CHEMICAL COMPONENTS 65 soluble class includes galacturonans(pectins), A sequence of a large number of units linked (1→3,1)4)-阝- glucans and arabinoxylans by peptide bonds is called a polypeptide The differences among amino acids lie in the Proteins side-chains attached to the carbon atom lying between their carboxyl and amino groups. Side Although an enormous range of proteins exists chains may be classified according to their capa in nature they are all composed of the same city for interacting with other amino acids by relatively simple units: amino acids. The diversity different mechanisms. The types of interaction of proteins comes about because the amino acids and the amino acids capable of engaging in them are arranged in different sequences and those are listed in Table 3. 5 sequences are of different lengths. There are only twenty amino acids commonly found in proteins TABLE 3.5 ereal proteins are important in human and Grouping of Amino Acid Residues According to their Capacity for animal nutrition, they provide the unique gas- retaining qualities in wheat four doughs and Type of interaction Amino acid bread,but in all organisms proteins are present (1)Covalent-disulphide bonding Cysteine/c which function as enzymes. within the growing Dissociated by oxidizing and reducing plant the genetic code is interpreted through the agents, e.g. performic aci synthesis and activation of enzymes, providing (2) Neutral- hydrogen bonding the means by which characteristics of individual Dissociated by strong H-bonding Gl species are expressed. When seen in the context agents, e.g. urea, dimethyl formamide Threonine of this function it is perhaps easier to appreciate the subtlety of the differences in behaviour that (3)Neutral -hydrophobic interaction Tyrosine can be achieved among what, at first sight, appear to be molecules of relatively simple construction Dissociated by ionic and non-ionic Phenylalanine The subtle functional differences are possible chain fatty acids because of the diversity of the properties of the amino acids and the relationships in which they Isoleucine are capable of engaging with other amino acids or even with lipid, carbohydrate and other mole cules. Additional variation comes about as a result (4)Electrostatic-acid hydrophilic ic hydrophilic of the environment in which a protein finds itsel A change in pH, temperature or ionic strengt Dissociated by acid, alkali, or salt Histidine can lead to a single protein species behaving in different ways Amino acids with short, aliphatic side-chains show very oluble in wate Structure Table from Simmonds, 1989 All amino acids have in common the presence of an alpha-amino group(nh,)and a carboxyl The order in which amino acids occur in group(-COOH). It is through the condensation polypeptide defines its ' primary structure. Because of these groups that neighb bouring amino acids of the range of interactions that can occur among are joined by a peptide bond as in Fig. 3.11 the side-chains, different sequences are capable of different interactions giving rise to a secondary structure. The units of secondary structure in turn react to give rise to the tertiary structure which defines the three-dimensional conformation FIG 3. 11 The pepti adopted as a result of side- chain interactions theCHEMICAL COMPONENTS 65 A sequence of a large number of units linked by peptide bonds is called a polypeptide. The differences among amino acids lie in the side-chains attached to the carbon atom lying between their carboxyl and amino groups. Side chains may be classified according to their capa￾city for interacting with other amino acids by different mechanisms. The types of interaction and the amino acids capable of engaging in them are listed in Table 3.5. soluble class includes galacturonans (pectins), (1+3,1+4)-P-glucans and arabinoxylans. Proteins Although an enormous range of proteins exists in nature they are all composed of the same relatively simple units: amino acids. The diversity of proteins comes about because the amino acids are arranged in different sequences and those sequences are of different lengths. There are only twenty amino acids commonly found in proteins. cereal proteins are important in human and animal nutrition, they provide the unique gas￾retaining qualities in wheat flour doughs and bread, but in all organisms proteins are present which function as enzymes. Within the growing plant the genetic code is interpreted through the synthesis and activation of enzymes, providing the means by which characteristics of individual species are expressed. When seen in the context Serine of this function it is perhaps easier to appreciate Cysteine the subtlety of the differences in behaviour that Tyrosine can be achieved among what, at first sight, appear Tryptophan Phenylalanine to be molecules of relatively simple construction. Proline The subtle functional differences are possible chain fatty acids Methionine because of the diversity of the properties of the Leucine Isoleucine amino acids and the relationships in which they Valine are capable of engaging with other amino acids Alanine* or even with lipid, carbohydrate and other mole- Glycine* Aspartic acid cules. Additional variation comes about as a result Lysine of the environment in which a protein finds itself. Arginine A change in pH, temperature or ionic strength Histidine can lead to a single protein species behaving in different ways. TABLE 3.5 Grouping ofAmino Acid Residues According to their Capacity for Interacting Within and Between Protein Chains Type of interaction (1) Covalent - disulphide bonding Dissociated by oxidizing and reducing agents, e.g. performic acid; 2-mercaptoethanol (2) Neutral - hydrogen bonding Asparagine Dissociated by strong H-bonding Glutamine agents, e.g. urea, dimethyl formamide Threonine Amino acid Cysteinelcy stine (3) Neutral - hydrophobic interaction Dissociated by ionic and non-ionic detergents, e.g. sodium salts of long (4) Electrostatic - acid hydrophilic - basic hydrophilic Dissociated by acid, alkali, or salt solutions Glutamic acid * Amino acids with short, aliphatic side-chains show very little hydrophobicity. Both glycine and alanine are readily soluble in water. Structure Table from Simmonds, 1989. All amino acids have in common the presence of an alpha-amino group (NH,) and a carboxyl group (-COOH). It is through the condensation of these groups that neighbouring amino acids are joined by a peptide bond, as in Fig. 3.11. The order in which amino acids occur in a polypeptide defines its ‘pm’may structure’. Because of the range of interactions that can occur among the side-chains, different sequences are capable of different interactions giving rise to a seconday structure. The units of secondary structure in turn react to give rise to the tertiary structure which defines the three-dimensional conformation adopted as a result of side-chain interactions. The 0 OH C-OH + Had -C- N￾I II FIG 3.11 The peptide bond