262 PRINCIPLES OF GENERAL PHYSIOLOGY 



of organic food stuffs as they pass through the organism, commencing our study 

 with proteins. 



Ttie Constitution of Proteins. As was first definitely and completely shown 

 by Emil Fischer (1899-1906), proteins, animal and vegetable, are composed of 

 a series of amino-acids united by elimination of water, as described on page 103 

 above. This work is of such fundamental importance that I have thought it 

 necessary to introduce a portrait of Fischer in Fig. 73. 



The following is a list of all the constituents isolated from various proteins 

 up to the present: 



Mono-amino-mono-carboxylic A cids. 



Glycine (amino-acetic), alanine (amino-propionic), amino-butyric, valine (amino- 

 isovalerianic), leucine (amino-isobutyl-acetic), isoleucine (-armno-/3-methyl-/:?-etli\ I 

 propionic), phenyl-alanine,, tyrosine (para-oxyphenyl-amino-propionic), serine (a- 

 amino /i-oxy-propionic), cystine (condensation of 2 molecules of a-amino-^8-thio- 

 propionic acid, the sulphur constituent of proteins). 



Mono-amino-di-carltoxylic A cids. 

 Aspartic (a-amino succinic), glutamic (a-amino-glutaric). 



Di-amino-mono-carboxylic A cids. 

 Arginine (a-aimno-8-guanido-valerianic acid). Guanidine is NH = C-NH . 



NH, 

 Lysine (di-amino-caproic) that is : 



NH, - CH, - CH 2 - CH, - CH, - CH(NH,) - COOH. 



Heterocycllc Compounds. 



Histidine (/3-iminazol-a-amino-propionic acid). 

 Proline (a-pyrrolidine carboxylic acid). Pyrrolidine is : 



C/HO L/ HO 



C/H.2 CHq 



NH 



Oxy-proline (y- or /i?-oxy-a-pyrrolidine carboxylic acid). 

 Tryptophane (constitution given on page 256). 



Ammonia. 



For further details see the monographs by Plimmer (1912, 1913). 



How far have all the constituents been accounted for ? The analysis of zein 

 by Osborne and his co-workers account for 85'4 per cent, of the total nitroircn 

 of the protein and, considering the inevitable losses in the mono-carboxylic acids, 

 the result must be regarded as very satisfactory. 



These amino-acids combine together in the way already indicated, that is : 



CO | OH H | NH 

 becomes, by elimination of water : 



CO-NH- 

 which is known as the "peplide linkage." 



In this way, Fischer has prepared a large number of peptides, containing two 

 or more amino-acids, in great variety. 



The proteins of the tissues may, therefore, be regarded as made up by various 

 selection out of the list given above, and in different relative proportion. Tims, 

 while gelatine contains 16 per cent, of glycine and 0'9 per cent, of glutamic acid, 

 gliadin of wheat contains no glycine, or extremely little, and 43 per cent, of 

 glutamic acid. 



We see that there are very te\vfree JV77, groups in a protein. 



Van Slyke and Birchard (1914), in fact, show that the only free NH 2 groups in all the 

 native proteins examined, a considerable number, amount to one-half of those of the lysine 

 contained therein. All the other amino-groups are condensed into peptide linkings. 





