Differences in Amino Acid Sequences of Gliadin and Glutenin!

Gliadin and glutenin were examined to identify peptides that differentiate the proteins. Pronase-resistant fragments from gliadin and glutenin have average molecular weights of 460 and 640, respectively, and in acid are readily converted to pyroglutamic acid (PGA) peptides. PGA-peptides isolated from pepsin-hydrolyzed Pronase digests contained glutamine (Gin) or glutamic acid; proline, serine, and glycine were other common residues. Several peptides were common to digests of both proteins; but most were unique, demonstrating sequence differences between gliadin and glutenin. Yield data suggested that most unique peptides were from a single protein or only a few different ones. Glycine occurs more frequently in the PGA-peptides from glutenin; and proline, in those from gliadin. That Gin is also positioned differently in the two proteins was evidenced by the enzymatic release of more PGA from glutenin than from gliadin and more PGA-Gin and PGA-GIn-Gln from gliadin than from glutenin. Some studies (1,2,3) have suggested that glutenin contains gliadin-like subunits joined by disulfide bonds, but others (4-7) have emphasized differences between the proteins. Recently, we showed by column chromatography (8) that enzymatic digestion of gliadin and glutenin produces many similar or identical fragments, as proposed by Ewart (3). The analyses, however, demonstrated that some peptides are characteristic of either gliadin or glutenin. Bietz et al. (9) also found evidence of both unique and similar peptides among isolated gliadin proteins. Their data suggested that minor sequence differences may be responsible for the different properties of gliadin and glutenin and, perhaps, for variability in gluten quality. Because structures of gliadin and glutenin remain unknown, however, the relationship between these proteins is only partially understood. In further pursuit of information on structural features that differentiate gliadin and glutenin, we have used extensive enzymatic digestion to destroy as many similar sequences as possible, and to degrade dissimilar structures to fragments more easily separated and characterized by existing methods. Ninhydrin-negative PGA peptides from Pronase and Pronase-pepsin digests were isolated and were found to include many sequences unique to gliadin or glutenin, as well as some common to both groups of proteins. 1presented at the 55th Annual AACC Meeting, Minneapolis, October 1970. Contribution from the Northern Regional Research Laboratory, Peoria, Ill. 61604. This is a laboratory of the Northern Marketing and Nutrition Research Division, Agricultural Research Service, U.S. Department of Agriculture. Mention of firm names or trade products does not constitute endorsement by the U.S. Department of Agriculture over others of a similar nature not mentioned. The following abbreviations are used: Ala, alanine; Arg, arginine; Asx, asparagine or aspartic acid; CySSCy, cystine; GIn, glutamine; Glu, glutamic acid; Glx, glutamine or glutamic acid; Gly, glycine; His, histidine, He, isoleucine; Leu, leucine; Lys, lysine; Met, methionine; MW, molecular weight; PGA, pyroglutamic acid; Phe, phenylalanine; Pro, proline; Ser, serine; Thr, threonine, TLC, thin-layer chromatography; Tyr, tyrosine; and Val, valine. Copyright © 1971 American Association of Cereal Chemists, Inc., 3340 Pilot Knob Road, St. PaUl, Minnesota 55121. All rights reserved.