PROTEINS 167 



many as eighteen of these amino-acid units have been caused to 

 unite together in this way, and the resulting compounds thus 

 artificially prepared have been found to possess the characteristic 

 properties of natural proteins. 



These artificially prepared, protein-like, substances have been 

 called " polypeptides," and the individual amino-acids which 

 unite together to form them are called " peptides." Thus, a 

 compound which contains three such units linked together is 

 called a " tripeptid "; one which contains four, a " tetrapeptid." 

 The use of the term " peptid " was suggested by the fact that these 

 amino-acids are produced from the hydrolysis of proteins by the 

 digestive enzyme pepsin. 



The peptid units of any such complex as those which have been 

 referred to in the preceding paragraphs may be linked together in a 

 great variety of ways. Thus, in a tetrapeptid containing units 

 which may be designated by the letters a, b, c, and d, the arrange- 

 ment may be in the orders abed, bacd, acbd, dbca, etc., etc. Sim- 

 ilarly, the same peptid unit may appear in the molecule in two or 

 more different places. Hence, the number of possible combina- 

 tions of amino-acids into protein molecules is very great. Further, 

 it is possible that the peptid units in natural proteins may be 

 united together through other linkages than the one illustrated 

 above, as they often contain alcoholic OH groups in addition to 

 the basic NH2 groups, and these OH groups may form ester- 

 linkages with the acid (COOH) groups of other units. Still other 

 acid and basic groups are present in some of the amino-acids 

 which have been found in natural proteins, so that the possibility 

 of variation in the polypeptid linkages is almost limitless. 



INDIVIDUAL AMINO-ACIDS FROM PROTEINS 



About twenty different amino-acids have been isolated from 

 the products of hydrolysis of natural proteins, and this number is 

 being added to from time to time, as the methods of isolation and 

 identification of these compounds are improved. Many of these 

 same amino-acids have been found in free form in plant tissues, 

 particularly in rapidly growing buds, or shoots, or in germinating 

 seeds, where they undoubtedly exist as intermediate products in 

 the transformation of proteins into other types of compounds. 



