosphate group in the coenzyme derived from thiamine (B) and the prevalence of phosphate and adenyl groups in the coenzymes derived from riboflavin (D) and niacin (F), showing the necessity for ATP in their synthesis.

enzymes. The flavin cofactor is bound covalently to the structure preventing disengagement during purification procedures.

Reactions Flavin enzymes are designed to remove (and add) electrons to and from substrates. In general, flavin coenzymes are stronger oxidizing agents than the pyrimidine coenzymes (NAD+, NADP+) and tend to participate in more complex reactions. Also, flavin coenzymes can accept single electrons from a donor, forming a semiquinone and allowing flavoproteins to take part in reactions that form free radicals. Having a single electron also allows favins to bind molecular oxygen as a hydroperoxyl complex.

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