TECHNOLOGY OF CEREALS dichroism, suggests a high content of a-helix Temperate cereals and low B-sheet content Under the classical nomenclature the necessity to reduce disulphide bonds would define B-,y B-Zeins and 8-zeins as glutelins. Indeed to regard all B-Zeins contribute 10-15% of total prolamin. insoluble cereal proteins as prolamins is not They are rich in methionine and cysteine and can universally accepted among protein chemists Such a classification can be extended to temp he presence of a reducing rate cereals but at the present time the tradi- agent indicating mutual association through di- sulphide bonds. No sequences are repeated and all tional Osborne classification is more widespread differ from those in a-zeins. The tertiary str especially in wheat proteins where the functional ture consists mainly of B-sheet, and ap aspects are particularly important. From two structure(B-turns and random coil) dimensional electrophoresis studies it has been established that up to 20 different polypeptides are found in glutenins(the glutelins of wheat y and 8-Zeins ee p. 69). An even greater number -up to 50 may be found in gliadins. One argument lamin.Like B-zeins, they require the presence of advanced for distinguishing between wheat pro- a reducing agent for extraction. Eight hex peptide sequences in the central domain are properties of the two classes when hydrated ranked by unique N-and c-terminal region gliadins behave as a viscous liquid and glutenins The repeat sequences are very hydrophilic, as a cohesive solid. Although both infiuence rendering the proteins very soluble when reduced. gluten behaviour, it is the larger polymeric glute 8-Zeins also require reduction before extraction nins that wield the One of the most attractive theories concerning but between 17 and 29 methionine residues occur the relationship between glutenin structure and here function is the linear gluten hypothesis(Fig 3. 13) It envisages a series of polymeric subunits joined Other tropica/ cereals head to tail by interchain disulphide bonds. The essential features of the subunits are terminal a Although only the prolamins of maize among the a-helices and central regions of many B-turns(B opical cereals have been studied extensively, avail- turns also occur in the body tissue protein elastin able evidence indicates that sorghum, pearl millet they are capable of much extension under tension and Job's tears contain essentially similar groups. and can return to their former folded condition on Stretching 0 ReLaxation region region The subunits are joined head-to-tail via S-S bonds to form polymers with molecular weights of p? everal million. The subunits are considered to have a conformation that may be stretched when tension is applied to the native conforma through elastic recoil. The N- and C- terminal ends of some high molecular weig interchain S-s bonds are located, are now thought to be alpha-helix rich domain domains are thought to be rich in repetitive beta- turn structures. The presence of rep eta-turn produced by courtesy of The Royal Society of Chen may confer elasticity. From D J. Schofield( 1986) structures may result in a beta-spiral structure mistry, London70 TECHNOLOGY OF CEREALS dichroism, suggests a high content of a-helix and low P-sheet content. p-Zeins p-Zeins contribute 10-15% of total prolamin. They are rich in methionine and cysteine and can agent indicating mutual association through di￾sulphide bonds' No sequences are repeated and all differ from those in a-zeins. The tertiary struc￾structure (p-turns and random coil). y and &Zeins y-zeins account for 5-10% of the total pro￾lamin. Like p-zeins, they require the presence of a reducing agent for extraction. Eight hexa￾peptide sequences in the central domain are flanked by unique N- and C-terminal regions. The repeat sequences are very hydrophilic, rendering the proteins very soluble when reduced. &-Zeins also require reduction before extraction, no sequences are repeated in the central regon but between 17 and 29 methiofie residues occuT here. Other tropical cereals Although only the prolamins of maize among the tropical cereals have been studied extensively, avail￾able evidence indicates that sorghum, pearl millet and Job's tears contain essentially similar groups. Temperate cereals Under the classical nomenclature the necessity to reduce disulphide bonds would define p-, y￾and 6-zeins as glutelins. Indeed to regard all insoluble cereal proteins as prolamins is not universally accepted among protein chemists. Such a classification can be extended to tempe￾tional Osborne classification is more widespread especially in wheat proteins where the functional aspects are particularly important. From two established that up to 20 different polypeptides are found in glutenins (the glutelins of wheat - see p. 69). An even greater number - up to 50 - may be found in gliadins. One argument advanced for distinguishing between wheat Pro￾lamins and glutelins is the different physical Properties of the two classes when hydrated: gliadins behave as a viscous liquid and glutenins as a cohesive solid. Although both influence gluten behaviour, it is the larger polymeric glute￾nins that wield the greater influence￾One of the most attractive theories concerning the relationship between glutenin structure and function is the linear gluten hypothesis (Fig. 3.13). It envisages a series of polymeric subunits joined head to tail by interchain disulphide bonds. The essential features of the subunits are terminal a-helices and central regions of many p-turns (p￾turns also occur in the body tissue protein elastin, they are capable of much extension under tension and can return to their former folded condition on be extracted Only in the presence Of a reducing rate cereals but at the present tirne the tradi￾ture consists main1y Of P-sheet, and aperiodic dimensional electrophoresis studies it has been Stretching Relaxat ion w- - L 5 0-helix flb 8- turn aad region region FIG 3.13 Schematic representation of a polypeptide subunit of glutenin within a linear concatenation. The subunits are joined head-to-tail via S-S bonds to form polymers with molecular weights of up to several million. The subunits are considered to have a conformation that may be stretched when tension is applied to the polymers, but when the tension is released the native conformation is regained through elastic recoil. The N- and C- terminal ends of some high molecular weight subunits, where interchain S-S bonds are located, are now thought to be alpha-helix rich domains, whereas the central domains are thought to be rich in repetitive beta-turn structures. The presence of repetitive beta-turn structures may result in a beta-spiral structure, which may confer elasticity. From D. J. Schofield (1986). Reproduced by courtesy of The Royal Society of Chemistry, London