158 INTERMEDIATE METABOLISM 



constituents it can be resolved by boiling with acids or by 

 the action of certain ferments. 



All Amino-acid is an organic acid in which a hydrogen 

 atom, other than that of the carboxylic group, is replaced 

 by an NHg group. In all the amino-acids occurring in 

 nature, such substitution occurs in the a position. 



The general formula of an amino-acid is therefore — 



I 

 R— C— COOH 



I 

 • H 



An amino-acid can be regarded not only as an acid con- 

 taining an NH2 group, but as a substituted ammonia. It 

 is therefore an acid at one point and a base at another. 

 For this reason the acid group of one amino-acid can, 

 under certain circumstances, combine with the basic group 

 of another, thus — 



CH3CHNH2COOH + NH2CH2COOH 



= CH3CHNH2CONHCH2COOH 



It will be observed that in this new compound there are 

 still a COOH group and an NH2 group intact. This process 

 of condensation can therefore, theoretically, be continued 

 indefinitely. The compounds thus formed are called di- 

 tri- poly-peptides, according to the number of amino- 

 acid molecules composing them. The most complex poly- 

 peptide hitherto made artificially contains eighteen amino- 

 acid molecules. 

 The following are the principal amino-acids : — 



The Principal Amino-acids 



I. Aliphatic Series. 



Glycine (amino-acetic acid) 



CH2NH2COOH 



