PROTEIDS AND THEIR CLASSIFICATION. 875 



These split products are all amido-acids, some of them belonging to the 

 fatty acid (aliphatic) series of carbon compounds, some to the aromatic 

 (carbocyclic) series, and some to the heterocyclic (pyrrol, indol) series. In 

 what may be considered the simplest proteids occurring in nature, namely, 

 the protamins found in the spermatozoa, only from four to six of these groups 

 occur, while in some of the more familiar proteids, such as serum-albumin 

 or casein, a much larger number is found. The a-amido-acids of which these 



H 



end-products consist all contain the grouping C NH 2 , and Fischer has 



COOH 



shown that such bodies possess the property of combining with one another 

 to make complex molecules containing two, three, or more groups of amido- 

 acids. The combination takes place with the elimination of water formed 

 by the union of the OH of the carboxyl (COOH) group in one acid and the 

 H of the amido (NH 2 ) group in another. Thus, two molecules of amido-acetic 

 acid (glycocoll) may be made to unite to form a compound, glycylglycin, as 

 follows : 



NH 2 CH 2 COOH + NH 2 CH 2 COOH H 3 O = NH 2 CH 2 CONHCH 2 COOH. 



Glycocoll. Glycocoll. Glycylglycin. 



Compounds of this kind are designated by Fischer as peptids. When formed 

 from the union of two amido-acids they are known as dipeptids, from three 

 as tripeptids, etc. The more complicated compounds of this sort, the poly- 

 peptids, begin to show reactions similar to those of the proteids. Some of 

 them give the biuret reaction, some are acted upon and split by proteolytic 

 enzymes. It seems justifiable, therefore, to consider proteids as essentially 

 polypeptid compounds of greater or less complexity, that is, they are acid- 

 amids formed by the union of a number of a-amido-acid compounds. This 

 conception of the structure of the proteid molecule explains a number of 

 their general characteristics, for instance: (1) The fact that they are all 

 decomposed and yield similar products under the influence of proteolytic 

 enzymes or boiling dilute acid. (2) The fact that the proteids are all so 

 alike in their general properties in spite of the great differences in the com- 

 plexity of their molecular structure. (3) The fact that they show both 

 basic and acid characters. (4) The fact that they all give the biuret reac- 

 tion* (see below). 



In addition to the amido-acids some proteids egg-albumin, for example 

 yield a carbohydrate body upon decomposition. The carbohydrate ob- 

 tained is an amido-sugar compound, usually glucosamin, C 6 H 15 NO 5 . It is 

 detected by its reducing action and by the formation of an osazone. It seems 

 probable, therefore, that some of the proteids at least contain such a group- 

 ing as part of the molecular complex, but at present it is undetermined how 

 many possess this peculiarity of structure. 



General Reactions of the Proteids. It is evident from what has been 

 said in the preceding paragraph that proteids may give different reactions 

 according to the kinds of groupings contained in the molecule. The reac- 

 tions common to all proteids are few in number, the most certain perhaps 

 being the biuret reaction, the hydrolysis by proteolytic enzymes or putre- 

 factive organisms, and the nature of the split products formed by these latter 

 hydrolyses or by the action of boiling dilute acids. A very large number 

 of reactions, however, have been described which hold for some or all of 

 the proteids usually found in the tissues and liquids of the body. These 

 reactions may be described under two heads: (1) precipitation of the proteid 

 when in solution ; (2) color reactions. 



* For further details see Cohnheim, "Chemie der Eiweisskorper," second 

 edition, 1904; or Hammarsten, "Physiological Chemistry/' translated by 

 Mandel, fourth edition, New York, 1904. 



