472 PROTOPLASM 



proportions vary considerably. Casein yields some 20 amino 

 acids. 



Proteins may be defined as substances that on hydrolysis yield 

 amino acids. The intermediate decomposition products, such 

 as the polypeptides, are not proteins in the strict sense but rather 

 are protein derivatives which in their turn yield amino acids. 



An amino acid may be defined as an acid in which one or more 

 of the hydrogen atoms, other than the carboxylic hydrogen, is 

 replaced by the amino radical NH2. For example, by replacing 

 one of the hydrogen atoms attached to the carbon atom in acetic 

 acid, CH3COOH, with the amino radical, NH2, glycine (glycocoll), 

 CH2NH2COOH, the simplest of the amino acids, results. Theo- 

 retically, it should be possible to replace two or all three of the 

 hydrogen atoms in the CH3 group of acetic acid by amino 

 radicals and get diamino and triamino acetic acid, but these 

 two compounds are unknown. The orientation of the atoms in 

 glycine is presumed to be as follows : 



H 



I 

 H— C— NH2 



OH 



While such structural formulas are the nearest possible 

 approach at present to atomic orientation in protein derivatives, 

 the amino group may not be present in the protein molecule as 

 such but possibly in the form of NH groups. 



The next homologue in the fatty-acid series after acetic acid is 

 propionic acid, CH3CH2COOH. It yields two monoamino 

 acids (i.e., acids with one amino radical each) — alpha and beta 

 aminopropionic acid, CH3CHNH2COOH and CH2NH2CH2- 

 COOH, respectively. These two amino acids are distinguished 

 by the position of the amino radical, NH2, which is attached to 

 the alpha carbon atom— the carbon adjacent to the earboxyl 

 group— in the first case, and to the beta carbon atom— next 

 but one from the earboxyl — in the second case. 



The relative positions of the amino and earboxyl radicals 

 determine the type of amino acid. These may be indicated as 

 follows : 



