192 PRACTICAL PHYSIOLOGY 



Proteins have been divided into a number of classes in a pro- 

 visional manner as our present knowledge of their chemical structure 

 does not permit of a strictly chemical classification. The arrange- 

 ment which has met with most general acceptance is as follows : 



(1) Protamines. 



(2) Histones. 



/Q\ All-mminC! ^ 



/A\ ni i, T " r Coagulable, or native proteins. 



(4) Globulins j 



(5) Glutelins ) - *. 



; ' .-,. ,. /T i \ r .tlant proteins. 



(6) Gliadms (Prolammes) j 



(7) Sclero-proteins (Albuminoids). 



(8) Phospho-proteins. 



(9) Conjugated proteins. 



(a) Gluco-proteins (Glyco-proteins). 

 (6) Nucleo-proteins. 

 (c) Chromo-proteins. 



(10) Derivatives of proteins (Hydrolysed proteins). 



These proteins have one thing in common, namely, that when 

 completely hydrolysed by mineral acids, alkalis or ferments, they 

 yield a variety of amino acids. During this process of hydrolysis 

 the large molecule of the protein is broken down and water is taken 

 up. 



The resulting amino acids, which vary, both in number and 

 amount, in the different proteins, are simple combinations in which 

 a hydrogen of a fatty acid is replaced by an amino group (NH 2 ) ; 

 thus, for example, propionic acid is CH 3 CH 2 COOH ; if now one of 

 the hydrogens is replaced by NH a we get alanine or amino propionic 

 acid, CH 3 CH(NH 2 )COOH. The great majority of the amino acids 

 found in the tissues have a hydrogen atom attached to the carbon 

 atom nearest the carboxyl group replaced by the NH 2 and they are 

 therefore a-amino acids. Further, some of the amino acids in place 

 of having a single NH 2 group attached have two. The amino acids 

 present may be divided into three classes. 

 I. Monoamino acids : 



(a) with one carboxyl group : glycine, alanine, leucine, etc. 

 (6) with two carboxyl groups : aspartic and glutamic 



acids. 



(c) of the aromatic series : tyrosine, phenylalanine. 

 II. Diamino acids : Lysine, arginine, (cystine), etc. 

 III. Heterocyclic amino acids : histidine, tryptophan, etc. 



