Scheme 5.

Photosensitized Oxidation

In the presence of a suitable sensitizer such as flavin, cysteine is oxidized to cysteic acid and methionine to methionine sulfoxide (Scheme 6) (11,15). Interestingly, the same oxidation products are obtained using chemical agents such as hypochlorite or hydrogen peroxide. The treatment of soy proteins with 0.5% hydrogen peroxide oxidized half of the methionine to sulfoxide (16).

Oxidation of Cysteine

The oxidation reaction especially important to the functions of organized protein systems involves the reduction and reoxidation of disulfides. This type of sulfhydryl-disulfide reaction plays a major role in dough mixing (1719). There is a direct correlation between the sulfhydryl-disulfide content and the rheological quality of doughs made from wheat flours of several varieties (20,21). However, the exact interpretation of disulfide exchange in the molecular model of dough forming remains unclear (22).

Another example of chemical and physical changes caused by reduction and reoxidation is exhibited by the whey protein, /i-lactoglobulin. The protein is heat sensitive and starts to polymerize and aggregate when heated above 65°C. The primary reaction is believed to be the reduction of the two disulfides and reoxidation of the thiols forming intermolecular or intramolecular interchange with the free cysteine-121 (23). Sulfhydryl-disulfide interchange also occurs between /i-lactoglobulin and K-casein, producing a complex that is susceptible to heat-induced calcium phosphate precipitation and resistant to rennin action (24,25).


Scheme 6.

Scheme 7.

The loss in functionality limits the usefulness of this abundant whey protein as ingredients in food processing. The development of recombinant technology provides a novel procedure to clone the /Mactoglobulin gene, and specifically modify the free cysteine, with the ultimate aim of improving its thermal stability and functional properties (26-28).

Autoxidation of Heme Iron

Myoglobin, the protein that determines the color of meat products, is known to undergo autoxidation. The bright-red exterior color of fresh meat is derived from oxymyoglo-bin, which contains a heme iron in the + 2 oxidation state with an oxygen molecule reversibly bound to the sixth coordination position. The Fe(II) is readily oxidized to Fe(III) to give ferrimyoglobin characteristic of the brown surface color of aged meat. The autoxidation of the heme iron has been shown not to occur by a simple one-electron transfer in which the iron is oxidized from + 2 to +3 state and 02 is reduced to O ~ 2 (29,30). Instead, autoxidation in this case involves a two-electron reduction of the oxygen to peroxide via an iron-dioxygen complex (Scheme 7).

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