Flavin flavins

The defenders of the carotenoid-photoreceptor-hypothesis have always understood the shape of these action spectra in the blue to mean that the bluelight receptor is a carotenoid. Indeed, in Fig. 6 3 it can be observed, that the three-peak absorption spectrum of trans-0-carotenoid (in hexene) agrees well with the observed action spectrum of the avena coleoptile (Fig. 3 5). However, there remains one loose end which has been the crucial point of controversy in this field, ever since Galston and Baker66 suggested in 1949 that the photoreceptor for phototropism might be a flavin Flavin absorbs in the near UV, /3-carotenoid does not. 

A comprehensive series of oxidation-reduction potential measurements have shown the FAD moiety to have the following one-electron couples PFl/PFIH = = —290 mV and PFIH 7PFIH2 = —365 mV while the FMN moiety exhibits the following PFl/PFl- = -110 mV and PFIH /PFIH = -270 mV. The FMN and FAD smiquinones were found to both be the neutral form as judged from absorption and ESR spectral data. The overlap of oxidized/semiquinone potential of the FAD moiety withkhe semiquinone/hydroquinone couple of the FMN moiety demonstrates the thermodynamic facilitation of flavin-flavin electron transfer via a one-electron mechanism. Stopped-flow kinetic data are also consistent with this view in... 

In the presence of H+, flavin semiquinone and superoxide yield peroxide and oxidized flavin flavin-H -F H+ -1- 02- flavin + H2O2... 

Flavin, flavine A dye prepared from quercitron bark. 

As for reduction processes, C02 free radicals were shown to react specifically with disulfide bonds (122). They were extensively used to study the redox properties of disulfide bonds, thiyl and disulfide free radicals in proteins. This is discussed in paragraph 5. However, they do react with thiol functions also (37). For proteins containing a prosthetic group, the reduction concerns also oxidized valencies of metals and flavins. Flavin adenine dinucleotide (FAD) or Flavin Mononucleotide (FMN). The proportion of reduced disulfide/reduced prosthetic group varies considerably with the protein. For instance, lipoamide dehydrogenase contains one disulfide bond close to a flavin (FAD). Free radicals can reduce only the flavin, although both are in the active site (123). In chicken egg white riboflavin binding protein, competitive formation of both disulfide and semireduced flavin is observed (124). 

Figure 11.9 Flavoenzyme catalyzed electron transfer and oxidation/oxygenation reactions The extensive conjugation of the isoaUoxazine ring system results in the yellow chromophore ( ax = 450 nm) in the oxidized flavin. Flavin semiquinones are stable radicals, because the unpaired electron is highly delocalized through the conjugated isoaUoxazine structure. The neutral semiquinone is blue = 570 nm) and the flavosemiquinone anion is red (A ax = 480 nm). The...

There are several lines of evidence from crystallographic studies, demonstrating that the two flavin domains are mobile. Superposition of the stmctures of wild-type and various point mutant stmctures of rat POR has shown that the relative orientation of, and distance between, the two flavin domains are variable, with the closest flavin-flavin distance ranging from 3.9 to... 

Peroxidases (POD) catalyze the oxidation of phenolics, whereas CDH catalyzes the reduction of quinones (which is generated electrochemically from diphenols). The prosthetic group of peroxidase is heme (22), whereas CDH contains two prosthetic groups one heme and one flavin (flavin adenine dinucleotide, FAD) (23-25), see Figure 2. The oxidation of sugars and the subsequent reduction of two-electron... 

The most usual choice for anodic enzyme has been glucose oxidase (GOx) [53], which, when under mediated ET conditions, can effectively electro-oxidize glucose. The enzyme carries a flavin (flavin adenine dinucleotide (FAD)) buried deep within the enzyme, which makes DET difficult. Although DET with GOx has been reported in many different studies [54-57], there is an ongoing debate as to whether true DET is achieved, or whether the observed bioelectrocatalytic currents are due to naturally mediated glucose oxidation by free FAD, which has diffused out firom the active centers of some partly denatured enzyme molecules. 

Furthermore, we have specified the flavin redox states by the terms flavoquinone, flavosemiquinone and flavohydroquinone in view of the polarographic reversibility, i.e. the quinonoid character, of the oxido-reduction. These terms are definitely superior to oxidized flavin, flavin radical, fully reduced flavin which are most widely used in biochemistry. In this context, we have tried to outline chemical evidence for the existence and potential biological relevance of several dihydroflavin isomers. The terms fully reduced and certainly leucoflavin should be dropped definitively in favour of dihydroflavin . Leucoflavin is in fact not leuco (i.e. colourless) but even more deeply (though, of course, less intensely) coloured (79) than oxidized flavin. Hence, this term should be abandoned for 1,5-dihydroflavin , which is identical with the product of the reversible reduction of flavoquinone, namely flavohydroquinone. 

Coemymes effecting transfer of hydrogen. These include the pyridine nucleotides, nicotinamide-adenine dinucleolide and nicotinamide-adenine dinucleolide phosphate the flavin nucleotides such as flavin-adenine dinucleotide and lipoic acid. 

FAD Flavin-adenine dinucleotide, fahl ore, CujSbSj. Tetrahedrite. 

The reduction of flavin in FAD is accompanied by loss of the characteristic yellow colour. The reduction-oxidation of flavo-proteins can thus be followed spectro-photomelrically. 

FMN See flavin mononucleotide, foamed plastics See cellular plastics. 

Figure 11.39 summarizes the reactions taking place in this amperometric sensor. FAD is the oxidized form of flavin adenine nucleotide (the active site of the enzyme glucose oxidase), and FAD1T2 is the active site s reduced form. Note that O2 serves as a mediator, carrying electrons to the electrode. Other mediators, such as Fe(CN)6 , can be used in place of O2. 

He/minthosporium (15). The mode of action is considered to be inhibition of the enzyme NADPH-cytochrome C reductase, which results in the generation of free radicals and/or peroxide derivatives of flavin which oxidize adjacent unsaturated fatty acids to dismpt membrane integrity (16) (see Enzyme inhibitors). 

Bacterial concentrations have also been determined by using the enzyme-catalyzed chemiluminescent reaction of reduced flavin mononucleotide (FMN) with oxygen and aldehydes. The detection limit was reported to be 10 ceUs of E. coli, which contains 7 x 10 g of FMN per ceU (303). 

An important advance with regard to light stabiUty was made with a group of yellow coumarin dyes with heterocycHc systems attached to the coumarin nucleus (4), eg, a greenish yellow cationic dye that is sold under the name Maxilon Brilliant Flavine 10 GFF [12221 -86-2] (Blue Wool 4), designated Cl Basic Yellow 40, available from several manufacturers. 

Paul Karrer chemistry research iato constitution of carotenoids, flavins, and vitamins A and B2... 

The chemistry of flavins, including several synthetic methods for the preparation of A/-D-ribityl-3,4-xyhdine (11) is reviewed in Reference 36. 

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). 

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