Chapter 8 8.2 Essential and nonessential amino acids An amino acid is defined as a compound that possesses both amino and carboxyl roups. Some amino acids are aminocarboxylic acids such as proline while others are sulphur containing amino acids, such as cysteine and methionine table 8.2). Over 10 amino acids have been isolated and identified from natural sources to date. The great majority of these naturally occurring amino acids have the amino group attached to the a-carbon carbon a to the carboxylic acid. With very few exceptions, the a-carbon also be ydrogen atom. The fourth bond of the a-carbon is joined to a group which has 100 variations. Thus, most of the naturally occurring amino acids differ only structure of the organic residue attached to the a-carbon An interesting and important fact is that almost all amino acids isolated from proteins L-configuration have the L-configuration at the a-carbon, although some amino acids isolated from microbiological sources are the mirror image isomers, ie in the D-configuration. We shall consider amino acid stereochemistry in more detail in section 8.3 Of the amino acids isolated from living material only about 20 are naturally occurring components of proteins. Some of these are shown in Table 8. 2. The remainder, non-proteinogenic amino acids, are found as intermediates or end products of One of the amino adids commonly found in protein hydrolysates is called cystine; it has the following structure HOOC- CH- CH2 -S-S- CH2- CH-COOH NH H2 of cysteine, where the thiol of two monomers spaced intervals in the polypeptide are j a disulphide bridge asic amino acid is cysteine and consequently the is not included here234 Chapter 8 8.2 Essential and nonessential amino acids a-carbon Lconfiguration dimer An amino acid is defined as a compound that possesses both amino and carboxyl groups. Some amino acids are iminocarboxylic acids, such as proline while others are sulphur containing amino acids, such as cysteine and methionine (Table 8.2). Over 100 amino acids have been isolated and identified from natural sources to date. The great majority of these naturally occumng amino acids have the amino group attached to the carbon a to the carboxylic acid. With very few exceptions, the a-carbon also bears a hydrogen atom. The fourth bond of the a-carbon is pined to a group which has over 100 variations. Thus, most of the naturally occurring amino acids differ only in the structure of the organic residue attached to the a-carbon. An interesting and important fact is that almost all amino acids isolated from proteins have the L-configuration at the a-carbon, although some amino acids isolated from microbiological sources are the mirror image isomers, ie in the Dconfiguration. We shall consider amino acid steremhemistry in more detail in section 8.3. Of the amino acids isolated from living material, only about 20 are naturally Occurring components of proteins. Some of these are shown in Table 8.2. The remainder, non-proteinogenic amino acids, are found as intermediates or end products of metabolism. One of the amino acids commonly found in protein hydrolysates is called cystine; it has the following structure: HOOC- CH-CH,-S-S-CH,-CH-COOH (CYSCYS) I I NH2 NH2 cystine It is clearly a dimer of cysteine, where the thiol groups have been oxidised to form a disulphide linkage. The dimer actually results because of two monomers at widely spaced intervals in the polypeptide are joined together by a disulphide bridge. Thus the basic amino acid is cysteine and consequently, the dimer is not included here