Flavopro tein

Conversion of inactive vitamin Bj2 to active 5 -deoxyadenosylcobal-amin is thought to involve three steps (see figure). Two flavopro-tein reductases sequentially convert Co in cyanocobalamin to the Co state and then to the Co state. Co is an extremely powerful nucleophile. It attacks the C-5 carbon of ATP as shown, expelling the triphosphate anion to form 5 -deoxyadenosyl-... 

Fig. 9. Absorption spectra of several flavopro-teins compared with an action spectrum of pho-totropism (dotted line C ),39))- The spectra are arranged with respect to the position of their UV-peaks (1) succinate dehydrogenase78), (2)lipo-amide dehydrogenase179), (4) lactate oxidase111), (5) D-amino acid oxidase111), (6) flavodoxin110), (7) old yellow enzyme2), (8) ferredoxin NADP+ reductase16), (9) oxynitrilase111), (10) L-amino acid oxidase111)...

V. Massey, S. Strickland, S.G. Mayhew, L.G. Howell, P.C. Engel, R.G. Matthews, M. Schuman, and P.A. Sullivan, Production of superoxide anion radicals in the reaction of reduced flavins and flavopro-teins with molecular oxygen. Biochem. Biophys. Res. Commun. 36, 891-897 (1969). 

Flavins are very versatile redox coenzymes. Flavopro-teins are dehydrogenases, oxidases, and oxygenases that catalyze a variety of reactions on an equal variety of substrate types. Since these classes of enzymes do not consist exclusively of flavoproteins, it is difficult to define catalytic specificity for flavins. Biological electron acceptors and donors in flavin-mediated reactions can be two-electron acceptors, such as NAD+ or NADP+, or a variety of one-electron acceptor systems, such as cytochromes (Fe2+/ Fe3+) and quinones, and molecular oxygen is an electron acceptor for flavoprotein oxidases as well as the source of oxygen for oxygenases. The only obviously common aspect of flavin-dependent reactions is that all are redox reactions. 

The first step in valine biosynthesis is a condensation between pyruvate and active acetaldehyde (probably hy-droxyethyl thiamine pyrophosphate) to yield a-acetolactate. The enzyme acetohydroxy acid synthase usually has a requirement for FAD, which, in contrast to most flavopro-teins, is rather loosely bound to the protein. The very same enzyme transfers the acetaldehyde group to a-ketobutyrate to yield a-aceto-a-hydroxybutyrate, an isoleucine precursor. Unlike pyruvate, the a-ketobutyrate is not a key intermediate of the central metabolic routes rather it is produced for a highly specific purpose by the action of a deaminase on L-threonine as shown in figure 21.10. 

The small protein thioredoxin supplies reducing equivalents to ribonucleotide reductase for the ribose ring reduction. Thioredoxin is itself reduced by another protein, thioredoxin reductase, a flavopro-tein. Reduced glutathione can also carry reducing equivalents to ribonucleotide reductase. In both cases, the ultimate source of reducing equivalents is NADPH. 

Riboflavin (vitamin B2) has been reported to improve the exercise capacity of a patient with Complex I deficiency. After conversion to flavin monophosphate and FAD, riboflavin functions as a cofactor for electron transport in Complex I, Complex II, and electron transfer flavopro-tein. Nicotinamide has been used because Complex I accepts electrons from NADH and ultimately transfers electrons to Q10. 

Mammalian NOS is a P-450-like enzyme that catalyzes the oxidation of L-arginine to L-citrulline and NO. This process is a two-step reaction that leads to a five-electron oxidation of L-arginine. The enzyme requires NADPH and O2 as substrates for both reaction steps, and iron protoporphyrin IX (heme), FMN, FAD, and tetrahydrobiopterin (H4B) as protein-bound cofactors. NOS is active as a homodimer and contains an N-terminal oxygenase (or heme) domain, a C-terminal flavopro-tein reductase domain, and a central calmodulin binding region... 

Under aerobic and anaerobic conditions, several reduction reactions can be catalyzed by the intact P450 monooxygenase system or only by its flavopro-tein component, NADPH-P450 reductase. 

Frerman FE, Goodman SI. Defects of the electron transfer flavoprotein and electron transfer flavopro-tein-ubiquinone oxidoreductase glutaric acidemia type II. In Scriver CR, Beaudet AL, Valle D, Sly WS, Childs B, Kinzler KW, et al, eds. The metabofic molecular bases of inherited disease, 8th ed. New York McGraw-Hill, 2001 2357-65. 

Some of the catalytic and structural properties of thioredoxin reductase as they relate to analogous properties of lipoamide dehydrogenase and glutathione reductase have been covered in Section II. The flavopro-tein, thioredoxin reductase, catalyzes the electron transfer from NADPH to thioredoxin, a protein of 12,000 molecular weight containing a single disulfide. The reductase has a reactive disulfide in addition to FAD. Thus, electron flow is from NADPH to the FAD-disulfide system of thioredoxin reductase, to the disulfide of thioredoxin, and finally to a variety of acceptor systems. 

The two-electron reduction can be catalyzed by carbonyl reductase and quinone reductase, whereas cytochrome P450 and some flavopro-teins act by single-electron transfers. The nonenzymatic reduction of quinones can occur, for example, in the presence of or some thiols such as glutathione. 

Figure 3-7. Sequence alignment of various enzymes in the flavopro-tein disulfide oxidoreductase family. The sequences of the NADP4-dependent enzymes are the glutathione reductase from E. coli (E-GR), human (H-GR), Pseudomonas aeruginosa (P-GR), mercuric reductase from Staphylococcus aureus (S-MR), P. aeruginosa Tn 501 (P-GR), and trypanothione reductase from Trypanosoma congolense (T-TR). The NAD+-dependent enzymes are dihydrolipoamide dehydrogenase from E. coli (E-DD), B. stearothermophilus (B-DD), yeast (Y-DD), and human (H-DD). Residue positions marked with an asterisk correspond to those that were targets of site-directed mutagenesis in the text.

Fig. 20.6. Succinate dehydrogenase contains covalently bound FAD. As a consequence, succinate dehydrogenase and similar flavopro-teins reside in the inner mitochondrial membrane where they can directly transfer elechons into the electron transport chain. The elechons are hansferred from the covalently bound FAD to an Fe-S complex on the enzyme, and then to coenzyme Q in the electron hansport chain (see Chapter 21). Thus, FAD does not have to dissociate from the enzyme to transfer its electrons. All the other enzymes of the TCA cycle are found in the mitochondrial mahix.

In addition to NADH dehydrogenase, succinic dehydrogenase and other flavopro-teins in the inner mitochondrial membrane also pass electrons to CoQ (see Fig. 21.5). Succinate dehydrogenase is part of the TCA cycle. ETF-CoQ oxidore-ductase accepts electrons from ETF (electron transferring flavoprotein), which acquires them from fatty acid oxidation and other pathways. Both of these flavo-proteins have Fe-S centers. a-Glycerophosphate dehydrogenase is a flavoprotein that is part of a shuttle for reoxidizing cytosolic NADH. 

Massey, V., Palmer, G. (1966). On the existence of spectrally distinct classes of flavopro-tein semiquinones. A new method for the quantitative production of flavoprotein semiquionones, Biochemistry, 5.- 3181. 

The enzyme is a cytochrome P-450 monooxygenase, which is reflected in its properties. It is membrane-bound, is dependent on NADPH and molecular oxygen, and displays a light-reversible inhibition by carbon monoxide. Cytochrome P-450 enzymes are dependent on a membrane-bound reductase (NADPH cytochrome P-450 reductase, EC 1.6.2.4), a flavopro-tein that is involved in the electron transfer from NADPH to the cytochrome P-450 heme group. 

Enzymes are proteins with catalytic activity, and many are holoenzymes, which are conjugated proteins, such as glyco-, hemo-, metallo-, and flavopro-teins. Enzyme-catalyzed metabolic reactions take place in complexes of the enzyme with the reactants (substrates). The concentration of such complexes at any moment is dependent on the concentrations of... 

Steinbacher S, Stumpf M, Weinkauf S, Rohdich F, Bacher A, Simon H (2002) Enoate reductase family. In Chapman SK, Perham RN, Scmthm NS (eds) Flavins and Flavopro-teins. Weber, p 941... 

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