Disulfide Bonds

Disulfide bonds between pairs of cysteine residues can cross-link different chains of a protein, which gives rise to covalent chain assembles. Disulfide bonds occur between different segments within a single polypeptide chain as well. These bridges play an important role in rheological properties of food proteins. For instance, the cohesive elastic character of wheat flour doughs is based on the disulfide bonds of glutenin, and the three-dimensional network of disulfide bonds in glutenin is responsible for the difficulties encountered during wet milling of corn. Rearrangement of disulfide bonds in a protein by sulfhydryl-disulfide interchange reactions plays an important role in the regulation of activities of enzyme and immunoglobulins. By the catalytic action of sulfhydryl groups in low molecular weight compounds, eg, cysteine, glutathione and mercaptoetha-nol, proteins can be linked to each other to form large aggregates through a chain reaction. However, in the presence of higher concentrations of these reducing compounds, disulfide bridges in proteins are reduced to sulfhydryl groups, thereby decreasing the molecular weights as well as viscosity.

The measurement of sulfhydryl and disulfide contents of proteins is a very useful tool to assess protein structure. Raman spectroscopy is advantageous because sulfhydryl and disulfide groups appear as peaks in the spectrum, and even solid samples such as gels can be used for analysis with minimum pretreatment (36).

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