Phosphorylation, adenosine riboflavin

Riboflavin (vitamin B2) 6,7-dimethyl-9-(D-l-ribityl)isoalloxazine (63), was discovered as a coloring matter in milk in 1879, but its importance was not then realized. Deficiency causes lesions of the eye and of the angle of the mouth. Riboflavin is phosphorylated by adenosine triphosphate (ATP) to give riboflavin 5 -phosphate (flavinadenine mononucleotide, FMN) and then flavinadenine dinucleotide (FAD) (64 R = riboflavin). These function as prosthetic groups in a number of flavoproteins which are dehydrogenation catalysts by virtue of the oxidation-reduction properties of the isoalloxazine system. 

Figure 3.8. Structures of vitamins or vitamin-derived molecules that function in oxidation-reduction reactions. The oxidation of these redox groups in the inner mitochondricil membrane contributes to the electron transport chain that carries electrons from the oxidation of glucose to oxygen and in the process pumps protons from one side to the other of the inner mitochondrial membrane (see Chapter 8 for details). The proton gradient thus formed is used to phosphorylate ADP to form 32 of the 36 ATPs resulting from the oxidation of one glucose molecule to six CO2 and six H2O molecules. A Vitamin B3, also called niacin or nicotinic acid, becomes converted to the amide (nicotinamide) and dressed up with a ribose sugar. Then, in a manner like that of riboflavin in B becomes phosphorylated to form nicotinamide mononucleotide (NMN) or further reacted with the addition of adenosine monophosphate (AMP) to form nicotinamide adenine dinucleotide (NAD). B Vitamin B2, also known as riboflavin, is shown converted to the forms involved in redox reactions such as those of the electron transport chain. (From Biochemistry, Second Edition, D. Voet and J. Voet, Copyright 1995, John Wiley Sons, New York. Reprinted with permission of John Wiley Sons, Inc.)...

Apart from dietary intake, riboflavin is also obtained from endogenous synthesis by microflora in the large intestine and is subsequently absorbed. Inside the cell, FMN is formed from vitamin B2 via adenosine triphosphate (ATP) phosphorylation and a flavokinase. FMN can be subsequently converted to FAD through a FAD synthetase also in the presence of ATP (Figure 37. IB). 

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