THE METABOLISM OF PROTEIN 603 



by the Van Slyke method; but in other cases, as in arginine, it fails to 

 give this and the other characteristic reactions of the amino group. That 

 the extra amino group in lysine reacts directly with nitrous acid explains 

 why various proteins when examined for amino nitrogen yield an amount 

 that is equal to half of the lysine nitrogen. 



It will further be observed that the amino acids are arranged in three 

 main groups: one basic, another neutral, and the third acid. The acids 

 of the basic group are three in number arid have an alkalinity similar to 

 that of ammonia. They have been called the hexone bases, because each 

 contains six carbon atoms. They are alone present in certain forms of pro- 

 tein called protamines. The neutral amino acids contain one amino group 

 and one carboxyl group, which exactly neutralize each other. This is 

 the largest group of amino acids, and is further subdivided into three: 

 one containing aromatic or benzene rings and including the very im- 

 portant amino acids, tyrosine and tryptophane; another containing the 

 so-called pyrrolidine ring; and the third, the largest of all, containing 

 the so-called aliphatic chains; that is, the chains characteristic of the 

 fatty acids and which may be either straight or branched. When the chains 

 are branched, the substance is called an isosubstance, as in isoleucine. 

 The acid amino acids, including glutamic acid and aspartic acid, are 

 characterized by containing tw^o carboxyl groups and only one amino 

 group. They therefore resemble acetic acid in acidity. 



It may be of assistance to some if we restate these chemical facts 

 from a .slightly different standpoint as follows: 



Glycine, or glycocoll, is aminoacetic acid, CH 2 NH 2 COOH. 



NH 2 



Alanine is glycine plus a methyl group, CH 3 CH ; it is therefore amino- 



COOH 



OH 



/ 

 propionic acid and is closely related to lactic acid, which is CH 3 CH . Many of 



COOH 



the other amino acids may be considered as derivatives of alanine^ thus: 



1. Serine is alanine with an "OH" (hydroxyl) group in place of one of the "H" 



NH 2 

 / 

 atoms of the methyl group, CH 2 OH-CH 



COOH 



2. Cysteine is alanine .with an "SH" (thio) group in this position, 



NH, 



CH 2 SH - CH 



COOH 



