Dioxygenase reaction

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. ... 

In some dioxygenase reactions, however, one atom each of oxygen molecule is incorporated into two different molecules of one substrate [Eq. (9)] or [Eq. (10)] into two different substrate molecules... 

When a benzene ring is cleaved by a dioxygenase reaction, hydroxylation of the benzene ring usually proceeds to form catechol or phenol derivatives. When catechol derivatives are cleaved by the action of individual dioxygenases, three modes of ring fission have been demonstrated by microbial enzymes85 ... 

The mechanism of a-ketoglutarate participation in the dioxygenase reaction has been proposed by Lindstedt and his co-workers6S,220) as shown in Eq. (27) with y-butyrobetaine hydroxylase. 

Incomplete aerobic transformations may involve cometabolic transformations and reactions resulting in recalcitrant dead-end metabolites. Cometabolic o-hydroxylation of MCPs and DCPs by a phenol monooxygenase has been shown, for example, in a Pseudomonas sp. (Knackmuss Hellwig, 1978) and by the toluene dioxygenase reaction in Pseudomonas putida (Spain Gibson, 1988 Spain et al., 1989)- Cometabolic transformation of CPs is also possible in aerobic mixed culture systems. Phenol- and toluene-enriched cultures completely removed 2,4-DCP, and the toluene enrichment also removed 2,4,6-TCP and PCP (Ryding et al., 1994). This PCP attack by the toluene enrichment involved an o-hydroxylation. 

The following reaction is catalyzed by a dioxygenase from E. coli. Tire dioxygenase reaction opens the ring and the intermediate is cleaved hydrolytically. Propose a structure for the intermediate and a mechanism for the hydrolytic cleavage. See Fleming et al.382... 

Phenylalanine is first converted to tyrosine by the monooxygenase phenylalanine hydroxylase a reaction involving the coenzyme tetrahydrobiopterin. The tyrosine is then converted first by transamination and then by a dioxygenase reaction to homogentisate, which in turn is further metabolized to fumarate and acetoacetate. 

Figure 20.7 Overview dioxygenase reactions in synthetic chemistry.

Figure 9 describes a proposed mechanism for the dioxygenase reaction based on enzymatic and structural studies. It is proposed that the substrate binds as a dianion, which ligates the metal ion. Oxygen would then react to form an organic peroxide anion, which would then form a eyerie peroxide intermediate. Trie different reaction specificity is proposed to result from attack of the peroxide oxygen on alternative carbons. For example, the Ni-ARD would direct attack of the peroxide anion at the C3 carbonyl to form a five-membered peroxide intermediate, while the Fe-ARD would attack at carbon 2 to form a four-membered cyclic peroxide. 

It is thus the intramolecular analogue to the a-ketoacid dioxygenase reaction discussed in the preceding section. The same requirements for ferrous ion and a reducing agent are observed. 

In the second sequence, degradation is mediated by a 2,3-dioxygenase reaction with the methyl group intact (Figure 6.27b), and this pathway is followed in the metabolism of alkylated benzenes such as ethylbenzene and isopropylbenzene (Eaton and Timmis 1986). 

Inhibition of Indoleamine 2,3-Dioxygenase Reaction by Superoxide Dismutase... 

Bacterial p-carotene hydroxylases, CrtZ, were characterized with respect to substrate specificity and reaction mechanism after expression in E. coli. The reactions depend on oxygen and are stimulated by 2-oxoglutarate, ascorbate, and Fe L which is typical for a dioxygenase reaction. Similar mechanistic properties were also found for bacterial ketolases encoded by crtW. These enzymes catalyze the introduction of... 

The oxygenated iron compound reported here requires much more detailed characterization. Nevertheless, our findings are quite consistent with the view that dioxygenase reactions in general involve a ternary complex of enzyme, oxygen, and organic substrate (7, 15). The role of tryptophan in the complex is now under investigation. 

It appears from the previous discussion that melanogenesis in vivo or in vitro is regulated by various factors. Hence, to study the dynamics of melanin formation (monooxygenase reaction, i.e. constructive metabolism) and breakdown (dioxygenase reaction, i.e. catabolism of melanin precursors), a non-enzymatic melanin synthesis from tyrosine and tryptophan, respectively, was devised by Roy et al. (227), using a prototype of a monooxygenase reaction, i.e. the Udenfriend reaction (Fe+ /EDTA/ ascorbic acid) (272). 

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