PROTEINS 



113 



PRODUCTS ON HYDROLYSIS— AMINO ACIDS 



Number and kind of amino acids 



\Mien a protein is iiydrolyzed by means of acid, alkali, or enzymes, 

 alpha amino acids, usually called amino acids, are obtained as products. 

 If the protein is a conjugated protein, such as casein, nucleoprotein, 

 etc., other products are obtained in addition. Only the amino acids, 

 however, come from the protein molecule proper; the additional com- 



Courtesy ol' W. C. Ko.se and Joiiniul of liioloijicul Chemistry. 

 Fig. 5-5. Threonine. 



pounds originate in the prosthetic group. The number of amino acids 

 that have been definitely obtained as a result of hydrolysis is about 

 twenty-five; about fifteen more have been reported, but their presence 

 in protein is either not well established or they occur only in unusual 

 protein materials, e.g., djencolic acid of the djencol bean and a,y-diamino- 

 butyric acid in the polymyxin antibit)tics (pp. 118, 119). Several others, 

 citrulline, homocysteine, and cystathionine, do occur free in the body 

 tissues and play an important role in intermediary metabolism. Amino 

 acids are classified according to the number of amino groups, carboxyl 

 groups, and other characteristics as shown on pp. 116-120. 



