Reoxidation behavior of wheat and rye glutelin subunits

The ability of HMW and LMW subunits of wheat glutelin to form a polymeric g luten network by intermolecular disulfide bonds is responsible for the uniq ue rheological properties and baking quality of wheat dough. Because the me chanism of gluten formation is wt fully understood, the reoxidation behavio r of HMW and LMW subunits of wheat glutelin and HMW subunits of rye gluteli n was studied. The subunits were isolated from wheat flour cv. Rektor (REK) and from rye flour cv. Danko (DAN) with a selective extraction and precipi tation method. For reoxidation, different oxidants (KBrO3 and KIO3), protei n 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 characterizati on of reoxidized products was achieved by the determination of the thiol co ntent with the Ellman's reagent, and of the M-r distribution by gel-permeat ion chromatography. The results demonstrated that both HMW and LMW subunits could be slowly reoxidized with KBrO3 to polymers with M-r up to several m illions. Yield and M-r distribution of polymers were dependent both on the protein concentration and on the molar ratio of oxidants to thiol groups. T he HMW subunits of wheat glutelin (HMW-REK) yielded slightly higher quantit ies of polymeric proteins than did the HMW subunits of rye (HMW-DAN). Reoxi dation with KlO(3) proceeded much faster than with KBrO3 and led to lower p roportions of polymerized proteins for HMW-REK and HMW-DAN. Obviously, more intra- and fewer intermolecular disulfide bonds were formed by reoxidation with KlO(3) compared with KBrO3. In contrast, LMW-REK was reoxidized with KlO(3) to higher amounts of polymeric aggregates, which indicated that LMW subunits formed intermolecular disulfide bonds with both KlO(3) and KBrO3. Independent of the protein type and the oxidant used for reoxidation, more inter- and fewer intramolecular disulfide bonds were formed when the protei n concentration was increased. Single subunits 5, 7, and 10 were isolated f rom 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 te ndency to form polymers than x-type subunit 7. The gamma -type subunit 10 w as characterized by a lower proportion of polymers after reoxidation than x -type subunits 5 and 7.