20 BIOLOGICAL CHEMISTRY 



Urea 



is the di-amide of carbonic acid and it can be considered as 

 formed from ammonium carbonate by the removal of two 

 molecules of water. The removal of one molecule of water 

 from ammonium carbonate produces the intermediate sub- 

 stance ammonium carbamate. The chemical characteristics 

 of urea will be discussed later (p. 182). 



OH ONH 4 ONH 4 NH 2 



=-H 2 c=0 ' Ha C = 



\ ~^ \ "" \ 



OH ONH 4 NH 2 NH 2 



Carbonic acid. Ammonium carbonate. Ammonium carbamate. Urea 



Other nitrogen-containing compounds such as uric acid, 

 creatine and creatinine will be considered later. 



Amino -acids, 



as their name suggests, consist of fatty acids to which an 

 amine group is attached. The amine group is attached to 

 the carbon atom next to the carboxyl group (a position). 

 The acidity of the carboxyl group is decreased by the presence 

 of the amino group and the alkalinity of the amino group is 

 decreased by the presence of the acid group. Thus we have 

 formed neutral substances which can unite with bases by their 

 acid group and with acids by their amine group. They are 

 therefore called amphoteric substances. 



The simplest amino acid is derived from acetic acid ; the 

 second member of the series is derived from propionic acid 

 and is of importance because of the many compounds into 

 which it enters and its relation to many other substances 

 (see Chapter VII). 



CH 3 COOH CH 2 (NH 2 )COOH CH 3 'CH 2 COOH 



Acetic acid. Amino-acetic acid or glycine. Propionic acid, 



CH 3 CH(NH 2 )COOH 



Amino-propionic acid or alanine. 



Unfortunately these simple formulae do not suffice for all 

 purposes. The presence of other groups complicates the 

 subject to a certain extent. Two amine groups occur in some 

 amino acids and an amino acid may be formed from an acid 

 containing two carboxyl groups. In the former case the 

 substance is more basic and in the latter case more acid than 

 the simple amino acids. 



Substituted alanines are formed by slight alterations in the 



