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Molecular Modeling of the N-terminal Regions of High Molecular Weight Glutenin Subunits 7 and 5 in Relation to Intramolecular Disulfide Bond Formation

¹Deutsche Forschungsanstalt für Lebensmittelchemie and Kurt-Hess-Institut für Mehl- und Eiweissforschung, Lichtenbergerstrasse 4, D-85748 Garching, Germany. ²USDA, Agricultural Research Service, Western Regional Research Center, 800 Buchanan Street, Albany, CA 94710. Mention of a product is for informational purposes only and is not meant to imply recommendation by the U. S. Department of Agriculture over others that may be suitable. ³Present address: Nestlé Ltd. Research Centre, PO Box 44, Lausanne 26, CH 1000, Switzerland.

Analyses of cystine peptides derived from the high molecular weight glutenin subunits (HMW-GS) 5 and 7 indicate that, in spite of a distinct sequence homology between the two subunits in the N-terminal region, different disulfide linkages of cysteine residues are present in these regions. To investigate the structural basis for these experimental results, the conformational structures of the polypeptide chains corresponding to the N-terminal regions (first 50 amino acids) of the wheat HMW-GS 5 and 7 were modeled by computer methods. Secondary structures were predicted by the method of Rost and Sander (1993) and, to the extent appropriate, applied to the constructed polypeptide chains. The resulting structures were energy-minimized and subjected to simulated heating and dynamic equilibration. In the final structure of subunit 5, the first two cysteines were located in a region of continuous α-helix. If folding to the helical form occurs rapidly during biosynthesis as expected, the distance between the sulfhydryl groups of these two cysteines would be great enough (≈2.2 nm) to make intramolecular disulfide bond formation unlikely. Although a somewhat similar region of α-helix was predicted for the subunit 7, in some predictions the helix was interrupted between the first two cysteines, and this break was assigned either extended structure or arbitrarily modeled as an inverse γ-turn. In the final structure of subunit 7 with the assigned inverseγ-turn, after energy minimization, heating, and dynamics, the two cysteines approached one another closely (≈0.4 nm). Formation of an intramolecular disulfide bond appeared a likely possibility. This model is in accord with experimental evidence for this latter intramolecular bond (Köhler et al 1993). In agreement with the modeling, an equivalent intramolecular disulfide bond of subunit 5 has not been found and experimental evidence for a different arrangement is presented.