MOLECULAR MODELING OF THE N-TERMINAL REGIONS OF HIGH-MOLECULAR-WEIGHTGLUTENIN SUBUNIT-7 AND SUBUNIT-5 IN RELATION TO INTRAMOLECULAR DISULFIDE BOND FORMATION

Analyses of cystine peptides derived from the high molecular weight gl utenin 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 r esults, the conformational structures of the polypeptide chains corres ponding 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 ext ent appropriate, applied to the constructed polypeptide chains. The re sulting structures were energy-minimized and subjected to simulated he ating 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 cys teines would be great enough (approximate to 2.2 nm) to make intramole cular disulfide bond formation unlikely. Although a somewhat similar r egion of alpha-helix was predicted for the subunit 7, in some predicti ons the helix was interrupted between the first two cysteines, and thi s 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 (approximat e to 0.4 nm). Formation of an intramolecular disulfide bond appeared a likely possibility. This model is in accord with experimental evidenc e for this latter intramolecular bond (Kohler et al 1993). In agreemen t with the modeling, an equivalent intramolecular disulfide bond of su bunit 5 has not been found and experimental evidence for a different a rrangement is presented.