The Flavin Coenzymes

Figure 5.3 The flavin coenzymes FAD and FMN. Note that in contrast to NAD+, flavins can be half-reduced to the stable radical FADH or fully reduced to the dihydroflavin shown.

We can only show here a few examples from the many organic redox systems that are found. In the complete reduction of the flavin coenzymes FMN and FAD (see p.l04),... 

Figure 15-7 The flavin coenzymes flavin adenine dinucleotide (FAD) and riboflavin 5 -phosphate (FMN). Dotted lines enclose the region that is altered upon reduction.

The attention of biochemists was first attracted to flavins as a result of their color and fluorescence. The study of spectral properties of flavins (Fig. 15-8) has been of importance in understanding these coenzymes. The biochemical role of the flavin coenzymes was first recognized through studies of the "old yellow enzyme"144 145 which was shown by Theorell to contain riboflavin 5 -phosphate. By 1938, FAD was recognized as the coenzyme of a different yellow protein, D-amino acid oxidase of kidney tissue. Like the pyridine nucleotides, the new flavin coenzymes were reduced by dithionite to nearly colorless dihydro forms (Figs. 15-7 and 15-8) revealing the chemical basis for their function as hydrogen carriers. 

Oxidation states of flavin coenzymes. The flavin coenzymes exist in three spectrally distinguishable oxidation states that account in part for their catalytic functions. They are the yellow oxidized form, the red or blue one-electron reduced form, and the colorless two-electron reduced form. Groups in red are those which are centrally involved in oxidation-reduction reactions. 

The functional end of the flavin coenzymes FMN and FAD is the tricyclic isoalloxazine system, with the numbering system shown in structure I, the air-stable, yellow, oxidized form. The other two functionally important redox states are the one-electron-reduced semiquinone, II (pKa = 8.4 for dissociation at N(5)), and the two-electron-reduced, colorless dihydroflavin, III. In the dihydro form N(5), C(4a), C(la), andN(l) form a diaminoethylene system and it was anticipated that nitrogen at the 5 and 1 positions would be key to coenzymatic function. 

Unlike the NAD+/NADH couple, the flavin coenzymes are not in free solution and are covalently linked to the dehydrogenases of their respective substrates. These enzymes are membrane-bound in close association with the respiratory chain in the inner mitochondrial membrane. 

In bacterial NADHrFMN oxidoreductase [15], the flavin coenzyme is bound relatively weakly. This enzyme serves to provide reduced FMN to luciferase in... 

Figure 7.1. Riboflavin, the flavin coenzymes and covalently bound flavins in proteins. Relative molecular masses (Mr) riboflavin, 376.4 riboflavin phosphate, 456.6 and FAD, 785.6.

Covalent binding of the flavin coenzymes is normally through the 8-a-methyl group. 8-Hydroxymethyl-riboflavin is formed by microsomal mixed-function oxidases (Section 7.2.5), but it is not known whether or not this is a precursor of covalently bound flavin coenzymes. A variety of amino acid residues may be involved in covalent binding of flavin coenzymes to enzymes, including the following ... 

Although the ribitol moiety is not involved in the redox function of the flavin coenzymes, both the stereochemistry and nature of the sugar alcohol are important. Although some riboflavin analogs have partial vitamin action. 

The metabolic function of the flavin coenzymes is as electron carriers in a wide variety of oxidation and reduction reactions central to aU metabolic processes, including the mitochondrial electron transport chain. Unlike the nicotinamide nucleotide coenzymes (Section 8.4.1), which act as cosubstrates, leaving the catalytic site of the enzyme at the end of the reaction, the flavin coenzymes remain bound to the enzyme throughout the catalytic cycle. 

Riboflavin deficiency is relatively common, yet there is no clear deficiency disease and the condition never seems to be fatal. This presumably reflects the high degree of conservation of riboflavin in tissues (Section 7.2.3). There is only a relatively small difference between the concentration of flavins at which tissues are saturated and the lowest levels in prolonged depletion of experimental animals. In deficiency, most of the flavin coenzymes released by the catabolism of enzymes are reutilized. 

Riboflavin, the flavin coenzymes and covalently bound flavins... 

The flavin coenzyme occurs in each of the oxidases discussed here as flavin adenine dinucleotide (FAD). The R group attached to is adenosyldiphosphoribityl. The flavin nucleus can exist in three redox states, each of which can adopt three ionization states (8, 9). Only two redox states—fully oxidized and fully reduced see Equation 1)—are kinetically important in the simple flavoprotein oxidases under discussion. 

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