Flavoproteins reactions catalyzed

Glutamate synthase (ferredoxin) [EC 1.4.7.1], also known as ferredoxin-dependent glutamate synthase, is an iron-sulfur flavoprotein, that catalyzes the reaction of L-glutamine with a-ketoglutarate (or, 2-oxoglutarate) and two reduced ferredoxin molecules to produce two... 

Glycerol-3-phosphate dehydrogenase [EC 1.1.99.5] is a flavoprotein that catalyzes the reaction of sn-glycerol 3-phosphate with an acceptor substrate to produce glycerone phosphate (dihydroxyacetone phosphate) and the reduced acceptor. 

The biological function of Factor-420 is to catalyze the electron transport between hydrogen and pyridine nucleotide in Methanobacteria (anaerob). These Arckaebacteria are obligate anaerobes. In this context some proposals have been put forward with respect to the evolution of biological reactions catalyzed by flavoproteins... 

In the overall reaction catalyzed by the mitochondrial respiratory chain, electrons move from NADH, succinate, or some other primary electron donor through flavoproteins, ubiquinone, iron-sulfur proteins, and cytochromes, and finally to 02. A look at the methods used to determine the sequence in which the carriers act is instructive, as the same general approaches have been used to study other electron-transfer chains, such as those of chloroplasts. 

The reaction catalyzed by the first of these is illustrated in Table 15-2 (reaction type F). The other two enzymes usually promote the reverse type of reaction, the reduction of a disulfide to two SH groups by NADPH (Eq. 15-22). Glutathione reductase splits its substrate into two halves while reduction of the small 12-kDa protein thioredoxin (Box 15-C) simply opens a loop in its peptide chain. The reduction of lipoic acid opens the small disulfide-containing 5-membered ring in that molecule. Each of these flavoproteins also contains within its structure a reducible disulfide group that participates in catalysis. 

The other classes of flavoproteins in table 10.2 interact with molecular oxygen either as the electron-acceptor substrates in redox reactions catalyzed by oxidases or as the substrate sources of oxygen atoms for oxygenases. Molecular oxygen also serves as an electron acceptor and source of oxygen for metalloflavoproteins and dioxygenases, which are not listed in the table. These enzymes catalyze more complex reactions, involving catalytic redox components, such as metal ions and metal-sulfur clusters in addition to flavin coenzymes. 

Fig. 7.4 Reactions associated with the thioredoxin system. Thioredoxin is a redox-regulating protein with a redox-active disulfide/dithiol within the conserved active site sequence -Cys-Gly-pro-Cys-. Thioredoxin reductase, a 55 kDa flavoprotein that catalyzes the NADPH-dependent reduction of thioredoxin (1) and thioredoxin oxidase (2), a flavin-dependent sulfhydry 1 oxidase that catalyzes the oxidative protein folding with the generation of disulfides...

Microsomes contain, in addition to the two cytochrome reductases just discussed, a flavoprotein which catalyzes the mixed function oxidation of secondary and tertiary amines to the hydroxylamines and amine oxides, respectively (333, 334). This flavoprotein, which contains about 2 moles of phospholipid and 1 mole of FAD per 70,000 g of protein, is specific for NADPH (333, 334) The enzyme is also able to catalyze the further oxidation of the hydroxylamines to nitrones (336). The reactions... 

Flavoprotein monooxygenases mainly use NAD(P)H as electron donor and insert one atom of molecular oxygen into then-substrates. Oxygen activation of flavoprotein monooxygenases involves the (transient) stabilization of a flavin C4a-(hydro) peroxide. This species performs either a nucleophilic or electrophilic attack on the substrate (Fig. 6). Oxygenation reactions catalyzed by flavoprotein monooxygenases include hydroxyla-tions, epoxidations, Baeyer-Villiger oxidations, and sulfoxida-tions (43). 

Manganese is required by mitochondrial superoxide dismutase. This enzyme catalyzes the same reaction catalyzed by the cytosolic form of the enzyme, namely the conversion of superoxide to hydrogen peroxide. Superoxide is a molecule of oxygen containing one additional electron. It is produced as a byproduct of various reactions in which molecular oxygen (O ) is involved. These reactions may include those of cytochrome c oxidase and various flavoproteins- The HOOH formed by superoxide is decomposed by catalase ... 

The hydrogens are accepted by FAD, which is covalently bound to the apoprotein via a histidine residue. In many flavoproteins, the flavin nucleotide is bound to the apoprotein not covalently but rather via ionic linkages with the phosphate group. The reducing equivalents of FADH2 are passed on to coenzyme Q (CoQ or Q) via the iron-sulfur centers. Thus, the overall reaction catalyzed by complex II is... 

In this review, the simple flavoenzymes will be discussed in groups according to the chemistry of a halfreaction - usually the biologically important half-reaction. This is different from the usual classifications based on the net reactions catalyzed or structural motifs. In addition to these enzymes, there are several flavoproteins that do not fit easily into any chemical group. These enzymes will be discussed at the end and include examples where more than one type of chemical conversion occurs, the enzyme contains additional prosthetic groups such as metal centers, or there is no evidence for the flavin being involved chemically in the reaction. 

Fig. 60. The respiratory chain of higher plants. Ubiquinone appears to serve as an electron reservoir. = probable site of ATP formation. SD = succinate dehydrogenase. It used to be assumed that, with the exception of the reaction catalyzed by SD, the hydrogen acceptor in dehydrogenation reactions was NAD+ and that the hydrogen then entered the respiratory chain in the form of NADH+H+. In reality the situation is more complicated since the lipoic acid oxidizing flavoproteid of the pyruvate dehydrogenase and the a-ketoglutarate dehydrogenase complexes—in both cases the same flavoproteid is involved—can establish direct contact with the flavoproteins of the respiratory chain just like succinate dehydrogenase. associated with encircled flavoproteins means that ATP can be formed as a result of transitions between the various flavoproteins, except those involving SD.

Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD and NADP, one- or two-elec-... 

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