22 PHYSIOLOGY FOR DENTAL STUDENTS. 



molecules, which can be further resolved into simple crystalline 

 substances. These have been called the building stones of the 

 protein molecule, and although they differ from one another in 

 many respects, they have one feature in common, namely, that 

 each consists of an organic acid having one or more of its hydro- 

 gen atoms' substituted by the radicle, NH 2 . Such substances are 

 called amino bodies. For example, the formula of acetic acid 

 is CH 3 CDOH. If for one of the H atoms there is substituted the 

 NH 2 group, we have CH,NH 2 COOH, which is amino acetic acid, 

 or glycine. The same sort of substitution may take place, not 

 alone in the simple organic acids containing one acid group, but 

 also in those containing two acid groups, as in amino-succinic 

 acid, COOH. CH 2 (NH 2 )COOH, or in acids containing the aro- 

 matic or benzene ring group, as in the case of tyrosine, 

 C 6 H 4 OH. C 2 H 3 . NH 2 COOH, or again there may be two amino 

 acid groups present, as in the diamino acid, ornithin or diamino- 

 valeric acid, C 4 H 7 (NH 2 ) 2 COOH. 



That the large and complex protein molecule is really built up 

 out of these amino bodies has been very conclusively shown by 

 Emil Fischer, who succeeded in causing two or more of them to 

 become united to form a body called a polypeptid. When several 

 amino bodies were thus synthesized, the polypeptid was found to 

 possess many of the properties of peptones, which we have just 

 stated are the earliest decomposition products of protein. 



Proteins differ from one another, not only in the nature of the 

 amino bodies of which they are composed (although certain of 

 these are common to all proteins), but also in the manner in 

 which the amino bodies are linked together. We shall see the 

 practical value of knowing what are the amino bodies in a given 

 protein when we come to the subject of dietetics (see p. 99). 



The proteins of the cell are classified into two groups. The 

 first includes the simple proteins, such as egg and serum albumin ; 

 and the second, the compound proteins, from which non-protein 

 groups can be split off. As primary cell constituents, the follow- 

 ing simple and compound proteins are important: albumin, 

 globulin, nucleoprotein, and the glycoproteins. They are all of 

 the nature of colloidal substances (sec p. 32), and therefore are 



