Flavin adenine dinucleotide forming enzyme

Methylene-Tetrahydrofolate Reductase The reduction of methylene-tetrahydrofolate to methyl-tetrahydrofolate, shown in Figure 10.7, is catalyzed hy methylene-tetrahydrofolate reductase, a flavin adenine dinucleotide-dependent enzyme during the reaction, the pteridine ring of the substrate is oxidized to dihydrofolate, then reduced to tetrahydrofolate by the flavin, which is reduced by nicotinamide adenine dinucleotide phosphate (NADPH Matthews and Daubner, 1982). The reaction is irreversible under physiological conditions, and methyl-tetrahydrofolate - which is the main form of folate taken up into tissues (Section 10.2.2) - can only be utilized after demethylation catalyzed by methionine synthetase (Section 10.3.4). 

Flavoprotein enzymes contain flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD) as prosthetic groups. FMN and FAD are formed in the body from the vitamin riboflavin (Chapter 45). FMN and FAD are usually tighdy—but not covalendy—bound to their respecdve apoenzyme proteins. Metalloflavopro-teins contain one or more metals as essential cofactors. 

Figure 17-5. Oxidative decarboxylation of pyruvate by the pyruvate dehydrogenase complex. Lipoic acid is joined by an amide link to a lysine residue of the transacetylase component of the enzyme complex. It forms a long flexible arm, allowing the lipoic acid prosthetic group to rotate sequentially between the active sites of each of the enzymes of the complex. (NAD nicotinamide adenine dinucleotide FAD, flavin adenine dinucleotide TDP, thiamin diphosphate.)...

A homogeneous electrochemical enzyme immunoassay for 2,4-dinitrophenol-aminocaproic acid (DNP-ACA), has been developed based on antibody inhibition of enzyme conversion from the apo- to the holo- form Apoglucose oxidase was used as the enzyme label. This enzyme is inactive until binding of flavin adenine dinucleotide (FAD) to form the holoenzyme which is active. Hydrogen peroxide is the enzymatic product which is detected electrochemically. Because antibody bound apoenzyme cannot bind FAD, the production of HjOj is a measure of the concentration of free DNP-ACA in the sample. 

Glucose oxidase (GOD) is a typical flavin enzyme with flavin adenine dinucleotide (FAD) as redox prosthetic group. Its biological function is to catalyze glucose to form gluconolaction, while the enzyme itself is turned from GOD(FAD) to GOD(FADH2). GOD was used to prepare biosensors in extensive fields. Many materials that can be used to immobilize other proteins can be suitable for GOD. GOD adsorbed on CdS nanoparticles maintained its bioactivity and structure, and could electrocatalyze... 

Most coenzymes have aromatic heterocycles as major constituents. While enzymes possess purely protein structures, coenzymes incorporate non-amino acid moieties, most of them aromatic nitrogen het-erocycles. Coenzymes are essential for the redox biochemical transformations, e.g., nicotinamide adenine dinucleotide (NAD, 13) and flavin adenine dinucleotide (FAD, 14) (Scheme 5). Both are hydrogen transporters through their tautomeric forms that allow hydrogen uptake at the termini of the quinon-oid chain. Thiamine pyrophosphate (15) is a coenzyme that assists the decarboxylation of pyruvic acid, a very important biologic reaction (Scheme 6). 

Riboflavin is the redox component of flavin adenine dinucleotide FAD. It is derived from FAD by hydrolysis of a phosphate ester link. The fully oxidised form of FAD is involved in many dehydrogenaze reactions during which it is converted to the fully reduced form. The fully oxidised state is restored either by another redox enzyme or by interaction with oxygen and hydrogen peroxide is liberated. The one-electron reduced, semiquinone form of FAD, is involved in some electron transfer steps. 

Most presently available MAOIs are irreversible inhibitors of the enzyme, forming a chemical bond with part of the enzyme or the flavin adenine dinucleotide cofactor. When treatment is stopped, inhibition continues for a time until MAO levels return to normal as the new enzyme is synthesized. Thus, phenelzine, isocarboxazid, and tranylcypromine are all irreversible, nonselective MAOIs. Clorgyline, however, is an irreversible, selective MAO-A inhibitor moclobemide is a reversible, selective MAOI l-deprenyl and pargyline are relatively selective, irreversible MAO-B inhibitors. 

Riboflavin (vitamin B2 6.18) consists of an isoalloxazine ring linked to an alcohol derived from ribose. The ribose side chain of riboflavin can be modified by the formation of a phosphoester (forming flavin mononucleotide, FMN, 6.19). FMN can be joined to adenine monophosphate to form flavin adenine dinucleotide (FAD, 6.20). FMN and FAD act as co-enzymes by accepting or donating two hydrogen atoms and thus are involved in redox reactions. Flavoprotein enzymes are involved in many metabolic pathways. Riboflavin is a yellow-green fluorescent compound and, in addition to its role as a vitamin, it is responsible for the colour of milk serum (Chapter 11). 

Milk XO has a molecular weight of c. 300 kDa and consists of two subunits. The pH optimum is about 8.5 and the enzyme requires flavin adenine dinucleotide (FAD), Fe, Mo and an acid-labile compound as co-factors cows deficient in Mo have low XO activity. The amino acid composition of XO has been determined by a number of workers at least five genetic polymorphic forms have been reported. 

Flavin adenine dinucleotide (FAD) (fig. 10.8) and flavin mononucleotide (FMN) are the coenzymatically active forms of vitamin B2, riboflavin. Riboflavin is the NI0-ribityl isoalloxazine portion of FAD, which is enzymatically converted into its coenzymatic forms first by phosphorylation of the ribityl C-5 hydroxy group to FMN and then by ade-nylylation to FAD. FMN and FAD are functionally equivalent coenzymes, and the one that is involved with a given enzyme appears to be a matter of enzymatic binding specificity. 

Amperometric biosensors based on flavin-containing enzymes have been studied for nearly 30 years. These sensors typically undergo several chemical or electrochemical steps which produce a measurable current that is related to the substrate concentration. In the initial step, the substrate converts the oxidized flavin adenine dinucleotide (FAD) center of the enzyme into its reduced form (FADH2). Because these redox centers are essentially electrically insulated within the enzyme molecule, direct electron transfer to the surface of a conventional electrode does not occur to a substantial degree. The classical" methods (1-4) of indirectly measuring the amount of reduced enzyme, and hence the amount of substrate present, rely on the natural enzymatic reaction ... 

FAD (flavine adenine dinucleotide) is located at the enzyme s active site (as). In oxidizing glucose, the cofactor is reduced to FADH2, but the oxidized form of an appropriate mediator (Mox) has a redox potential sufficient to re-oxidize the cofactor whilst itself being reduced. In its simplest embodiment, the reduced mediator (Mred) diffuses from the active site at the enzyme to an electrode surface poised at a potential sufficiently oxidizing to re-oxidize the mediator ... 

Fig. 2. Flavin adenine dinucleotide, oxidised form, (a) X is H in the ordinary free coenzyme, (b) In cases of covalent attachment to the enzyme, X is the imidazole N (I) or N (3) of a histidine residue, or the S of...

---