TECHNOLOGY OF CEREALS secondary and tertiary structures of a protein ably. the distinction formalized by Osborne that hange in response to the environment but the remains unquestionably valid is that between primary structure remains unaltered unless its albumins and globulins on the one hand and length is reduced by hydrolysis prolamins and glutelins on the other. In composi All the interactions listed in Table 3. 5 can tion there is a marked difference due mainly to contribute to tertiary structure but the most the extremely high content of proline and gluta- stable types are the covalent disulphide bonds mine in the less soluble fractions (the name formed by oxidation of sulphydryl prolamine' reflects this characteristic). An interacting cysteine/cystine residues extremely low lysine content is also characteristic Such bonds also occur between cysteine/cystine of insoluble cereal proteins residues on different polypeptides giving rise to a stable structure involving more than one poly- Soluble proteins peptide Inter-peptide links can thus produce in a protein a fourth or quaternary level of structure These are found in starchy endosperm Disulphide bonds are stronger than non-cova- aleurone and embryo tissues of cereals. They lent bonds but they are nevertheless capable of account for approximately 20% of the total pro- entering into interchange reactions with sub- tein of the grain. Albumins are usually more stances containing free sulphydryl groups. prevalent than globulins. The amino acid com Such reactions are of great importance in dough position of soluble proteins is similar to that of proteins found in most unspecialized plant cells suggesting that they include those that constitute Cereal proteins the cytoplasm found in most cells. They are a complex mixture including The complexity of cereal proteins Is enormous 1. metabolic enzymes and the determination of the structure of gluten 2. hydrolytic enzym the protein complex responsible for the dough forming capacity of wheat four has been described as one of the most formidable problems 4. phytohaemaglutenins (proteins that clot red ver faced by the protein chemist (Wrigley and Bietz, 1988). To simplify their studies cereal Globulins may also arise as storage proteins chemists have sought to separate the proteins occurrI ng in protein bodies, particularly in oat into fractions that have similarities in behaviour, and rice endosperm. In other cereals, storage composition and structure. As protein studies proteins arising in protein bodies are exclusively have proceeded and knowledge has accumulated of the insoluble types (payne and rhodes, 1982) the validity of earlier criteria of classiacation nas The number of individual proteins in the been, and continues to be, challenged oluble categories is large. By two-dimensional plant proteins is that which Osborne( 1907)made rated in aqueous extracts from ave been sepa- ne of the most significant means of classifying electrophoresis 160 component on the basis of solubility Water soluble proteins and a different pattern of 140 components have were described as albumins, saline soluble as been separated from the 0.5 M NaCl extracts (lei globulins,, aqueous alcohol soluble as ' prolamins' and reeck, 1986) and those that remained insoluble as glutelins There are differences in amino acid composI- Enzymes ion between proteins in the Osborne classes( see Ch. 14)but there is also heterogeneity within each Enzymes may be considered in the context of class and this may be as significant as between the stage of the grain's life cycle. Thus,most class differences. Newer analytical methods have enzyme activity during maturation is concerned hown that the solubility classes overlap consider- with synthesis, particularly the synthesis of storage66 TECHNOLOGY OF CEREALS secondary and tertiary structures of a protein ably. The distinction formalized by Osborne that change in response to the environment but the remains unquestionably valid is that between primary structure remains unaltered unless its albumins and globulins on the one hand and length is reduced by hydrolysis. prolamins and glutelins on the other. In composi￾All the interactions listed in Table 3.5 can tion there is a marked difference due mainly to contribute to tertiary structure but the most the extremely high content of proline and gluta￾stable types are the covalent disulphide bonds mine in the less soluble fractions (the name formed by oxidation of sulphydryl group on ‘prolamine’ reflects this characteristic). An interacting cysteinekystine residues. extremely low lysine content is also characteristic Such bonds also occur between cysteinekystine of insoluble cereal proteins. residues on different polypeptides giving rise to Soluble proteins a stable structure involving more than one poly￾peptide. Inter-peptide links can thus produce in a protein a fourth or quaternary level of structure. These are found in starchy endosperm, Disulphide bonds are stronger than non-cova- aleurone and embryo tissues of cereals. They lent bonds but they are nevertheless capable of account for approximately 20% of the total pro￾entering into interchange reactions with sub- tein of the grain. Albumins are usually more stances containing free sulphydryl groups. prevalent than globulins. The amino acid com￾Such reactions are of great importance in dough position of soluble proteins is similar to that of formation. proteins found in most unspecialized plant cells suggesting that they include those that constitute the cytoplasm found in most cells. They are a complex mixture including: Cereal proteins The complexity of cereal proteins is enormous 1. metabolic enzymes; 2. hydrolytic enzymes; and the determination of the structure of gluten -the protein complex responsible for the dough- 3. enzyme inhibitors; forming capacity of wheat flour - has been 4. phytohaemaglutenins (proteins that clot red described as one of the most formidable problems blood cells). ever faced by the protein chemist (Wrigley and Bietz, 1988). To simplify their studies cereal Globulins may also arise as storage proteins, chemists have sought to separate the proteins occurring in protein bodies, particularly in oat into fractions that have similarities in behaviour, and rice endosperm. In other cereals, storage composition and structure. As protein studies proteins arising in protein bodies are exclusively have proceeded and knowledge has accumulated, of the insoluble types (Payne and Rhodes, 1982). the validity of earlier criteria of classification has The number of individual proteins in the been, and continues to be, challenged. soluble categories is large. By two-dimensional One of the most significant means of classifying electrophoresis 160 components have been sepa￾plant proteins is that which Osborne (1907) made rated in aqueous extracts from wheat endosperm on the basis of solubility. Water soluble proteins and a different pattern of 140 components have were described as ‘albumins’, saline soluble as been separated from the 0.5 M NaCl extracts (Lei ‘globulins’, aqueous alcohol soluble as ‘prolamins’ and Reeck, 1986). and those that remained insoluble as ‘glutelins’. There are differences in amino acid composi- Enzymes tion between proteins in the Osborne classes (see Ch. 14) but there is also heterogeneity within each Enzymes may be considered in the context of class and this may be as significant as between the stage of the grain’s life cycle. Thus, most class differences. Newer analytical methods have enzyme activity during maturation is concerned shown that the solubility classes overlap consider- with synthesis, particularly the synthesis of storage