APPENDIX. 



PROTEINS AND THEIR CLASSIFICATION. 



Definition and General Structure. Proteins or albumins are complex 

 organic compounds containing nitrogen which, although differing much in 

 their composition, are related in their properties. They are formed by 

 living matter, and occur in the tissues and liquids of plants and animals, 

 of which they form the most characteristic constituent. On ultimate analy- 

 sis they are all found to contain carbon, hydrogen, oxygen, and nitrogen; 

 most of them contain also some sulphur, and some, in addition, phosphorus 

 or iron. As usually obtained, they leave also some ash when incinerated, 

 showing that they hold in combination some inorganic salts. Percentage 

 analyses of the most common proteLas of the body show that the above 

 named constituents occur in the following proportions : 



Carbon 50 to 55 per cent. 



Hydrogen 6.5 to 7.3 " " 



Nitrogen . . .' 15 to 17.6 " " 



Oxygen 19 to 24 " " 



Sulphur 0.3 to 2.4 " " 



The clearest insight into the structure of the protein molecule has been 

 obtained by a study of its decomposition products. When submitted to the 

 action of proteolytic enzymes, or putrefaction, or acid at high temperatures, 

 the large molecules split into a number of simpler bodies in consequence of 

 hydrolytic cleavage. These end-products are very numerous, and, while 

 they differ somewhat for the different proteins, yet a number of them are 

 the same or similar for all proteins. The great variety in the end-products is 

 an indication of the complexity of the molecule, while their similarity is proof 

 that the various proteins are all built, so to speak, upon a common plan, by the 

 union of certain groupings which may be more numerous in one protein than 

 in another. This fact becomes evident from a brief consideration of the prod- 

 ucts obtained by hydrolytic cleavage with acids. The groupings represented 

 by the following compounds may be supposed to exist preformed in protein 

 molecules, some possibly containing them all, some only a portion of the 

 list, while the different groups vary in their proportional amounts in the 

 various proteins (see p. 793) : 



Amino-acids. 



1. Glycocoll or glycin (amino-acetic acid). 



2. Alanin (aminopropionic acid). 



3. Valin (aminovalerianic acid). 



4. Leucin (aminocaproic acid). 



5. Isoleucin (aminocaproic acid). 



6. Norleucin (aminocaproic acid). 



7. Lysin (diaminocaproic acid). 



8. Serin (oxyaminopropionic acid). 



9. Cystein (aminothiopropionic acid). 



10. Phenylalanin (phenylaminopropionic acid). 



11. Tyrosin (oxyphenylaminopropionic acid). 



12. Tryptophan (indolaminopropionic acid). 



13. Histidin (imidazolaminopropionic acid). 



14. Arginin (guanidinaminovalerianic acid). 



15. Aspartic acid (aminosuccinic acid). 



16. Glutaminic acid (aminoglutaric acid). 



17. Prolin (pyrollidin-carboxylic acid). 



18. Oxyprolin (oxypyrollidin-carboxylic acid). 

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