CLASSIFICATION OF PROTEINS ICl 



Proline 



H^C 



C^H 



m. 



CHCOOH 



NH 



Oxyproline 



HOHC 



H^C 



CH2 



CHCOOH 



NH 



Classification of Proteins 



Proteins are divided into two main groups, 



1. Simple Proteins, conforming to the definition of a 

 protein given above. Such are fibrinogen of blood, myosin 

 of muscle, casein of cheese. These are classified into sub- 

 groups, e.g. albumins, globulins, etc., according to their 

 solubility and precipitabihty by certain reagents. 



2. Conjugated Proteins. — In these the protein molecule 

 is hnked with a non-protein molecule ; with nucleic acid, 

 for instance, in nucleo-proteins. 



Hydrolysis of Proteins 



In the breakdown of proteins to amino-acids certain 

 intermediate stages are recognised. The disruption of the 

 protein molecule is a gradual process, involving the suc- 

 cessive subdivision of ever-shortening chains of amino- 

 acids. The diminution in size of the molecules is accom- 

 panied by a physical change involving increase in solubihty 

 and decrease in precipitabihty. 



The first recognisable change is that the molecule, if 

 originally completely insoluble, becomes soluble in dilute 

 acid or alkah, but the solution is easily precipitated and 

 is coagulated by heat. In this stage it is called a meta- 

 protein. It then becomes soluble in water, is not coagulated 

 by heat, and requires half-saturation with ammonium 

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