The chemical and biological versatility of riboflavin.

Since their discovery and chemical characterization in the 1930s, flavins have been recognized as being capable of both one- and two-electron transfer processes, and as playing a pivotal role in coupling the two-electron oxidation of most organic substrates to the one-electron transfers of the respiratory chain. In addition, they are now known as versatile compounds that can function as electrophiles and nucleophiles, with covalent intermediates of flavin and substrate frequently being involved in catalysis. Flavins are thought to contribute to oxidative stress through their ability to produce superoxide, but at the same time flavins are frequently involved in the reduction of hydroperoxides, products of oxygen-derived radical reactions. Flavoproteins play an important role in soil detoxification processes via the hydroxylation of many aromatic compounds, and a simple flavoprotein in liver microsomes catalyses many reactions similar to those carried out by cytochrome P450 enzymes. Flavins are involved in the production of light in bioluminescent bacteria, and are intimately connected with light-initiated reactions such as plant phototropism and nucleic acid repair processes. Recent reports also link them to programmed cell death. The chemical versatility of flavoproteins is clearly controlled by specific interactions with the proteins with which they are bound. One of the main thrusts of current research is to try to define the nature of these interactions, and to understand in chemical terms the various steps involved in catalysis by flavoprotein enzymes.