Molecular modeling of the n-terminal regions of high molecular weight glutenin subunits 7 and 5 in relation to intramolecular disulfide bond formation

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 Rest 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 a-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 (approximate to 2.2 nm) to make intramolecular disulfide bond formation unlikely. Although a somewhat similar region of alpha-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 gamma-turn. In the final structure of subunit 7 with the assigned inverse gamma-turn, after energy minimization, heating, and dynamics, the two cysteines approached one another closely (approximate to 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 (Kohler 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.