Reoxidation behavior of wheat and rye glutelin subunits

The ability of HMW and LMW subunits of wheat glutelin to form a polymeric gluten network by intermolecular disulfide bonds is responsible for the unique rheological properties and baking quality of wheat dough. Because the mechanism of gluten formation is not fully understood, the reoxidation behavior of HMW and LMW subunits of wheat glutelin and HMW subunits of rye glutelin was studied. The subunits were isolated from wheat flour cv. Rektor (REK) and from rye flour cv. Danko (DAN) with a selective extraction and precipitation method. For reoxidation, different oxidants (KBrO3 and KIO3), protein concentrations (0.5, 1.0, and 2.0%), solvent compositions, pH values (2.0 and 8.0), and reaction times (0-360 min) were compared. The characterization of reoxidized products was achieved by the determination of the thiol content with the Ellman's reagent, and of the M(r) distribution by gel-permeation chromatography. The results demonstrated that both HMW and LMW subunits could be slowly reoxidized with KBrO3 to polymers with M(r) up to several millions. Yield and M(r) distribution of polymers were dependent both on the protein concentration and on the molar ratio of oxidants to thiol groups. The HMW subunits of wheat glutelin (HMW-REK) yielded slightly higher quantities of polymeric proteins than did the HMW subunits of rye (HMW-DAN). Reoxidation with KIO3 proceeded much faster than with KBrO3 and led to lower proportions of polymerized proteins for HMW-REK and HMW-DAN. Obviously, more intra- and fewer intermolecular disulfide bonds were formed by reoxidation with KIO3 compared with KBrO3. In contrast, LMW-REK was reoxidized with KIO3 to higher amounts of polymeric aggregates, which indicated that LMW subunits formed intermolecular disulfide bonds with both KIO3 and KBrO3. Independent of the protein type and the oxidant used for reoxidation, more inter- and fewer intramolecular disulfide bonds were formed when the protein concentration was increased. Single subunits 5, 7, and 10 were isolated from HMW-REK by preparative acid-PAGE and were reoxidized with KBrO3 for 360 min. The M(r) distribution indicated that x-type subunit 5 had a greater tendency to form polymers than x-type subunit 7. The y-type subunit 10 was characterized by a lower proportion of polymers after reoxidation than x-type subunits 5 and 7.