Enzymecatalyzed Oxidation

Lipoxygenase-Catalyzed Peroxidation

The action of the enzyme lipoxygenase (EC1.13.11.12) in food and food-related systems leads to the formation of hydroperoxides in the aerobic pathway, and dimers and oxodienoic acids in the anaerobic reaction (31,32). The enzyme substrate must be unsaturated fatty acids containing ci's,cis-l,4-pentadiene system, and the reaction is regio-specific and stereospecific in contrast to autoxidation. As the result of free-radical formation in the intermediate process, proteins present in the same system may be oxidized, and subsequent protein-protein polymerization yields many compounds similar to those outlined in autoxidation (33). Oxidized lipids also cause chemical changes in some sensitive amino acids in proteins, and cross-linking reaction between proteins and aldehydes generated from the breakdown of hydroperoxide.

Soybean lipoxygenase has been utilized to bleach flour by incorporating soy flour in wheat flour. The oxidative destruction of pigments in flour by the addition of lipoxygen ase is believed to be a free-radical mediated reaction in which the alkyl or peroxy radicals are involved. Besides the bleaching effect, using soy flour in bread making often results in an improvement of the dough-forming properties (34-36). It has been proposed that in a peroxidizing system, the radicals enter into coupled oxidation of cysteine residues in the flour proteins and cause structural changes in the dough (Scheme 8).

The hydroperoxides formed from lipoxygenase-cata-lyzed oxidation of unsaturated lipids are converted to flavor compounds by the enzyme hydroperoxide lyase, which has been identified and purified from cucumber and tomato fruits, watermelon seedlings, pear, soybean seedlings and leaf tissues. The major flavor compound in cucumber, (¿£,Z)-2,6-nonadienal, is one of the products of this enzymatic rearrangement. Other aldehydes also originate from fatty acid through similar pathways.

Enzymatic Browning Reactions

Polyphenol oxidase (EC1.10.3.1) is an enzyme of great importance in that it not only causes the browning of cut or bruised fruits and vegetables but also plays a major role in the curing of tea, coffee, and tobacco. The enzyme functions as a monooxygenase in the ortho hydroxylation of monophenols to dihydroxyphenols, and a two-electron oxidase in the oxidation of the o-diphenols to o-quinone (Scheme 9) (37-39). The enzyme has broad substrate specificity. Some of the common substrates include catechol, 4-methylcatechol, dopamine, pyrogallol, catechin, caffeic acid, chlorogenic acid,p-cresol, tyrosine, andp-hydroxycin-namic acid. The product of the oxidation of diphenols is o-quinone, which polymerizes to form colored pigments (40-42).

In the oxidation of dopamine, the primary substrate found in banana, the dopamine quinone undergoes nonen-zymatic rearrangement to indole-5,6-quinone that in turn is polymerized to form melanin (43,44).

The development of flavor (and color as well) in the fermentation of tea is related to the oxidation of phenolic compounds. The most important reaction is the oxidation of flavanols by polyphenol oxidase. The oxidized flavanols condense to form theaflavin, which is one of the major constituents of tea flavor (45,46). Similar type of desired activity is also evidenced in coffee cocoa, prunes, dates, and dark raisin production.

The oxidized flavanols can also undergo Strecker degradation with amino acids to form various aldehydes. Flavor compounds, such as isobutanal, 2-methylbutanal, isovaleraldehyde, and phenyacetaldehyde present in fermented tea aroma have been suggested to be the products of degradation of valine, isoleucine, leucine, and phenylalanine. The oxidation of flavanols to the o-quinone form

Lipoxygenase

Scheme 8.

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