Succinate flavin

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. 

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. 

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). 

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),... 

Such a process is supposed to occur within the limits of Q-cycle mechanism (Figure 23.2). In accord with this scheme ubihydroquinone reduced dioxygen in Complex III, while superoxide producers in Complex I could be FMN or the FeS center [12]. Zhang et al. [24] also suggested that the Q-cycle mechanism is responsible for the superoxide production by the succinate-cytochrome c reductase in bovine heart mitochondria and that FAD of succinate dehydrogenase is another producer of superoxide. Young et al. [25] concluded that, in addition to Complex III, flavin-containing enzymes and FeS centers are also the sites of superoxide production in liver mitochondria. 

Studies of protein film electrocatalysis have also been illuminating. For example, succinate dehydrogenase displays an unusual optimal potential for activity. The enzyme contains four redox sites a flavin, a... 

The oxygen reactivity of flavohydroquinone bound to apoflavoprotein dehydrogenases can vary considerably from fast (flavodoxins), moderate (xanthine oxidase) to nil (succinate dehydrogenase) Most, but not all, flavoprotein dehydrogenases contain one or more types of metal prosthetic groups, e.g. xanthine oxidase contains also Fe and Mo. Since these metal ions are involved in electron flux, their possible participation in the reaction with O2 cannot be excluded. Much evidence, however, indicates that the flavin is involved in the one-electron reduction of Oj, as shown in Equation (5). 

The covalent 8a-N(3)-histidyl FAD of mitochondrial succinate dehydrogenase functions as a two-electron acceptor in the oxidation of succinate to fumarate and as a one-electron donor in the reduction of the iron-sulfur centers of the enzyme. Recent ESR spectroscopic data have shown the covalent flavin semiquinone... 

Ackrell, B. A. C., McIntire, W., Edmondson, D. E., Kearney, E. B. The semiquinone form of succinate dehydrogenase and other Sa-substituted flavoenzymes. In Flavins and flavoproteins (Massey, V., Williams, C. H. eds.) pp. 488-491, New York, Amsterdam, Oxford, Elsevier/North Holland 1982... 

Acyl-CoA dehydrogenases (Flavin) Succinic dehydrogenase (Flavin) Opposite enoyl reductase (NADPH)... 

Mitochondrial succinate dehydrogenase, which catalyzes the reaction of Eq. 15-21, contains a flavin prosthetic group that is covalently attached to a histidine side chain. This modified FAD was isolated and identified as 8a-(Ne2-histidyl)-FAD 219 The same prosthetic group has also been found in several other dehydrogenases.220 It was the first identified member of a series of modified FAD or riboflavin 5 -phosphate derivatives that are attached by covalent bonds to the active sites of more than 20 different enzymes.219... 

The flavin of NAD dehydrogenase was an obvious candidate for a carrier, as was ubiquinone. However, the third loop presented a problem. Mitchell s solution was the previously discussed Q cycle, which is shown in Fig. 18-9. This accomplishes the pumping in complex III of 2 H+/ e, the equivalent of two loops.111 However, as we have seen, the magnitude of Ap suggests that 4 H+, rather than 2 H+, may be coupled to synthesis of one ATP. If this is true, mitochondria must pump 12 H+/ O rather than six when dehydrogenating NADH, or eight H+/ O when dehydrogenating succinate. 

The dehydrogenases that remove electrons from succinate or NADH contain flavins as prosthetic groups. NADH... 

With the help of spectrophotometric techniques, Britton Chance and others showed that any of the normal respiratory substrates can reduce all of the cytochromes in the membrane. Either succinate or NADH-linked substrates also can reduce all of the UQ. The flavins, however, are reduced only partially by succinate but almost completely by a combination of succinate and malate. These observations fit the idea that all of the cytochromes and UQ are part of a common network to which various flavoprotein dehy-drogenases can feed electrons. 

Electron transfer to 02 occurs stepwise, through a series of flavoproteins, cytochromes (heme-proteins), iron-sulfur proteins, and a quinone. Most of the electron carriers are collected in four large complexes, which communicate via two mobile carriers— ubiquinone (UQ) and cytochrome c. Complex I transfers electrons from NADH to UQ, and complex II transfers electrons from succinate to UQ. Both of these complexes contain flavins and numerous iron-sulfur centers. Complex III, which contains three cyto-... 

Answer The flavin nucleotides, FMN and FAD, would not be synthesized. Because FAD is required by the citric acid cycle enzyme succinate dehydrogenase, flavin deficiency would strongly inhibit the cycle. 

Electrons from succinate, glycerol 3-phosphate, and the flavin-dependent steps of fatty acid oxidation enter the chain in the following way ... 

The electron transferring non-heme iron proteins can be strictly differentiated from those non-heme iron proteins and polypeptides such as ferritin and ferrichrome which act in biological transfer and storage of iron. They can be distinguished also from iron-flavoproteins, such as succinic dehydrogenase, which contain flavin in addition to the iron constituent. Nevertheless, in many chemical and physical aspects, the non-heme iron moiety of the iron-flavoproteins exhibits behavior similar to that of electron-transferring non-heme iron proteins. 

Fumarase. The development and use of this immobilized enzyme by Tanabe Seiyaku for production of L-malic acid is very similar to that of aspartase ( 3). Lysed Brevibacterium ammoniagenes or B. flavin cells are treated with bile acid to destroy enzymatic activity which converts fumarate to succinate. As with aspartase, the cells can be immobilized in polyacrylamide or k-carrageenan gels. Using a substrate stream of 1 M sodium fumarate at pH 7.0 and 37°C, L-malic acid of high purity has been produced since 1974 by a continuous, automated process (3,39) for example, using a 1000-L fixed-bed bioreactor, 42.2 kg L-malic acid per hour was produced continuously for 6 months. 

Succinate dehydrogenase is the only enzyme of the citric acid cycle which is bound to the inner membrane of mitochondria. It is also one of three flavoproteins known in which flavin is covalently linked to the protein. The other two are monoamine oxidase of the outer membrane of liver mitochondria (138) and Chromatium cytochrome c-552 (139). 

Physical measurements support a molecular weight of approximately 200,000. This value is also in accord with gel exclusion studies on Sephadex G-200. Thus the enzyme contains 1 mole of flavin and 4 g-atoms of nonheme iron per mole.. . . The sedimentation velocity of the beef heart enzyme at 10-15 mg protein/ml is 6.5 S. . . This preparation could oxidize succinate in the presence of ferricyanide or phenazine methosulfate (PMS) as electron acceptor but was unable to transfer elec-to be unable to interact with the respiratory chain. 

Wang et al. 146) reported in 1956 the isolation of succinate dehydrogenase from heart muscle preparations treated with succinate and cyanide. The enzyme was extracted with 20% aqueous ethanol at pH 9.0. The final product, after adsorption on calcium phosphate gel and ammonium sulfate fractionation, was stated to be electrophoretically homogeneous and to contain 1 mole of flavin and 4 g-atoms of iron per 140,000-... 

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