Double bond cleaving dioxygenases

Among the synthetic rubbers, only poly(cw-l,4-isoprene) rubber has been reported to be biodegradable. It is also known that poly(cA-l,4-isoprene) is quite susceptible to oxidative degradation at the double bond. Thus, carbonyl ends are produced by the double bond scission as shown in Fig. 14.5 (Harayama et al, 1992). As shown in this figure, double bond cleaving dioxygenases require a transition metal as a cofactor for their interaction with dioxygen. 

Carotene cleavage enzymes — Two pathways have been described for P-carotene conversion to vitamin A (central and eccentric cleavage pathways) and reviewed recently. The major pathway is the central cleavage catalyzed by a cytosolic enzyme, p-carotene 15,15-oxygenase (BCO EC 1.13.1.21 or EC 1.14.99.36), which cleaves p-carotene at its central double bond (15,15 ) to form retinal. Two enzymatic mechanisms have been proposed (1) a dioxygenase reaction (EC 1.13.11.21) that requires O2 and yields a dioxetane as an intermediate and (2) a monooxygenase reaction (EC 1.14.99.36) that requires two oxygen atoms from two different sources (O2 and H2O) and yields an epoxide as an intermediate. ... 

Plant apocarotenoids have a wide variety of structures and functions. As expected, there is a small gene family of CCDs with different cleavage sites and somewhat promiscuous substrate selection. Some CCDs are stereo-specific, for example, 9-cis epoxycarotenoids are the substrates for NCEDs (9-cis expoxy dioxygenases) that produce the precursor of ABA biosynthesis, xanthoxin. Both linear carotenoids (lycopene) and cyclic carotenoids are substrates for cleavage at various double bonds including the central 15-15 and eccentric 5-6, 7-8, 9-10, 9 -10, and 11-12 bonds. Some CCDs cleave both linear and cyclic carotenoids and may cleave the same molecule twice, e.g., both 9-10 and 9 -10 positions. 

Marasco, E. and C. Schmidt-Dannert (2008). Identification of bacterial carotenoid cleavage dioxygenase homologs that cleave the interphenyl u,(J double bond of stilbene derivatives via a monooxygenase reaction. Chembiochem. 9(9) 1450-1461. 

The enzymatic reaction mechanism was determined by incubating a-carotene 6, a non-symmetric substrate of the enzyme, under a 02 atmosphere in H2 0 followed by isolation and characterization of derivatives of the cleavage product 2 (6). Accordingly, the enzyme cleaving the central double bond of 1 was found to be a non-heme iron monooxygenase (P-carotene 15,15 -monooxygenase) and not dioxygenase as termed earlier (Fig. 3). From the chemical point of view this enzymatic reaction is very unusual for various reasons (i) the reaction... 

The occurrence of xanthophyll dioxygenase, which releases xanthoxin from xanthophyll, was found using the ABA-deficient mutant vpl4oi Z. mays.650 Xanthophyll dioxygenase cleaves the double bond between C-ll (C-11 ) and C-12 (C-12 ) of 9-Z-violaxanthin (43) and 9 -Z-neoxanthin to give xanthoxin (5).651 The mRNA of xanthoxin dioxygenase in the leaves of P. vulgaris increased upon water stress and decreased upon... 

Degradation of L-tryptophan in most organisms proceeds via L-kynurenine, 3-hydroxy-L-kynurenine, 3-hydroxyanthranilic acid and quinolinic acid to acetyl Co A and CO2 (Fig. 244). Anthranilic acid formed as an intermediate may be recycled to L-tryptophan (see above). The ring of 3-hydroxyanthranilic acid is cleaved by a dioxygenase (C 2.5). The x-amino-/3-carboxymuconic acid-e-semialdehyde formed either undergoes a cis trans isomerization of the Zl -double bond and cyclization to quinolinic acid, a compound synthesized in microorganisms and plants from aspartic acid and D-glyceraldehyde-3-phosphate (D 16.2). On the other hand o -amino-/3-carboxymuconic acid-e-aldehyde may be de-carboxylated and is then the immediate precursor of NH3, acetic acid and COg. 

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