Riboflavin coenzymes

A hypothesis for the oxidation of purines in the presence of this enzyme has been elaborated by Bergmann and his colleagues. It postulates that the purine, often in one of its less prevalent tautomeric forms, is adsorbed on the protein, or the riboflavin coenzyme, of the enzyme then hydration occurs under the influence of the electronic field of the enz5rme, and this must involve a group that is not sterically blocked by the enzyme but which is accessible to the electron-transport pathway of the riboflavin moiety. Finally, the secondary alcohol is assumed to be dehydrogenated in this pathway to give a doubly... 

Br Riboflavin Coenzyme in oxidation and reduction reactions prosthetic group of flavoproteins Lesions of corner of mouth, lips, and tongue seborrheic dermatitis... 

A recent interlaboratory comparison of HPLC and microbiological methods for total riboflavin revealed significant variability between the 13 participating laboratories (42). The extraction and hydrolysis of the riboflavin coenzymes were cited as the most likely sources of this variability. A later intercomparison (70) of riboflavin methods showed lower variability between laboratories, although coefficients of variability (CV) of 12-40% were still reported. 

Control over tissue concentrations of riboflavin coenzymes seems to be largely by control of the activity of flavokinase, and the synthesis and catabolism of flavin-dependent enzymes. Almost all the vitamin in tissues is enzyme bound, and free riboflavin phosphate and FAD are rapidly hydrolyzed to riboflavin. If this is not rephosphorylated, it rapidly diffuses out of tissues and is excreted. 

Because flavin coenzymes are widely distributed in intermediary metabolism, the consequences of deficiency maybe widespread. Because riboflavin coenzymes are involved in the metabohsm of folic acid, pyridoxine, vitamin K, and niacin, deficiency will affect enzyme systems other than those requiring flavin coenzymes. With increasing riboflavin deficiency, tissue concentrations of FMN and FAD fall, as does flavokinase activity, thus further decreasing FMN concentrations. FMN concentrations are decreased proportionally more than FAD concentrations. Decreases in the activities of enzymes requiring FMN generally follow the fall in tissue concentrations, whereas the FAD-dependent enzymes are more variably affected. ... 

Metabolic Role. Riboflavin coenzymes are required for most oxidations of carbon-carbon bonds (Fig. 8.29). Examples include the oxidation of succinyl CoA to fumarate in the Krebs cycle and introduction of a,jS-unsaturation in /3-oxidation of fatty acids. Riboflavin is also required for the metabolism of other vitamins, including the reduction of 5,10-methylene tetrahydrofolate to 5-methyl tetrahydrofolate (Fig. 8.49), and interconversion of pyridoxine-pyridoxal phos-phate-pyridoxamine (Fig. 8.33). Because oxi-dation/reductions that use FAD or FMN as the coenzyme constitute a two-step process, some flavin coenzyme systems contain more than one FAD or FMN. 

Industry and Business. Scientists in industry traditionally carry out a significant percentage of metabolic engineering research, a good portion of which is directed to health care products, such as pharmaceuticals. Successful scientific projects include creating of lysine, riboflavin, coenzyme Q-IO, the aminoshikimate pathway, and beta-carotene by meta-bolically engineered microorganisms. 

Niacin, a member of the B-complex, is a collective term which includes nicotinic acid and nicotinamide, both natural forms of the vitamin with equal niacin activity. In the body, they are active as nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) and serve as coenzymes, often in partnership with thiamin and riboflavin coenzymes, to produce energy within the cells, precisely when needed and in the amount necessary. 

To date no definite evidence has been obtained of a riboflavin coenzyme associated with kynurenine hydroxylase. In view of the difficulty in purif dng this enz3rme this is hardly conclusive. 

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