Alkenes enzyme activators

Several enzymes that halogenate organic substrates are well known and these enzymes have been studied extensively, especially those involving alkenes, alkynes, active methylene compounds, electron-rich heterocycles (pyrroles, indoles), and phenols [1,103-105]. Both chloroperoxidase and bromoperoxidase are widespread in the... 

The reason for this must come from the way in which the substrate interacts with the osmium-ligand complex. However, the detailed mechanism of the asymmetric dihydroxylation is stUl far from clear-cut. What is known is that the ligand forms some sort of chiral pocket, like an enzyme active site, with the osmium sitting at the bottom of it. Alkenes can only approach the osmium if they are correctly aligned in the chiral pocket, and steric hindrance forces the alignment shown in the scheme above. The analogy with an enzyme active... 

Biocatalytic asymmetric epoxidation of alkenes catalyzed by monooxygenases cannot be performed on a preparative scale with isolated enzymes due to their complex nature and their dependence on a redox cofactor, such as NAD(P)H. Thus, whole microbial cells are used instead. Although the toxic effects of the epoxide formed, and its further (undesired) metabolism by the cells catalyzed by epoxide hydrolases (Sect. 2.1.5), can be reduced by employing biphasic media, this method is not trivial and requires bioengineering skills [1151]. Alternatively, the aUcene itself can constitute the organic phase into which the product is removed, away from the cells. However, the bulk apolar phase tends to damage the cell membranes, which reduces and eventually abolishes all enzyme activity [1152]. 

Alkane oxidation via a hydroperoxide was suggested many years ago, and seems to be operative in Acinetobacter sp. strain M-1 that has, in addition, a rather unusual range of substrates that include both n-alkanes and -alkenes. The purified enzyme contains FAD and requires copper for activity (Maeng et al. 1996). 

It is noteworthy that, in contrast to mammalian systems, the majority of bacterial strains exhibited sufficient activity even when the cells were grown under non-optimized conditions. Since enzyme induction is still a largely empirical task, cells were grown on standard media in the absence of inducers. Furthermore, all attempts to induce epoxide hydrolase activity in Pseudomonas aeruginosa NCIMB 9571 and Pseudomonas oleovorans ATCC 29347 by growing the cells on an alkane (decane) or alkene (1-octene) as the sole carbon source failed [27]. 

The haem peroxidases are a superfamily of enzymes which oxidise a broad range of structurally diverse substrates by using hydroperoxides as oxidants. For example, chloroperoxidase catalyses the regioselective and stereoselective haloge-nation of glycals, the enantioselective epoxidation of distributed alkenes and the stereoselective sulfoxidation of prochiral thioethers by racemic arylethyl hydroperoxides [62]. The latter reaction ends in (i )-sulfoxides, (S)-hydroperoxides and the corresponding (R)-alcohol, all In optically active forms. 

Thus, exposure to any of these enzyme inducers concurrent with or after exposure to diazinon may result in accelerated bioactivation to the more potent anticholinesterase diazoxon. The extent of toxicity mediated by this phenomenon is dependent on how fast diazoxon is hydrolyzed to less toxic metabolites, a process that is also accelerated by the enzyme induction. Similarly, concurrent exposure to diazinon and MFO enzyme-inhibiting substances (e.g., carbon monoxide ethylisocyanide SKF 525A, halogenated alkanes, such as CC14 alkenes, such as vinyl chloride and allelic and acetylenic derivatives) may increase the toxicity of diazinon by decreasing the rate of the hydrolytic dealkylation and hydrolysis of both parent diazinon and activated diazinon (diazoxon) (Williams and Burson 1985). The balance between activation and detoxification determines the biological significance of these chemical interactions with diazinon. 

In a somewhat related work, Nolte, Rowan et al. [13] described in 2003 a rotaxane complex that mimics the ability of processive enzymes to catalyze multiple rounds of reaction while the polymer substrate stays bound. The catalyst, which consists of a substrate-binding cavity incorporating a manganese] 111) porphyrin complex acting as the catalytic center, can oxidize alkenes complexed within the toroid cavity, provided a ligand has been attached to the outer face of the toroid to both activate the porphyrin complex and prevent it from being able to oxidize alkenes outside the cavity. 

However, the enzymology of alkene and alkyne hydration is not well known. Recently, Meckenstock et al. (1999) discovered that the enzyme responsible for anaerobic hydration of acetylene contains a tungsten atom and an [Fe-S] cluster. This may hint that the enzyme uses the tungsten as a Lewis acid to activate the double bond. Possibly, the [Fe-S] cluster then serves to deliver a hydroxide as known in many common metabolite hydrations (Flint and Allen, 1996). Having introduced an oxygen moiety in an initial hydration, anaerobic bacteria may now be able to continue the biodegradation of such compounds. 

The selective catalytic epoxidation of alkenes has become the most important reaction catalyzed by heme proteins in organic synthesis. As described above, the monoxygenase activity of a heme peroxidase is restricted to CPO due to the open substrate access of the ferryl subunit for this enzyme. HRP catalyzes epoxidations only after mutagenetic variations, as shown for the substrate trans-P-methylstyrene [234]. An exception of this rule is the regioselective epoxidation of (T.TJ-piperylpiperidide, which is successfully catalyzed by native HRP [265]. 

Previously proposed mechanisms of the biosynthesis of certain chlorinated compounds have invoked electrophilic bromination of alkenes followed by passive chloride attack [62], Although this mechanism could explain the origin of adjacent brominated and chlorinated carbons, it does not readily account for compounds containing chlorine only. Thus, with the discovery of chloroperoxidase activity of the vanadium enzyme, the origin of specific chlorinated marine natural products can now be addressed. 

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