Flavin hydroperoxide intermediate

NADPH oxidase activity is minimized by stabilizing the flavin hydroperoxide intermediate rather than inhibiting flavin reduction in the absence of substrate. NADPH reduces FAD in the reductive half-reaction and NADP remains bound, shielding the hydroperoxide. The subsequent reaction with O2 is much more rapid than that of FMO, with rate constants close to 10 moH 1 s or higher. Transient kinetic studies strongly... 

Lanosterol biosynthesis begins with the selective epoxidation of squalene to give (35)-2,3-oxidosqualene, catalyzed by squalene epoxidase. Molecular O2 provides the source of the epoxide oxygen atom, and NADPH is required, along with a flavin coenzyme. The proposed mechanism involves reaction of FADH2 with O2 to produce a flavin hydroperoxide intermediate (ROOH), which transfers an oxygen to squalene in a pathway initiated by nucleophilic... 

Eckstein, J. W., Hastings, J. W., and Ghisla, S. (1993). Mechanism of bacterial bioluminescence. 4a,5-Dihydroflavin analogs as models for luciferase hydroperoxide intermediates and the effect of substituents at the 8-position of flavin on luciferase kinetics. Biochemistry 32 404 111. 

These include the mitochondrial respiratory chain, key enzymes in fatty acid and amino acid oxidation, and the citric acid cycle. Reoxidation of the reduced flavin in oxygenases and mixed-function oxidases proceeds by way of formation of the flavin radical and flavin hydroperoxide, with the intermediate generation of superoxide and perhydroxyl radicals and hydrogen peroxide. Because of this, flavin oxidases make a significant contribution to the total oxidant stress of the body. 

The mechanism suggested in Ref. [129] does not imply the formation of an intermediate product with an open ring. Active oxaziridine-4a,5-flavin benzoxide formed from flavin hydroperoxide is assumed to be the intermediate. This mechanism based on the electrophilic type of the oxaziridine system is justified in multiple examples, united in the following diagram [128] ... 

The mechanisms for passage of electrons out of the cosubstrate undergoing oxidation and into the flavin undergoing reduction have been scrutinized carefully in the past decade, as have those for reoxidation of flavin and passage of electrons to the cosubstrate that experiences net reduction. Depending on substrate structure a variety of covalent intermediates have been postulated or demonstrated directly thus 02 reacts with dihydroflavins to form 4a flavin hydroperoxides IV... 

O Reduced flavin adenine dinucleotide reacts with molecular oxygen to give a hydroperoxide intermediate. 

Mechanistically, it has been proposed that the reaction proceeds predominantly via epoxidation of the aromatic species 28, which leads to unstable arene-oxides 29-31 (equation 47). Rearrangement of the arene-oxides 29-31 involving the migration of a hydride anion (NIH-shift) forms the phenolic product 32 or 33. Alternative flavin-dependent oxidases have been proposed to involve a hydroperoxide intermediate. ... 

Because the 4a-flavin hydroperoxides are intermediates in both flavoprotein oxidase and mono-oxygenase chemistry/7 possible mechanisms for their formation are fundamental to an understanding of their biogenesis. 

A variation on this model has recently been proposed by Kelly et al. who suggested that the hydroxy group of the Criegee intermediate could not be immobilized in such a mechanism, and that unreasonable steric constraints would be imposed for many of the substrates transformed reported for these enzymes. A new tautomer of the the flavin hydroperoxide was proposed as part of an alternative scheme (lower cycle, Fig. 16.5-16) in which an intermediate trioxane decomposes to yield the lactone and flavin hydrate. 

The bulk of free reduced flavin does not go through a hydroperoxide intermediate when reacting with O2 (Scheme 20). Instead, the reaction is autocatalytic." ° Oxidized flavin, initially formed through the slow... 

Hydroxylases can synthesize unstable intermediates because of the very strong oxygen-donating ability of flavin hydroperoxides. These intermediates can then perform normally inaccessible reactions. One example of this is RebH. RebH chlorinates Trp at the 7-position (Equation (17)) in the synthesis of the antibiotic rebeccamycin. 

Because it is a two-component hydroxylase, reduced flavin is a substrate of luciferase. After it binds, it reacts with O2, forming a hydroperoxide, and then aldehyde binds binding of aldehyde to the free enzyme prevents flavin binding and luminescence. The flavin hydroperoxide bound to luciferase is very stable and may be trapped for many hours by adding long-chain alcohols instead of aldehydes. The hydroperoxide intermediate was prepared in this way using C-labeled its NMR spectrum was critically important in confirming... 

The reduced enzyme-substrate complex reacts with molecular oxygen forming a flavin hydroperoxide, which hydroxylates 2-aminobenzyl-CoA at the 5-position at 80 s (Scheme 33). In the absence of the reductase flavin, an aromatic product is obtained. However, in its presence, the hydroxylated intermediate is reduced rapidly and undergoes tautomerization to give the product 2-amino-5-oxocyclohex-l-enecarboxyl-CoA. 

Figure 2 (a) X-ray crystal structure of MHPCO (2.1 A, PDB 3gmc). The monomer is shown with FAD and MHPC bound at the active site, (b) The active site of MHPCO. (c) Proposed mechanism for the oxidative ring-opening reaction catalyzed by MHPCO with flavin hydroperoxide (18) acting as an electrophile, (d) An alternative mechanism for the catalytic transformation through a ketene intermediate (23). 

The Baeyer-Wlliger oxidation of ketones affords esters (from open-chain ketones) and lactones (from cyclic ketones), respectively. It is typically carried out using nucleophilic oxidants, such as peracids or dihydrogen peroxide in the presence of bases (29). Nucleophilic oxidants are required because the first step of the reaction mechanism is the addition of the peroxidic oxidant to the ketone s carbonyl function, affording the so-called Criegee-intermediate (Scheme 10). The subsequent cleavage of the 0 0 bond in the latter, and the concerted shift of yield the ester (lactone) product. In enzymatic Baeyer-Vlliger oxidations, flavin hydroperoxides such as 26 act as the nucleophilic oxidant (30). 

The flavin coenzyme reacts with molecular oxygen to form an intermediate species called flavin hydroperoxide. The oxygen of flavin hydroperoxide can be attacked by the % electrons of an alkene group. The subsequent nucleophilic attack of oxygen to the carbocation generates the epoxide ring (Figure 1.42). Epoxides are found as intermediate products in some biosynthetic pathways. The compound... 

Flavin hydroperoxide can also be attacked by the % electrons of an aromatic group. For example, in the oxidation of tryptophan, the intermediate (kynurenine) is hydrox-ylated via an electrophilic aromatic substitution with the reactive oxygen of flavin hydroperoxide (Figure 1.43). 

The reaction is initiated by reaction of catalyst 19 with molecular oxygen to give flavin hydroperoxid 20. Once in the cycle, this hydroperoxide can be regenerated by H2O2. The hydroperoxide 20 transfers an oxygen to the sulfide via the hydrogen-bonded transition state 21 to give sulfoxide and hydroxyflavin intermediate 22. 

Elimination of OH from 22 produces the aromatic 1,4-diazine 23. The latter species, which becomes the catalytic intermediate, reacts with hydrogen peroxide to regenerate the catalytic flavin hydroperoxide. The advantage of the N,N,N-1,3,5-trialkyl flavin system over the N,N,N-3,5,10-trialkylated analogs is that the elimination of OH (22 23) is fast because of aromatization. In the N,N,N-3,5,10-... 

To overcome the difficulty encountered in Hamilton s mechanism. Dolphin in 1974 proposed an interesting alternative (293) where no open-ring product intermediate is formed. His mechanism involves a reactive oxazi-ridine-4a,5-flavin, obtainable from flavin hydroperoxide. ... 

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