THE METABOLISM OF PROTEIN 639 



characterized by containing two 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 



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, 



CH 2 SH - CH 



COOH 



Two cysteine molecules united at the ' ' S " groups give cystine. 

 NH 2 1 



CH 2 S - CH 



COOH 



NH 2 



3. Phenylalanine has a C 6 H 4 (phenyl) group, CH 2 C 6 H 5 -CH 



COOH 

 NH 2 



4. Tyrosine has a C 6 H 4 OH (phenol) group, CH 2 C 6 H 4 OH - CH 



COOH 

 C 



5. Tryptophane has a C 6 H 4 CH (indole) group : 



NH 

 C CH 2 - CH - NH 2 - COOH. 



C 6 H 4 CH 



NH 

 CH 



N NH 



6. Histidine has a CH C - (imidazole) group: 



