FadH protein

Unsaturated fatty acids can also be degraded by the 3-oxidation pathway. The FadB protein possesses cw-P-enoyl-CoA isomerase activity, which converts cis-3 double bonds to trans-2 (Fig. 8). A 2,4-dienoyl-CoA reductase encoded by fadH is also required for the metabolism of polyunsaturated fatty acids (Fig. 8). This protein is a 73-kDa monomeric, NADP" -dependent, 4Fe-4S flavoprotein. The FadH protein can utilize compounds with either cis or trans double bonds at the 4-position. An epimerase activity of FadB allows for the utilization of D-hydroxy fatty acids. The epimerase is actually a combination of a Z)-P-hydroxyacyl-CoA dehydratase and the crotonase (hydratase) activities, resulting in the conversion of the d to the L enantiomer (Fig. 8). 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

For example, acyl-CoA dehydrogenases are thought to form an anionic species of reduced FAD (FADH ) which is tightly bound to the protein.149... 

The purified E. coli protein has a molecular weight of 49 kD. It does not require any divalent cation for activity. It contains two different noncovalently bound chromophores that absorb light. One chromophore is flavin adenine dinucleotide (FADH- or FADH2). The other is 5,10-methenyltetrahydrofolyl polyglutamate (MTHF). The absorption of light by the chromophores is essential for the enzymatic reversal of the pyrimidine dimer back to the original pyrimidine monomers. However,... 

An average of one mediator was boimd per f 2-75 kDa of enzyme. The mediators used have redox potentials 0.07 V to 0.55 V positive of the FAD/FADH enzyme s redox potential (—0.05 V relative to the NHE at pH 7), but those which have a redox potential more negative than glucose oxidase enzyme do not mediate electron transfer. Electrons can be relayed both by tunneling and by motion of the mediator in and out of the protein chains. For distances >8 A, tunneling rates decrease ex-... 

Electron flow within the protein proceeds from the reducing substrate (NADPH) to FAD to FMN to heme. The initial reduction forms fully reduced FAD (FADH2), which then transfers one-electron to FMN forming the neutral FAD semiquinone (FADH ) and the anionic FMN semiquinone (FMN ). When the fatty acid substrate is bound within a long hydrophobic channel adjacent to the distal face of the heme, it induces a low-spin to high-spin change and an increase in the heme potential [93]. This probably involves displacement of the water molecule in the sixth coordination position [100] and a decrease in the local dielectric con-... 

The vaiue given is for free FAD/FADH. The Ei of the protein-bound coenzyme varies. 

Special electron carriers ferry the electrons from one complex to the next. Electrons are carried from NADH-Q oxidoreductase to Q-cytochrome c oxidoreductase, the second complex ot the chain, by the reduced form of coeti2 ymt2 Q (Q), also known as ubiquinone because it is a ubiquitous quinone in biological systems. Ubiquinone is a hydrophobic quinone that diffuses rapidly within the inner mitochondrial membrane. Cytochrome c. a small soluble protein, shuttles electrons from Q-cytochrome c oxidoreductase to cytochrome c oxidase, the final component in the chain and the one that catalyses the reduction of Oi. Electrons from the FADH generated bv... 

Recall that FADH i is formed in the citric acid cycle, in the oxidation of sue cinate to fumarate by succinate dehydrogenase (p. 487). This enzyme is of the swccineite-Q reductase complex (Complex 11), an integral membrane protein of the inner mitochondrial membrane. FADH does not leave the complex. Rather, its electrons are transferred to Fe-S centers and then toQ for entry into the electron-transport chain. The succinate-Qreductase complex, in contrast with NADFI-Q oxidoreductase, does not transport protons. Consequently, less ATP is formed from the oxidation of FADH than from NADH. 

Oxidative phosphorylation The process by which adenosine triphosphate (ATP) is synthesized from a hydrogen ion gradient across the mitochondrial inner membrane. The hydrogen ion gradient is formed by the action of protein complexes in the mitochondrial membrane that sequentially transfer electrons from the rednced cofactors nicotinamide adenine dinucleotide (NADH) and FADH to molecnlar oxygen. Movement of hydrogen ions back into the mitochondrion via ATP synthase drives the synthesis of ATP. 

In eukaryotic cells aerobic metabolism occurs within the mitochondrion. Acetyl-CoA, the oxidation product of pyruvate, fatty acids, and certain amino acids (not shown), is oxidized by the reactions of the citric acid cycle within the mitochondrial matrix. The principal products of the cycle are the reduced coenzymes NADH and FADH, and C02. The high-energy electrons of NADH and FADH2 are subsequently donated to the electron transport chain (ETC), a series of electron carriers in the inner membrane. The terminal electron acceptor for the ETC is 02. The energy derived from the electron transport mechanism drives ATP synthesis by creating a proton gradient across the inner membrane. The large folded surface of the inner membrane is studded with ETC complexes, numerous types of transport proteins, and ATP synthase, the enzyme complex responsible for ATP synthesis. 

Pulse radiolysis was also used to elucidate the mechanism of catalytic action of monodehydroascorbate reductase, an enzyme containing FAD and using Nicotinaminde atjenine dinucleotide (NADH) as reductant. The substrate is dehydroascorbate radical produced by pulse radiolysis (130). The authors show that this radical reacts with the protein to give the FADH radical and that the... 

Fig. 4. Structure of Escherichia coli photolyase. (A) Ribbon diagram representation. The MTHF antenna is exposed on the surface, whereas the FADH catalytic cofactor is buried within the core of the a-helical domain. (B) Surface potential representation. Blue, basic residues red, acidic residues white, hydrophobic residues. Note the positively charged groove running diagonally the length of the protein and the hole (marked by a square) with asymmetric charge distribution along the side walls and leading to the flavin located in the bottom. (See Color Insert.)...

Spectral fittings to the experimental data were made with the non-linear least-squares method to obtain the principal g-values gx = 2.00429(3), gy = 2.00389(3), gz =2.00216(3) yielding giso= 2.00345(3). These values differed only slightly between FAD and the protonated FADH in several investigated protein-bound... 

Both the semiquinone and the fully reduced forms can exist free in solution as either neutral or anionic forms with p values of 8.5 and 6.5, respectively. Both semiquinones of POR are found as the blue, neutral form in the pH range 6.5-S.5. In this review, the fully reduced forms are referred to as FMNH2 and FADH2. However, the protonation states of the fully reduced forms in POR are unknown. Those of the homologous proteins, FNR and flavodoxin, are anionic and it should be kept in mind that the fully reduced flavins in POR may also be in the anionic forms, FADH- and FMNH-. 

After the T S transition in the ion-radical pair (O Fig. 29-2b), an ordinary chemical transformation on the singlet state PES occurs (O Fig. 29-2c). It involves abstraction of hydrogen atom from the N1 atom of FADHj and a proton abstraction from the nearest histidine residue in order to create H2 O2 by reduction of the superoxide anion. This process, accompanied by the formation of hydrogen peroxide, can occur only in the singlet state. The final phase of the catalytic cycle (not shown in O Fig. 29-2) consists of a subsequent proton transfer from FADH ion back to histidine across the system of H-bonds in water-protein chain (Prabhakar et al. 

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