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Structure of Glutenin : Achievements at the Northern Regional Research Center

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Structure of Glutenin : Achievements at the Northern Regional Research Center Ann. Technol., agric., I980, 29 (2), 249-277. 4909 Structure of glutenin : Achievements at the northern regional research center J. A. BIETZ and F. R. HUEBNER Northern Regional Research Center, A gricultural Research Science and Education Administration U.S. Department 01 Agriculture Peoria, Illinois 6I604 United States Summary The Northern Regional Research Center (~·RRC), Peoria, Illinois, is a laboratory of the U.S. Department of Agriculture's Science and Education Administration. For more than 20 years studies have been carried out on wheat glutel1 and its major fractions, gliadin and glutenin. This review summarizes studies of glutenin at ~ RRC and integrates this information with other laboratories' results to show how glutertirt structure relates to wheat's functional properties. In early studies at RRC, glutenin was isolated and shown to be a heterogeneous mixture of high molecular weight (.M:W) molecules containing ordered and unordered structures. Mole­ cules are asymmetric, and have high surface areas favoring interactions and association. Elec­ trophoresis showed that reduced glutenin contains many subunits joined by intermolecular disulfide bonds; hydrogen and hydrophobic bonds also contribute to glutenin's structure. Three distinct types of glutenin subunits were isolated and characterized, and genetic control of glu­ tenin's unique high MW subunits was established. Wheats having different qualities differ in subunit composition. Glutenin differs from low-MW gliadins, but its ethanol-soluble subunits are identical to subunits of high MW gliadin; a hypothesis was developed showing how different subunit types assemble into glutenin. High MW native glutenin molecules are related to wheat quality. These molecules are oriented by mixing, and form disulfide and noncovalent bonds with other flour proteins, resulting in a continuous gluten network in dough. Gluten is cohesive because of the many covalent and nOn- covalent bonds between subunits; it is elastic because of changing interactions and conformations under stress, combined with a tendency to returl1 to a minimun energy state. Key words: wheat - glutenin - structure - breadmaking - quality. (1) The mention of firm names or trade products does not imply that they are endorsed or recom­ mended by the U.S. Department of Agriculture over other :firms or similar products not mentioned. Purchased by U. S. Dept. 01 Agriculture for Official Use J. A. BIETZ, F. R. HUEBNER Introduction It is now more than 160 years since wheat gluten was first separated into two distinct fractions on the basis of differential solubility in alcohol, and more than 70 years since Osborne's classical studies, after which the term" glutenin" became accepted for its alcohol-insoluble fraction. Glutenin is widely recognized as an important contributor to wheat quality and functional properties but, in spite of numerous studies and perhaps due to its innate complexity, conside­ rable divergence of opinion still exists as to the nature and even the existence of glutenin. In part, this problem is due to glutenin's intractability: it is extremely difficult to solubilize, isolate, or purify without drastically changing its properties. As a result, many reports have considered only part of glutenin, and results and interpretations have differed. There is also no one good, simple definition for glutenin: it has meant different things to many people. For now, let us define glutenin as the complex, high-molecular-weight (MW) alcohol-insoluble fraction of wheat endosperm proteins made up of numerous subunits, joined both cova­ lently and non-covalently, and including characteristic high MW subunits that are absent in all other wheat protein classes. This is not a simple definition, but it is broad and dYnamic enough to allow incorporation of new knowledge. The orthern Regional Research Center ( RRC) is located at Peoria, Illinois. It is part of the U.S. Department of Agriculture's Science and Education Admi­ nistration (formerly the Agricultural Research Service). Research into the nature and structure of glutenin and other wheat proteins has been on-going at the laboratory for more than 20 years. During this time, as we have probed glutenin structure with new techniques, our understanding of its nature has improved. Our laboratory has published more than 50 research reports concerning glutenin composition, structure, properties, and quality. Other laboratories also have made significant and pioneering advances, of course, as detailed in recent reviews (WALL, 1979b; KHAN and BUSHUK, 1978; KASARDA et al., 1976; BrETz et al., 1973; HUEB ER, 1977, 1978; HUEB ER et al., 1977). This paper reviews RRC research on glutenin and introduces new ideas and interpretations that permit a unified view of glutenin's structure. The many contributions of our colleagues at other laboratories, as detailed in the above-mentioned reviews, and their continued cooperation are gratefully acknowledged. Initial studies Studies at RRC concerning the nature of wheat gluten and its components were first published in 1959; as new biochemical techniques became available during the next few years, additional studies Yielded valuable new information about glutenin's structure. JONES et al. (1959) described methods for preparation of gluten and for its fractionation into gliadin and glutenin; these methods are still widely used today. J a rES et al. also demonstrated, using moving boundary electrophoresis, that gliadin and glutenin are uniquely different (Fig. I); glutenin had only one major peak, similar to the" ex. " peak of gluten, and was shown to STRUCT RE OF GL TENIK 25 1 be associated with the gluten-like properties of wheat. The single peak observed for glutenin seems somewhat surprising considering current knowledge of ist heterogeneity and complexity; but, in fact, it may be an early indication that glutenin's subunits join randomly since their mass jcharge ratio is relatively constant. ltracentrifugal analysis (JO~ES et al., 1961, 1964) found glutenin to be heterogeneous, having a weight average MW of 1.5-2 million, and to contain molecules with MW's ranging from about 50 000 to several million. MW's remained constant in several disaggregating solvents, suggesting that chemical a a w 0 A ~ ~ .=-8--------- <:50 24 ho 59 2018 ( 2 I 21 : a ---~{'P"- -(-----01-90 -10-(-(---- FIG. 1. - Moving boundary electrophoresis 0/ gliadin extracted trom gluten ball with 70 p. roo ethanol (A) and 0/ gliadin (B) and glutenin (C) separated by neutralization 0/ an acidic 70 p. roo ethanol solution. (From JONES et al., 1959). bonds, rather than physical aggregation, were responsible for glutenin's large molecular size. IELSEN et al. (1962) confirmed native glutenin's MW distribu­ tion, but they also showed that disulfide bond cleavage reduced its MW to about ~o 000. Reduced glutenin had lost its elastic and cohesive properties, showing that disulfide cross-links contribute to glutenin's native structure. Good information on the amino acid composition of glutenin became available (WOYCHlK et al., 1961; Wu and DIMLER, 1963), and Jo- ~ES et al. (1963) found that gluten could be fractionated into glutenin and gliadin by gel filtration on Sephadex G-75. The novel technique of starch gel electrophoresis (SGE) in aluminium lactate buffer (WOYOHIK et al., 1961; JONES et al., 1963) showed that native glutenin could not enter the starch gel. After reductive cleavage of glutenin's disulfide bonds, however, 20 or more components were released from glutenin (WOYCHIK et al., 1964) (Fig. 2). Some were similar to FIG. 2. - Starch gel electrophoresis (SGE) ot reduced gliadin (A) and glutenin (B). (Prom WOYCHIK et al., 1964). 'B 252 J. A. BIETZ, F. R. HURB ER gliadin, suggesting that glutenin was formed through disulfide bonding of gliadins; other components were obviously different. Thus, intermolecular disulfide bonding was proposed as a principal factor in glutenin's structure and unique rheological properties, which confirmed the studies of IELSE T et al. (1961). It was proposed that gliadin modified glutenin's properties through disulfide interchange reactions. BECKWITH and WALL (1966) extended this concept by studying the reoxidation of reduced glutenin at various concentrations. Viscosity measurements during reduction suggested that both inter- and intra-molecular disulfides were present while reoxidation studies showed that appropriate ratios of these two types of disulfides are essential for glutenin's cohesive-elastic properties. Early studies at RRC also demonstrated, however, that noncovalent bonds contribute to gluten's functionality: side-chain amide groups were shown to participate in hydrogen bonding between protein molecules and between proteins and solvents (BECKWITH et al., 1963). Thus, these studies established methods for isolating and characterizing glutenin, and showed that it is a high MW, heterogeneous polymer of many different polypeptide subunits held together by disulfide bonding and non-covalent a sociations which were essential to glutenin's cohesive-elastic properties. Physical studies Several studies at RRC have characterized glutenin using the physico­ chemical techniques of intrinsic viscosity determinations, sedimentation velocity, titration. ultraviolet difference spectra, optical rotatory dispersion (ORD), osmotic pressure, circular dichroism (CD), and infrared absorption. TAYLOR and CLUSKEY (1962) found that glutenin behaves as a flexible, randomly coiled polyelectrolyte of high viscosity in solution, making it the major
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