Flavin derivatives phosphorylation

ELAVOPROTEINS. Flavin is an essential substance for the activity of a number of important oxidoreductases. We discuss the chemistry of flavin and its derivatives, FMN and FAD, in the chapter on electron transport and oxidative phosphorylation (Chapter 21). 

The procedure to phosphorylate riboflavin derivatives on a preparative scale has recently been improved . These preparations, and also commercial FMN, contain a considerable amount of riboflavin phosphate isomers, which are difficult to separate by column chromatography. This problem is emphasized in the chemical synthesis of FAD where the yield is rather low (20-25 %). In this context, it is surprising that a modification of the synthesis of FAD from FMN published by Cramer and Neuhoeffer has not been noticed by workers in the flavin field. According to Cramer and Neuhoeffer, the yield of the chemical synthesis of FAD is drastically improved ( 70 % pure FAD). The procedure was successfully applied in the author s own laboratory (yield 60-70%). It is expected that the improved procedure of the FAD synthesis will stimulate the active-site directed studies on flavoproteins because the problem of separating FMN or FAD from their synthetic by-products has already been solved by use of FMN- or FAD-specific affinity column... 

The study of bioenergetics involves the study of (1) the processes by which reduced nicotinamide and flavin nucleotides, generated primarily from the oxidation of carbohydrates (Chap. 11) and lipids (Chap. 13), are oxidized ultimately by molecular oxygen via the mitochondrial electron-transport chain, and (2) the mechanism by which this oxidation is coupled to ATP synthesis. The synthesis of ATP in this way is referred to as oxidative phosphorylation, in contrast to phosphorylation of ADP via soluble enzymes. The latter involves intermediate phosphate derivatives of the substrate and is known as substrate-level phosphorylation (Chap. 11). 

By phosphorylation, riboflavin is converted to flavin mononucleotide (FMN), from which flavin adenine dinucleotide (FAD) is derived by adenylation. FMN and FAD are the cofactors of the broad class of flavoenzymes coupling the two-electron oxidation of many organic substrates to the one-electron transfers of the respiratory chain. 

Flavin binding proteins. Flavin-binding chromoproteins are collectively referred to as flavoproteins. Their chromophores are derived from riboflavin (vitamin B2 Fig. 15), which is phosphorylated by the flavokinase enzyme to yield flavin mononucleotide (FMN) and, in a second ATP-dependent reaction, FAD pyrophosphorylase attaches an AMP... 

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