PROTEINS 



129 



character to the compound, while the carboxyl group gives it acidic 

 properties. AVhen two amino acids are joined together, water is elimi- 

 nated, and the acids are linked tlirough the carboxyl group of one and 

 the amino group of the other. TJiis is a fundamental point and should 

 be clearly noted. For example, the combination of glycine and alanine 

 may be represented thus: 



NH2'CH-C0:0H + HjNH-CHCOOH 

 H CH3 



NH2-CH-';C0-NH:-CHC00H -f H2O 



I ■ " I 



H CH3 



Glycine Alanine Glycyl-alanine 



The linkage between the carboxyl group of the glycine and the amino 

 group of the alanine is a peptide linkage. In the formula it is enclosed 

 by (lotted lines. A combination of two amino acids is known as a 

 dipcptide. If three amino acids are joined together, a tripeptide is ob- 

 tained; four amino acids give a tetrapeptide. 



There is obviously another dipeptide of these two amino acids in which 

 the carboxyl group of the alanine is linked to the amino group of the 

 glycine: 



NH2-CH-;C0-NH:CHC00H 

 I I 



CH3 H 



Alanyl-glycine 



If two molecules of glycine or two of alanine are combined, two more 

 peptides, glycyl-glycine and alanyl-alanine, respectively, are obtained. 

 Thus with two amino acids there are four possible peptides. 



A tripeptide consists of three amino acids joined together by two 

 peptide linkages, as is illustrated by the tripeptide, tyrosyl-lysyl-aspartic 

 acid: 



NH2-CH-iC0-NH-;CH-:C0-NH-:CHC00H 

 I ■-- I I 



CH2 (0112)3 CH2 



I I I 



C6H4-OH CHj-NHz COOH 



Tyrosyl-lysyl-aspartic acid 



There are 27 possible tripeptides with three amino acids, provided each 

 acid is used once, twice, or three times in a given combination. The 

 number of combinations (polypeptides) that can be obtained by use of 

 25 different amino acids is almost unlimited. Sherman stated, "If a 

 protein be imagined made up of 30 molecules of 18 different amino acids, 

 one taken twice, one three times, another 3, one 4, one 5 times and 13 



