Flavin dehydrogenases

The reaction can be coupled to the oxidation of either NADH or NADPH by the presence of a flavin dehydrogenase, which is usually present complexed with the luciferase ... 

In contrast to the flavin oxidases, flavin dehydrogenases pass electrons to carriers within electron transport chains and the flavin does not react with 02. Examples include a bacterial trimethylamine dehydrogenase (Fig. 15-9) which contains an iron-sulfur duster that serves as the immediate electron acceptor167 169 and yeast flavocytochrome b2, a lactate dehydrogenase that passes electrons to a built-in heme group which can then pass the electrons to an external acceptor, another heme in cytochrome c.170-173 Like glycolate oxidase, these enzymes bind their flavin coenzyme at the ends of 8-stranded a(i barrels similar... 

Scrutton, N. S., 1994, a/p barrel evolution and the modular assembly of enzymes emerging trends in the flavin dehydrogenase/oxidase family, BioEssays 16 1159122. 

In contrast to the nicotinamide nucleotide dehydrogenases, the prosthetic groups FMN and FAD are firmly associated with the proteins, and the flavin groups are usually only separated from the apoen2yme (protein) by acid treatment in water. However, in several covalently bound flavoproteins, the enzyme and flavin coen2ymes are covalently affixed. In these cases, the flavin groups are isolated after the proteolytic digestion of the flavoproteins. 

Covalently Bound Flavins. The FAD prosthetic group in mammalian succinate dehydrogenase was found to be covalently affixed to protein at the 8 a-position through the linkage of 3-position of histidine (102,103). Since then, several covalently bound riboflavins (104,105) have been found successively from the en2ymes Hsted in Table 3. The biosynthetic mechanism, however, has not been clarified. 

Oxidation of P-nicotinamide adenine dinucleotide (NADH) to NAD+ has attracted much interest from the viewpoint of its role in biosensors reactions. It has been reported that several quinone derivatives and polymerized redox dyes, such as phenoxazine and phenothiazine derivatives, possess catalytic activities for the oxidation of NADH and have been used for dehydrogenase biosensors development [1, 2]. Flavins (contain in chemical structure isoalloxazine ring) are the prosthetic groups responsible for NAD+/NADH conversion in the active sites of some dehydrogenase enzymes. Upon the electropolymerization of flavin derivatives, the effective catalysts of NAD+/NADH regeneration, which mimic the NADH-dehydrogenase activity, would be synthesized [3]. 

The electrodeposited film of flavin derivatives would be utilized as a functional material in combination with number dehydrogenases and pyridine coenzymes for the detection of great number of analytes. 

Fe= Catalase Flavin adenine dinucleotide (FAD) Hydrogen atoms Succinate dehydrogenase... 

Note that flavin coenzymes can carry out either one-electron or two-electron transfers. The succinate dehydrogenase reaction represents a net two-electron reduction of FAD. 

Akiyama, S. K, and Hamme.s, G. G., 1981. Elementary. steps in die reaction mechani.sm of die pyruvate dehydrogenase mnltienzyme complex from Escherichia coli Kinetics of flavin reduction. Biochemistry 20 1491-1497. 

As its name implies, this complex transfers a pair of electrons from NADH to coenzyme Q a small, hydrophobic, yellow compound. Another common name for this enzyme complex is NADH dehydrogenase. The complex (with an estimated mass of 850 kD) involves more than 30 polypeptide chains, one molecule of flavin mononucleotide (FMN), and as many as seven Fe-S clusters, together containing a total of 20 to 26 iron atoms (Table 21.2). By virtue of its dependence on FMN, NADH-UQ reductase is a jlavoprotein. 

Step 1 of Figure 29.3 Introduction of a Double Bond The /3-oxidation pathway begins when a fait)7 acid forms a thioester with coenzyme A to give a fatty acyl Co A. Two hydrogen atoms are then removed from C2 and C3 of the fatty acyl CoA by one of a family of acyl-CoA dehydrogenases to yield an a,/3-unsaturated acyl CoA. This kind of oxidation—the introduction of a conjugated double bond into a carbonyl compound—occurs frequently jn biochemical pathways and usually involves the coenzyme flavin adenine dinucleotide (FAD). Reduced FADH2 is the by-product. 

Complex II contains four peptides, the two largest form succinate dehydrogenase, the largest has covalently boiuid flavin adenine dinucleotide (FAD) which reacts with succinate, and the other has three iron-sulphur centers. Smaller subunits anchor the two larger subunits to the membrane and form the UQ binding site. Ubiquinone is the electron acceptor but complex II does not pump protons (see below). 

Formate dehydrogenases are a diverse group of enzymes found in both prokaryotes and eukaryotes, capable of converting formate to CO2. Formate dehydrogenases from anaerobic microorganisms are, in most cases, Mo- or W- containing iron-sulfur proteins and additionally flavin or hemes. Selenium cysteine is a Mo- ligand. 

Four of the B vitamins are essential in the citric acid cycle and therefore in energy-yielding metabolism (1) riboflavin, in the form of flavin adenine dinucleotide (FAD), a cofactor in the a-ketoglutarate dehydrogenase complex and in succinate dehydrogenase (2) niacin, in the form of nicotinamide adenine dinucleotide (NAD),... 

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.)...

Catalysis by flavoenzymes has been reviewed and various analogues of FAD have been prepared e.g. P -adenosine-P -riboflavin triphosphate and flavin-nicotinamide dinucleotide ) which show little enzymic activity. The kinetic constants of the interaction between nicotinamide-4-methyl-5-acetylimidazole dinucleotide (39) and lactic dehydrogenase suggest the presence of an anionic group near the adenine residue at the coenzyme binding site of the enzyme. ... 

Zellner G, A Jargon (1997) Evidence for a tungsten-stimulated aldehyde dehydrogenase activity of Desulfovi-brio simplex that oxidizes aliphatic and aromatic aldehydes with flavins as coenzymes. Arch Microbiol 168 480-485. 

Sugimoto M, M Tanaba, M Hataya, S Enokibara, JA Duine, F Kawai (2001) The first step in polyethylene glycol degradation by sphingomonads proceeds via a flavoprotein alcohol dehydrogenase containing flavin adenine dinucleotide. J Bacterial 183 6694-6698. 

Alcohol dehydrogenases found in certain microorganisms utilize a pyrroloquino-line quinone (PQQ) or flavin cofactor to pass electrons released upon oxidation of alcohols to the heme electron-acceptor protein, cytochrome c. These membrane-associated alcohol dehydrogenases form part of a respiratory chain, and the energy from fuel oxidation therefore contributes to generation of a proton gradient across... 

There have been a number of reports of electrocatalysis of alcohol oxidation using immobilized PQQ-dependent alcohol dehydrogenases or flavin-containing alcohol dehydrogenases or oxidases with dissolved mediators in solution. Co-immobihzing the mediator with the enzyme is advantageous, as set out in Section 17.1, and several such strategies have been employed for electrocatalytic alcohol oxidation. 

---