Peroxidizing dioxygenases

Although there are a great many naturally occurring peroxides and hydroperoxides which presumably arise from dioxygenation of unsaturated precursors (162), only two of the enzymes responsible have been classified so far (163). 

This enzyme is found in a wide variety of plants and exists in various iso-enzymic forms having different regioselectivities (163). 

Two of the four iso-enzymes of soybean, lipoxygenase-1 (eq. 37) and -2 give the 13-hydroperoxy- and a mixture of the 9- and 13-hydro-peroxydienoic acids, respectively (163). Another iso-enzyme gives the 

Any valid proposal for the mechanism of catalysis by lipoxygenase-1 must take into account the following observations 163, 165, 166). 

There are also other mechanistic implications, As each lipoxygenase contains only one, tightly bound iron atom, the stereochemistry of lipoxygenation must preclude any mechanism which requires direct metal participation in both acts of hydrogen abstraction and oxygen addition. Moreover, it is generally agreed that the reaction is free radical in nature 165, 172, 173) and that the enzyme is activated to its ferric oxidation state by the hydroperoxide product in a Haber-Weiss reaction (eq. 39) 174). 

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In 1990, Triantaphylidds and coworkers reported on the preparative enzymatic synthesis of hnoleic acid (135) hydroperoxide 24a using soybean lipoxygenase-1. In this dioxygenation asymmetry is induced by the catalyst, the enzyme. The reaction was later used by Dussault and also by Baba and coworkers as key step in the preparation of more complex peroxides. The enzyme is a non-heme iron dioxygenase which catalyzes the incorporation of dioxygen into polyunsaturated fatty acids to yield E,Z conjugated diene hydroperoxides 24a-d. With this enzymatic method, the hydroperoxide 24a could... 

These results indicate that the Fusarium lipoxygenase differs from the soybean lipoxygenase in various respects soybean lipoxygenase is a nonheme iron-containing dioxygenase and has a molecular weight of 102,000, optimum pH of 6.5 to 7,0 and isoelectric point of pH 5.4. The soybean enzyme is not inhibited by cyanide and catalyzes the peroxidation of linoleic acid and linolenic acid at equal rates74-76,193. ... 

Figure 18-22 Some possible intermediates in the action of extradiol (left) and intradiol (right) aromatic dioxygenases. Although the steps depict the flow of pairs of electrons during the formation and reaction of peroxide intermediates, the mechanisms probably involve free radicals whose formation is initiated by 02. The asterisks show how two atoms of labeled oxygen can be incorporated into final products.

A different type of concerted reaction involves the bacterial cytochrome c peroxide, where two hemes are coupled together, so that hydrogen peroxide undergoes a two-electron reduction to water without the formation of radical species. In a number of dioxygenases, dioxygen is reduced to peroxide by concerted electron transfer from [2Fe-2S] and non-heme Fe11 centres. 

Regardless of the peroxide transfer reaction mechanism of Equation 45, its occurrence suggests that the 4a-FlEtOO species might serve as a biomimetic dioxygenase reagent. This has been found to be the case (Equation 48) (53). Presumably, the reaction of Equation 48 occurs through the intermediacy of a peroxy adduct (Equation 49). 

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. 

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