A\UNO ACIDS , PEPTIDES AND PROTEINS 



215 



Since the number of plant species examined for various amino acids is still very 

 small, it is difficult to reach definite conclusions on the distribution of the amino acids 

 that are not generally found in proteins. Apparently, gamma-aminobutyric acid, pipecolic 

 acid (20), beta gamma-dihydroxyglutamic acid (21) and alpha -amino-adipic acid (22) are 

 rather widely distributed. Others are of only very limited distribution. For example, 

 theanine has been detected only in tea leaves (23), azetidine-2-carboxylic acid appears to 

 be limited to certain species of the Liliaceae family where it was detected in twenty-five 

 per cent of the eighty-nine species examined (24). The results indicate that it will not be 

 possible to predict the distribution of these amino acids from the taxonomic classifica- 

 tion of the plants in which they occur. 



TABLE 6. HYDROXY AMINO ACIDS 



1. homoserine 



2. gamma-hydroxy valine 



3. gamma-hydroxy glutamic acid 



4. beta-gamma-dihydroxy glutamic acid 



5. gamma-methyl-gamma hydroxy- 

 glutamic acid 



HO(CH2)2CH(NH2)C02H 



HOCH2CH(CH3)CH(NH2)C02H 



H02CCH(OH)CH2CH(NH2)C02H 



H02CCH(OH)CH(OH)CH(NH2)C02H 



CH, 

 I ^ 

 HO2CCCH2CHCO2H 



OH NH2 



6. 5-hydroxy pipecolic acid 



PEPTIDES AND PROTEINS 



HO-CH 



I 



CH 



CH, 



\ 



CH, 



r, CH-COpH 



\ ^ 

 N 



H 



Peptides and proteins are condensation polymers of amino acids in which the elements 

 of water have been eliminated between an amino and a carboxyl group. If the molecular 

 weight of the compound is below 6000 it is generally classified as a peptide. All available 

 evidence indicates that the amino acids of the proteins have the L-configuration and that 

 amide bonds form only between alpha amino and alpha carboxyl groups of amino acids. 

 Sixteen to twenty different amino acids are usually found on hydrolysis of a given protein. 

 The amino acids in proteins are linked together to form linear polymer chains with cross- 

 links possible between the sulfhydryl groups of cysteine, resulting in disulfide bridges. 

 Greater variations are found with peptides. For example, peptides are known that contain 

 D-amino acids, others have non-amino acid substituents and some are known in which 

 not all the amide links are between the alpha amino and alpha carboxyl groups. 



