Flavin involvement

In bacterial NADHrFMN oxidoreductase [15], the flavin coenzyme is bound relatively weakly. This enzyme serves to provide reduced FMN to luciferase in 

At the active site of beef heart mitochondrial succinate dehydrogenase (EC 1.3.99.1), the FAD is covalently linked by C-8a to nitrogen of a histidine residue (Fig. 2b) [20]. In the catalytic reaction, removal of a proton from C-3 of the succinate may be followed by attack of the 3-carbanion on N-5 of the FAD to form an intermediate adduct, which breaks down with loss of a proton from C-2 of the succinate, giving fumarate and a reduced FAD moiety [21]. This mechanism is not certain, but it is established that the succinate loses two non-equivalent hydrogen atoms by a trans elimination (Fig. 4) [22], In other enzymes, different types of covalent attachment of the FAD are known [23]. 

The flavodoxins, which resemble the larger flavoprotein dehydrogenases [24], are well-characterized flavoproteins [25] in which oxidation involves formation of a semiquinone radical (Fig. 3b) [26], 

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The evidence for flavin involvement as deduced from action, absorption, and fluorescence spectra, as well as via non-spectroscopic methods, is then evaluated. It is concluded that all experimental results indirectly support the flavin hypothesis, but that direct proof will have to await the isolation and in vitro characterization of the chromophore. 

Pyridine nucleotide-dependent flavoenzyme catalyzed reactions are known for the external monooxygenase and the disulfide oxidoreductases However, no evidence for the direct participation of the flavin semiquinone as an intermediate in catalysis has been found in these systems. In contrast, flavin semiquinones are necessary intermediates in those pyridine nucleotide-dependent enzymes in which electron transfer from the flavin involves an obligate 1-electron acceptor such as a heme or an iron-sulfur center. Examples of such enzymes include NADPH-cytochrome P4S0 reductase, NADH-cytochrome bs reductase, ferredoxin — NADP reductase, adrenodoxin reductase as well as more complex enzymes such as the mitochondrial NADH dehydrogenase and xanthine dehydrogenase. 

The stability of the radical species FMNH and FADH is the source of the most dramatic distinctions between the nicotinamide cofactors and the flavins. This stability makes possible a number of reactions for flavins involving singleelectron transfers that are rendered essentially impossible by the high energy of the corresponding nicotinamide radical. The source of the relative stability of the... 

Biological activity of flavin enzymes is determined by the capacity of their prosthetic groups— flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN)—to take part in the reversible process of oxidation-reduction. They perform the role of electron carriers from the substrate to either other carriers or to oxygen. The function of flavin involves two main aspects activation of CH bonds and uncoupling of electrons. Figure 4 shows a flavin redox system. In enzymes, flavins are often coordinated with Mo, Fe, or Co ions. 

Riboflavin can be assayed by chemical, en2ymatic, and microbiological methods. The most commonly used chemical method is fluorometry, which involves the measurement of intense yeUow-green fluorescence with a maximum at 565 nm in neutral aqueous solutions. The fluorometric deterrninations of flavins can be carried out by measuring the intensity of either the natural fluorescence of flavins or the fluorescence of lumiflavin formed by the irradiation of flavin in alkaline solution (68). The later development of a laser—fluorescence technique has extended the limits of detection for riboflavin by two orders of magnitude (69,70). 

Anhydrotetracycline oxygenase from Streptomjces aureofaciens which cataly2es the conversion of anhydrotetracycline to dehydrotetracycline, has been isolated and characterized as a flavin-dependent oxygenase (83). It consists of two subunits of mol wt = 57, 500 based on SDS/polyacrylamide—gel electrophoresis. The cosynthetic factor 1 of Streptomjces aureofaciens involved in the reduction of 5a,lla-dehydrochlortetracycline to chlortetracycline, has been identified as 7,8-didemethyl-8-hydroxy-5-deazariboflavin. This work was aided by comparison of spectral data with that of an authentic sample obtained from the hydrolysis of coenzyme F-420 (84). 

Oxidative substitutions at ring junction positions in various tetrahydro-5-deaza-pterins (79JA6068) and -flavins (77JA6721) have been studied, e.g. to give (13), and the oxidation-reduction reactions of 5-deazaflavins (e.g. 78CL1177, 80CPB3514) across the 1,5-positions, e.g. (19) (20), are involved in their co-enzymic role in enzymic oxidations (see Section... 

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. 

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

This thiol-disulfide interconversion is a key part of numerous biological processes. WeTJ see in Chapter 26, for instance, that disulfide formation is involved in defining the structure and three-dimensional conformations of proteins, where disulfide "bridges" often form cross-links between q steine amino acid units in the protein chains. Disulfide formation is also involved in the process by which cells protect themselves from oxidative degradation. A cellular component called glutathione removes potentially harmful oxidants and is itself oxidized to glutathione disulfide in the process. Reduction back to the thiol requires the coenzyme flavin adenine dinucleotide (reduced), abbreviated FADH2. 

Lanosterol biosynthesis begins with the selective conversion of squalene to its epoxide, (35)-2,3-oxidosqualene/ catalyzed by squalene epoxidase. Molecular 02 provides the source of the epoxide oxygen atom, and NADPH is required, along with a flavin coenzyme. The proposed mechanism involves... 

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. 

Ghisla, S., Entsch, H., Massey, V., and Husein, M. (1977). On the structure of flavin-oxygen intermediates involved in enzymic reactions. Eur. J. Biochem. 76 139-148. 

An additional component is the iron-sulfur protein (FeS nonheme iron) (Figure 12-6). It is associated with the flavoproteins (metallofiavoproteins) and with cytochrome b. The sulfur and iron are thought to take part in the oxidoreduction mechanism between flavin and Q, which involves only a single e change, the iron atom undergoing oxidoreduction between Fe " and Fe k... 

Flavins — Riboflavin is first of all essential as a vitamin for humans and animals. FAD and FMN are coenzymes for more than 150 enzymes. Most of them catalyze redox processes involving transfers of one or two electrons. In addition to these well known and documented functions, FAD is a co-factor of photolyases, enzymes that repair UV-induced lesions of DNA, acting as photoreactivating enzymes that use the blue light as an energy source to initiate the reaction. The active form of FAD in photolyases is their two-electron reduced form, and it is essential for binding to DNA and for catalysis. Photolyases contain a second co-factor, either 8-hydroxy-7,8-didemethyl-5-deazariboflavin or methenyltetrahydrofolate. ... 

The anaerobic degradation of some hydroxybenzoates and phenols involves reductive removal of the phenolic hydroxyl group. The enzyme that dehydroxylates 4-hydroxybenzoyl-CoA in Thauera aromatica is a molybdenum-flavin-iron-sulfur protein (Breese and Fuchs 1998), and is similar to the enzyme from the nonsulfur phototroph Rhodopseudomonas palustris that carries out the same reaction (Gibson et al. 1997). 

Hartmans S, MJ van der Werf, JAM de Bont (1990) Bacterial degradation of styrene involving a novel flavin adenenine dinucleotide-dependent styrene monooxygenase. Appl Environ Microbiol 41 1045-1054. 

Xun L, ER Sandvik (2000) Characterization of 4-hydroxyphenylactate 3-hydroxylase (HpaB) of Escherichia coli as a reduced flavin adenine dinucleotide-utilizing monooxygenase. Appl Environ Microbiol 66 481-486. Zaar A, J Gescher, W Eisenreich, A Bacher, G Fuchs (2004) New enzymes involved in aerobic benzoate metabolism m Azoarcus evansii. Mol Microbiol 54 223-238. 

Matsubara T, T Ohshiro, Y Nishina, Y Izumi (2001) Purification, characterization, and overexpression of flavin reductase involved in dibenzothiophene desulfurization by Rhodococcus erythropolis D-1. Appl Environ Microbiol 67 1179-1184. 

The release of iron from intracellular ferritin stores is thought to involve the reduction of Fe to Fe " (Funk et al., 1985) and one would expect this reduction to be fecilitated by the low oxygen tension, increased levels of reducing species and the low pH shown by nuclear magnetic resonance (NM to be as low as 6.9 after only 6 h of cold storage (Fuller et al., 1988). Exogenous redox-active quinones such as adriamycin have been shown to catalyse lipid peroxidation in the presence of ferritin under hypoxic conditions (Vile and Winterbourne, 1988), and lipid peroxidation is stimulated in micro-somes in the presence of purified ferritin and flavin... 

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