Dioxygenase mechanism

Scheme 10.21 Dioxygenase mechanism of epoxyquinone formation. The 180 isotope labels are omitted for clarity in the lower pathway.

Scheme 10.23 Possible dioxygenase mechanism for the transformation of protoasukamycin to asukamycin.

Braaz R, W Armbruster, D Jendrossek (2005) Heme-dependent rubber oxygenase Roxa of Xanthomonas sp. cleaves the carbon backbone of poly(cw-l,4-isoprene) by a dioxygenase mechanism. Appl Environ Microbiol 71 2473-2478. 

Nishinaga further demonstrated that Con(salpr) [salpr = bis(3-salicylidineamino-propyl)amine] effected the oxygenation of 4-aryl-2,6-di-tert-butylphenols to yield a-ketohydroperoxycobalt(III) complexes 66Sh The free hydroperoxides were obtained by treatment with silica gel. These hydroperoxides are analogous to the peroxide intermediate proposed in the dioxygenase mechanism 33K... 

The closely related PmB enzyme catalyzes a similar reaction (Fig. 6a) and binds its substrate differently (64) in this case, the C —bond of 7-chloro-L-Trp is cleaved (without insertion of O2) and Naismith and coworkers have shown that the L-typtophan binds in a different orientation to that of X. cam-pestris TDO (Fig. 6b). This system may provide important clues as the details of the dioxygenase mechanism are imraveled. 

Scheme 9.13 Oxygen activation by an Ru-polyoxometalate A dioxygenase mechanism.

The major natural sources of vitamin A in the diet are certain plant carotenoid pigments, such as p-carotene, and the long-chain retinyl esters found in animal tissues. p-Carotene is converted to vitamin A primarily in the intestinal mucosa. The biosynthetic process involves two soluble enzymes p-carotene 15,15 -diox-ygenase and retinaldehyde reductase. The first enzyme catalyzes the cleavage of P-carotene at the central double bond by a dioxygenase mechanism to yield two molecules of retinaldehyde (Cl) the aldehyde is then reduced to retinol (Al). 

Khan, M M.T. Shukla, R.S. (1992) Ascorbate-dependent monoxygenase vs dioxygenase mechanism for Ru(iii)-EDTA catalyzed oxidation of adamantane by molecular-oxygen, J. Mol Catal, 11,221-8. 

Since the mass spectra of the O-enriched stipitatonic acid and other tropo-lone metabolites isolated showed only M + 2 peaks and no A/ + 4 peak, which would have been required by the dioxygenase mechanism, the monooxygenase pathway was favoured. 

Scheme 8.12 Oxygen activation by a Ru-polyoxometalate a dioxygenase mechanism...

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