20 THE CHEMISTRY AND PHYSICS OF THE CELL 



tained in these different ways indicates that there are definite lines of 

 cleava<i'e in the protein molecnle along which separation takes place, 

 independent of the nature of the agency at work, and that the sub- 

 stances obtained represent, as the Germans figuratively say, the 

 "building stones" of the entire molecule. 



These substances all have in common one important point : each one 

 is an acid, which has a NIIo group substituted for a hydrogen atom on 

 the carhon nearest the acid radical (the a-position). It makes no dif- 

 ference what the rest of the radicals are, whether they are simple 

 chains (leucine), or members of the cyclic or aromatic series (ty- 

 rosine), or sulphur-containing bodies (cystine), withou.t exception 

 this relation of a NIL group to an acid radical is constant, as in this 

 formula: XH, 



/ 

 R— CH— COOH. 



Through this arrangement every one of the constituents of the 

 protein molecule is provided with a group with a strong basic char- 

 acter and a group with a strong acid character, and hence it is pos- 

 sible for them to unite with one another in indefinite numbers, and, 

 because of the great variety of individuals, in practically an infinite 

 number of combinations. It is believed that it is in just this way that 

 the protein molecule is built up. By artificially uniting various 

 cleavage products Emil Fischer has succeeded in producing large 

 molecules made up of several amino-acid radicals (called by him 

 "polypeptids")3 which show some of the characteristics of the pep- 

 tones, and this is the nearest that investigators have yet come to synthe- 

 sizing a protein molecule. The union is accomplished by the split- 

 ting off of water, corresponding to the addition of water that occure 

 when the protein molecule undergoes cleavage. It may be illustrated 

 by showing the formation of the simplest polypeptid, gbjcijighjcine. 



NH, o NH, o 



CHo — C— l OH + HI HN — CH; — COOH = CH2 — C — HN — CHo — COOH + H:0. 

 (glycocoll) (glycocoll) (glycylglycine) 



For these reasons it is believed that the protein molecule consists 

 of great nunibers of amino-acid groups, comhined ivith one another 

 through their hasic and acid radicals, and that the various proteins 

 are different from one another because they contain different num- 

 bers or varieties of amino-acids. For example, the glohin of hemo- 

 globin yields no glycocoll on hydrolysis, while gelatin yields 16.5 per 

 cent. On the other hand, gelatin is free from tyrosine. Some of 

 the protamins (proteins obtained chiefly from spermatozoa) yield as 

 high as 58 to 84 per cent, of arginine, while the simpler amino-acids 

 Avith but one N (mono-amino-acids) are scanty, and most varieties 

 are lacking. 



It will be noticed that when two amino-acids unite, as seen in 



3 Reviewed by Fisolior, in Rer. cliMit. Cliom. Gosoll., 1906 (."^9). ri.50. 



