PROTEINS 121 



HO— HC CH, 



HjCs^^^^CH-COOH 



N 

 H 



L-Hydroxyproline 



Every amino acid contains at least one amino (NHo) group and one 

 carboxyl (COOH) grouj). (Proline and hydroxyproline may be re- 

 garded as modified amino acids, in which the amino group has been 

 linked to a second carbon, thus forming a ring compound.) One of the 

 amino groups in the above list is always attached to the alpha carbon, 

 hence the name alpha amino acids. Acids with the amino group attached 

 to carbons other than the alpha carbon occur in nature (e.g., beta-alanine 

 in pantothenic acid, beta-lysine in certain antibiotics, and gamma- 

 aminobutyric acid in biological fluids) . Other types of amino acids will 

 probably be found as more plants and microorganisms are investigated. 



The part of the formula other than NHl.CH-COOH is called the side 

 chain and is represented by the letter R. Amino acids differ with respect 

 to their side chains, and, hence, it is this part of the molecule that im- 

 parts distinctive features to the compound. Since the most important 

 chemical and physiological properties of amino acids are attributable to 

 the side chains, the student should note these carefully and become familiar 

 with the groups contained therein. Some of these distinguishing groups 

 are: 



It is evident that the alpha carbon is asymmetric in all of the amino 

 acids except glycine; hence there are two structural forms of the acids. 

 The two forms may be represented by the general formulas where R 



COOH COOH 



I I 



HjN-C-H H-C-NH, 



I I 



R R 



L-Forrn D-Form 



