The flavins are nature's most complicated and versatile enzyme cofactors. There are three forms of flavin present in significant concentrations in your body right now:

riboflavin or vitamin B6 - consists of a three ring head group called isoalloxazine covalently attached to a linearized molecule of ribose. Riboflavin is not used, as such, by the body. Instead it is a metabolic precursor to other flavin cofactors.

flavin mononucleotide, or FMN - consists of a molecule of riboflavin with a phosphate group on the far end of the ribose chain from the head group. This form serves as an enzyme cofactor which binds noncovalently to a variety of enzymes. The anionic terminal phosphate group serves as an anchor for the enzyme to bind the flavin using salt bridges, although most enzymes bind FMN by interacting with several other parts of the molecule as well.

flavin adenine dinucleotide, or FAD - This molecule is formed when a molecule of FMN and a molecule of adenosine triphosphate are joined together by an enzyme called, strangely enough, FAD synthetase. The end result is a riboflavin-diphospho-adenosine molecule, with the diphsophate group attached to the far end of the ribose on the flavin side and a hydroxyl group of the adosine's circular ribose on the other. These molecules also serve as enzyme cofactors.

This is just a list of the most common flavin cofactors, several other more obscure versions have been detected over the years, and are also present in your body at very low concentrations. 6-hydroxy-FAD, 5-deazaflavin 3-sulfonate, and 8-histidinyl-FMN are worthy of mention, perhaps in a later node.

With very few exceptions, flavins are involved in biological electron transfer reactions. They occupy a unique position in cellular metabolism, as they have the ability to transfer electrons either one or two at a time. You often hear it said that flavins can 'take in a dime and spit out two nickels.' (Well, at least I often hear it at biochemistry seminars.) The ability of the flavin to perform either single or double electron transfer varies greatly from one enzyme to another, and the factors which determine whether a flavoprotein can do both or either is a currently a topic of much debate among biophysicists.

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