32 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



are found among the final cleavage products of most proteins ; but 

 there are a good many more in addition, for instance : 



Serine (amind-oxypropionic acid, CH 2 OH.CHNH 2 .COQH) ; 



Amino-valeric acid ; 



Amino-succinamic acid (asparagine) ; 



Amino-succinic acid (aspartic acid) ; 



Amino-pyrotartaric acid (glutamic acid), 



some of which are derived from fatty acids of a different series from 

 those first enumerated. 



But in all these cases there is only one replacement of an atom 

 of hydrogen by NH 2 ; hence they are called monoamino-acids. 



Passing to the next stage in complexity, we come to another 

 group of amino-acids which are called diamino-acids : that is, fatty 

 acids in which two hydrogen atoms are replaced by NH 2 groups. Of 

 these we may mention lysine, ornithine, arginine, and histidine. 



Lysine is diamino-caproic acid. 



Caproic acid is C 3 H H .COOH. 



Mono-amino-caproic acid, or leucine, we have already learnt is 

 C 5 H 10 .NH 2 .COOH. 



Lysine or diamino-caproic acid is C 5 Hy(NH 2 ) 2 COOH. 



Ornithine is di-amino-valeric acid, and the following formulae will 

 show its relationship to its parent fatty acid : 

 C 4 H 9 .COOH is valeric acid. 

 C 4 H 7 (NH 2 ) 2 .COOH is diamino-valeric acid or ornithine. 



Arginine is a somewhat more complex substance, which contains 

 the ornithine radical. It belongs to the same group of substances as 

 creatine, another important cleavage product of the protein molecule. 



Creatine is methyl-guanidine acetic acid, and has the formula 



HN\ 



C -N(CH 3 )CH 2 .GOOH 

 H 2 N/ 



On boiling it with baryta water, it takes up water (H 2 0) and splits 

 at the dotted line into urea [CO (NH) 2 ] and sarcosine, as shown 



below. 



H 2 N\ 



C=0 



NH.CH,.CH 9 .COOH 



H 2 N/ 



[urea] [sarcosine or methyl-glyciiie] 



Arginine decomposes in a similar way, urea being split off on 

 the left, and ornithine instead of sarcosine on the right. Arginine is 

 therefore a compound of ornithine with a urea group. 



Histidine, the last member of this class, has the formula 



