364 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



The amino acids can be combined together in any order, e.g. a-b- 

 c-d, b-c-a-d, etc. Consequently an enormous number of isomers 

 is possible. 



The chief evidence which we possess in support of this polypeptide 

 constitution of the proteins is (i) the hydrolysis of these polypeptides 

 by trypsin and other proteoclastic enzymes into their constituent 

 units in the same way as the natural proteins are hydrolysed, (2) the 

 isolation of polypeptides from the natural proteins, e.g. glycyl-tyrosine. 



Classification. 



The known proteins are classified according to their origin, solu- 

 bility, coagulability on heating and other physical properties without 

 reference to their chemical composition. This classification is, how- 

 ever, borne out by their actual chemical composition as far as it is 

 known. 



The following is the classification adopted by the Chemical and 

 Physiological Societies in 1 907 : 



1. Protamines. 



2. Histones. 



3. Albumins^i 



~. , .. vCoagulable Proteins. 



4. GlobulmsJ 



5. Glutelins. 



6. Gliadins. 1 / 



7. Scleroproteins. 



8. Phosphoproteins. 



9. Conjugated Proteins. 



(a) Nucleoproteins. 

 (&) Glucoproteins. 

 (c) Chromoproteins. 



10. Derivatives of Proteins. 



(a) Metaproteins. 



(b) Proteoses. 



(c) Peptones. 



(d) Polypeptides. 



1 Prolamins is the American terminology of this group. 



