Vitamin B2 (riboflavin; section 11.7) is important in a wide variety of oxidation and reduction reactions. A few enzymes contain riboflavin itself, while others contain a riboflavin derivative: either riboflavin phosphate (sometimes called flavin mononucleotide) or flavin adenine dinucleotide (FAD; Figure 2.14). When an enzyme contains riboflavin, it is usually covalently bound at the active site. Although riboflavin phosphate and FAD are not normally covalently bound to the enzyme, they are very tightly bound, and can be regarded as prosthetic groups. The resultant enzymes with attached riboflavin are collectively known as flavoproteins.
As shown in Figure 2.15, the riboflavin moiety of flavoproteins can undergo two reduction reactions. It can accept one hydrogen, to form the flavin radical (generally written as flavin-H*), followed by a second hydrogen, forming fully reduced flavin-H2.
Some reactions involve transfer of a single hydrogen to a flavin, forming flavin-H*, which is then recycled in a separate reaction. Sometimes two molecules of flavin each accept one hydrogen atom from the substrate to be oxidized. Other reactions involve the sequential transfer of two hydrogens onto the flavin, forming first the flavin-H* radical, then fully reduced flavin-H2.
As discussed in section 22.214.171.124, the reoxidation of reduced flavins in enzymes that react with oxygen is a major source of potentially damaging oxygen radicals.
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