Flavin adenine dinucleotide reduced, reoxidation

In the biological oxidation-reduction system, reduced NAD (i.e., NADH) is reoxidized to NAD by the riboflavin-containing coenzyme FAD flavin-adenine dinucleotide). 

Three types of subunit proteins constitute such dehydrogenase complexes (1) a TPP-dependent decarboxylase, which converts the a-keto acid to an a-hydroxyalkyi-TPP complex (2) a transacylase core, which contains lipoyl residues that are acylated by the a-hydroxyalkyl-TPP and (3) a flavin adenine dinucleotide (FAD)-dependent dihy-drolipoyi dehydrogenase, which reoxidizes the reduced Hpoyl... 

FIGURE 15.4 The structures of riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD). Even in organisms that rely on the nicotinamide coenzymes (NADH and NADPH) for many of their oxidation-reduction cycles, the flavin coenzymes fill essential roles. Flavins are stronger oxidizing agents than NAD and NADP. They can be reduced by both one-electron and two-electron pathways and can be reoxidized easily by molecular oxygen. Enzymes that use flavins to carry out their reactions—flavoenzymes—are involved in many kinds of oxidation-reduction reactions. 

The oxidation reactions involved are catalyzed by a series of nicotinamide adenine dinucleotide (NAD+) or flavin adenine dinucleotide (FAD) dependent dehydrogenases in the highly conserved metabolic pathways of glycolysis, fatty acid oxidation and the tricarboxylic acid cycle, the latter two of which are localized to the mitochondrion, as is the bulk of coupled ATP synthesis. Reoxidation of the reduced cofactors (NADH and FADH2) requires molecular oxygen and is carried out by protein complexes integral to the inner mitochondrial membrane, collectively known as the respiratory, electron transport, or cytochrome, chain. Ubiquinone (UQ), and the small soluble protein cytochrome c, act as carriers of electrons between the complexes (Fig. 13.1.1). 

For the reduction of alkenes or alkynes to alkanes in laboratory we use metal catalysts such as Pt or Pd and often high pressures. The heating of alkane precursors with these metal catalysts reoxidizes alkanes to alkenes. In biosynthesis these reactions proceed with special reagents like flavine adenine dinucleotide FAD or its reduced form FADH2. 

Scheme 10.6. The oxidation of an amino acid (G = alkyl or alkaryl) by an oxidase utilizing flavin adenine dinucleotide (FAD). The latter is reduced to FADH2 as the amino acid is oxidized to imine. The imine is hydrolyzed to ammonia and a-ketocarboxylic acid. The reduced dinucleotide (FADH2) is reoxidized by oxygen (O2) to FAD. The reduction product in the second reaction is hydrogen peroxide, H2O2 (or its equivalent).

---