Hydroperoxide cleavage enzyme

Figure 10-20 Lipoxygenase Catalyzed Formation of Aroma Compounds in Cucumber. Source Reprinted from Biochim. Biophys. Acta., Vol. 441, T. Galliard, D.R. Phillips, and J. Reynolds, The Formation of cw-3-nonenal, mwu-2-nonenal and Hexanol from Linoleic Acid Hydroperoxide Isomers by a Hydroperoxide Cleavage Enzyme System in Cucumber (Cucumis Sativus) Fruits, p. 184, Copyright 1976, with permission from Elsevier Science.

Galliard, T., et al. 1976. The formation of cw-3-none-nal, frarw-2-nonenal and hexanal from linoleic acid hydroperoxide isomers by a hydroperoxide cleavage enzyme system in cucumber (Cucumis sativus) fruits. Biochim. Biophys. Acta 441 181-192. 

Yeast-derived saturated short-medium chain and branched-chain aldehydes are formed from sugar metabolism, fatty acid metabolism and branched-chain amino acid metabolism (Fig 8D.7). In addition, hexanal, as well as hexenal isomers, are formed during the pre-fermentative stages of winemaking by the sequential action of grape lipoxygenase and hydroperoxide cleavage enzyme on linoleic and linolenic acid, respectively (Crouzet 1986). 

The hydroperoxide cleavage enzyme from cucumber fruit is optimally active at pH 6.5, is very heat-labile, and attacks both 9- and 13-hydroperoxide isomers with equal facility. Subcellular localization studies have shown that the enzyme is associated mainly with the plasmalemma, Golgi body, and endoplasmic reticulum membranes (Wardale et al., 1978). 

It is known that volatile compounds found in fruits are mainly derived from three biosynthetic pathways in many plants [25] The formation of the hedonically important short-chain aldehydes and alcohols, such as di-3-hexenol, takes place through the action of lipases, hydroperoxide lyases, and cleavage enzymes on lipid components, followed by the action of alcohol dehydrogenases [26]. 

Although LOX from tomato fruits forms predominantly 9-hydroperoxides from linoleic and linolenic acids (Matthew et al., 1977), the cleavage enzyme from tomato does not attack these positional isomers but, rather, is specific for the 13-hydroperoxy isomers, producing hexanal or c/5-3-hexanal, respectively (Galliard and Matthew, 1977). Thus one can rationalize the formation of both Cg and C9 volatiles aldehydes in cucumber extracts with less specificity of LOX and cleavage enzymes and the absence of C9 volatiles in tomato with the substrate specificity of the cleavage enzyme. 

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

Such a species cannot be ruled out in reactions of iron-EDTA complexes with hydroperoxides recently described by Bruice and coworkers (27). On the other hand, a hydroperoxide complex that reacts with the substrate such that bond formation fiom O to substrate is concerted with 0-0 bond breaking, as proposed by Klinman for dopamine P-monooxygenase (18), could provide compensation for the cost of 0-0 bond cleavage in the transition state. In fact, it is interesting to speculate that for each of these enzymes, the mechanism by which the substrate is oxidized may be dependent on the reactivity of the substrate. One could envision certain substrates that would react with the metal-bound hydroperoxide ligand prior to or concerted with 0-0 bond cleavage. This possibility is difficult to assess because of our lack of information concerning the reactivity of HQ2" when complexed to different metal ions. 

The class of mononuclear dioxygenases [30] can e.g. perform hydroperoxidation of lipids, the cleavage of catechol and dihydroxylation of aromatics. A prominent example is naphthalene dioxygenase, which was the first identified by its crystal structure. It contains iron and a Rieske (2Fe-2S) cluster and is commonly referred to as a Rieske-type dioxygenase [33]. The iron in this case is flanked by two histidines and one aspartic acid residue. Among the mononuclear iron enzymes, the 2-His-l-carboxylate is a common motif, which flanks one-side of the iron in a triangle and plays an important role in dioxygen activation [34] (Fig. 4.17). 

The first step in the formation of the leukotrienes is cleavage of arachidonic add from membrane phospholipids. 5-I.ipoxygcnasp, which is a calcium-dependent enzyme, catalyzes the introductionof Oi into the arachidonic add and the conversion of the resulting hydroperoxide to an epoxide. Leukotriene A, contains an epoxide group. Leukobiene (LTA ) can be converted to leukotriene B (LTB4) by enzymatic hydrolysis. LTB is a dihydroxy fatty acid it regulates the functions... 

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