Flavin enzymes

Glucose oxidase (GOD) is a typical flavin enzyme with flavin adenine dinucleotide (FAD) as redox prosthetic group. Its biological function is to catalyze glucose to form gluconolaction, while the enzyme itself is turned from GOD(FAD) to GOD(FADH2). GOD was used to prepare biosensors in extensive fields. Many materials that can be used to immobilize other proteins can be suitable for GOD. GOD adsorbed on CdS nanoparticles maintained its bioactivity and structure, and could electrocatalyze... 

Since long retention times are often applied in the anaerobic phase of the SBR, it can be concluded that reduction of many azo dyes is a relatively a slow process. Reactor studies indicate that, however, by using redox mediators, which are compounds that accelerate electron transfer from a primary electron donor (co-substrate) to a terminal electron acceptor (azo dye), azo dye reduction can be increased [39,40]. By this way, higher decolorization rates can be achieved in SBRs operated with a low hydraulic retention time [41,42]. Flavin enzyme cofactors, such as flavin adenide dinucleotide, flavin adenide mononucleotide, and riboflavin, as well as several quinone compounds, such as anthraquinone-2,6-disulfonate, anthraquinone-2,6-disulfonate, and lawsone, have been found as redox mediators [43—46]. 

How the NOS isoforms compare to these related dual flavin enzymes is a matter of ongoing investigation. Characterization of the neuronal NOS revealed that it normally exists in its one-electron reduced form and maintains an air-stable, flavin semiquinone radical (Stuehr and Ikeda-Saito, 1992), as seen for NADPH-cytochrome P450 reductase. It is unknown which flavin in NOS contains the odd electron, although precedent argues that it probably resides on... 

Flavin redox states in a dual flavin enzyme. (Left) Single-electron reduction of the isoalloxazine ring generates the semiquinone radical, while reduction by two electrons generates the fully reduced species. (Right) Five possible oxidation levels of a dual flavin enzyme, where the FMN reduction potential is held at a more positive value relative U) FAD. The flavins can theoretically accept a maximum of four electrons obtained from two NADPH. However, in NADPH-cytochrome P450, reductase, full reduction of the flavins is not normally reached when NADPH serves as the reductant. 

Components of the electron transport chain in bacteria have been shown to include b- and c-type cytochromes, ubiquinone (fat-soluble substitute quinone, also found in mitochondria), ferredox (an enzyme containing nonheme iron, bound to sulfide, and having the lowest potential of any known electron-canying enzyme) and one or more flavin enzymes. Of these a cytochrome (in some bacteria, with absorption maximum at 423.5 micrometers, probably Cj) has been shown to be closely associated with the initial photoact. Some investigators were able to demonstrate, in chromatium, the oxidation of the cytochrome at liquid nitrogen temperatures, due to illumination of the chlorophyll. At the very least this implies that the two are bound very closely and no collisions are needed for electron transfers to occur. 

FAD ready for a subsequent catalytic cycle. This enzyme catalyzed Baeyer-Villiger oxidation bears great resemblance to the analogous chemical reaction performed by peroxides or peracids, which act as nucleophiles. Globally these flavin-enzymes can perform the same reactions as peracids, i.e. epoxidations, Baeyer-Villiger-reactions and nucleophilic heteroatom oxidation [26-28]. 

Riboflavin is absorbed by the gut and enters the bloodstream, where close to half of it is loosely bound to serum albumin. The main site of absorption is the ileum. As might be expected, when large doses (20- 60 mg) of riboflavin are eaten, much of the dose is promptly excreted in the urine (Zempleni et ai, 1996). The FAD CO factors based on riboflavin are called flavins enzymes using flavins as a cofactor are called flavopioteins. 

Burch, H. B., Lowry, O H., Padilla, A. M and Coombs, A M.n956). Effects of a riboHavin deficiency and realimentation on flavin enzymes of tissues. /, Sioi Cheni. 223,29-45. 

Figure 7.10 (a) Structure of flavin and of FAD. (b) Oxidative half-reaction of flavin enzymes, (c) Half-reduced flavin in two ionisation states os... 

The first study was reported by Matsue and coworkers (41) and aimed at creating micropattems of diaphorase—a flavin enzyme on a glass sub-... 

We have observed three types of effects on the structures of enzymes as the temperature is lowered in cryosolvents (1) no apparent change in the protein conformation, with the possible exception of decreased mobility of the surface side chains (2) conformational transitions, usually marked by little effect on the catalytic properties and (3) increased association of subunits. In most cases no detectable effects of decreasing temperature on the enzyme s structure have been detected by such procedures as monitoring the intrinsic fluorescence (16), or intrinsic visible absorbance in the case of flavin enzymes (Fink and... 

Figure 2-2 illustrates the irreversible inhibition of the flavine-enzyme monamine oxidase (MAO) by the antihypertensive agent pargyline. This enzyme is important in the catabolism of catechol- and other biogenic amines, including epinephrine and norepinephrine to their corresponding aldehydes. Equation 2.8 illustrates the oxidation of monamines by way of an imino intermediate while the oxidized flavine prosthetic group FAD is simultaneously reduced. 

No active substance could be detected in plants treated with carboxin six weeks after the treatment. The major part of the residue found was sulfoxide. Actually, sulfoxide can occur in the plant in two ways. On the one hand, carboxin is oxidised relatively rapidly in the soil, and the plant takes up its sulfoxide, and, on the other hand, carboxin is metabolised within the plant to sulfoxide, presumably by enzyme systems producing hydrogen peroxide, such as riboflavin or flavin enzymes (Lyr et al., 1975a). It has been proved by extraction with hot dimethyl sulfate that sulfoxide formed in the plant is gradually bound in the form of a water-insoluble complex to lignin and is thus detoxified. No hydrolysis of carboxin in the plant has been observed (Chin et al., 1973). 

USE As an electron carrier in place of the flavine enzyme of Warburg in the hexosemonophosphate system Dickens, foe. oil. In the prepn of succinic dehydrogenase Green et of.. J. Biot Chem. 217, 551 (1955). 

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