Oxidized flavins, structures

The structures of EX and El were deduced by resolution of El into apoenzyme and free flavin-substrate adduct. The structure of this adduct was determined as 5-cyanoethy 1-1,5-dihydro FAD and that of EX was deduced to be a cationic imine resulting from elimination of NO2" from the initial 5-nitroethy 1-1,5-dihydro FAD adduct formed in the process controlled by k2 by nucleophilic attack of nitroethane carbanion on the position of oxidized flavin. The chemistry of flavin reduction by nitroethane carbanion at the active site of D-amino acid oxidase is given by the following scheme (Equation 19) in which the kinetically important... 

Several of the reductases mentioned here belong to the same structural family (the FNR family), and they are mechanistically related to each other (9). A two-electron reduction of the flavin by NAD(P)H in these enzymes typically involves the transient formation of an oxidized flavin-reduced pyridine nucleotide charge-transfer complex, which is followed by hydride transfer. After reduction, the flavin can transfer its electrons to different redox partners. With NADH cytochrome b5 reductase, this transfer occurs in separate single-electron transfer steps. With... 

Fig. 4.80. Structure of the activated C(4a)-(hyclro)peroxyflavin formed upon 2-electron reduction of the oxidized flavin and its subsequent reaction with molecular oxygen.

In the MAO-catalyzed reaction, intermediate structure 10 is converted directly to structure 12. As shown in figure 2, the electron acceptor for the cytochrome P450-catalyzed reaction is the perferryl 0X0 species (Fe O), while the electron acceptor for the MAO-catalyzed reaction is the oxidized flavin moiety (FAD). 

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...

Oxidation of P-nicotinamide adenine dinucleotide (NADH) to NAD+ has attracted much interest from the viewpoint of its role in biosensors reactions. It has been reported that several quinone derivatives and polymerized redox dyes, such as phenoxazine and phenothiazine derivatives, possess catalytic activities for the oxidation of NADH and have been used for dehydrogenase biosensors development [1, 2]. Flavins (contain in chemical structure isoalloxazine ring) are the prosthetic groups responsible for NAD+/NADH conversion in the active sites of some dehydrogenase enzymes. Upon the electropolymerization of flavin derivatives, the effective catalysts of NAD+/NADH regeneration, which mimic the NADH-dehydrogenase activity, would be synthesized [3]. 

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. 

Trivially, photo-excitation will drastically enhance the oxidation potential of the flavin chromophore and might give rise to a great variety of reversible chemical reactions, depending on the structure of the environment and/or the pathway of potential e - as well as H -conductivity. It must be emphasized, that the oxidative action of the flavin triplet Tj is by no means confined to 1 e -uptake from suitable aromatic... 

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