TDK METABOLISM OF PROTEIN 603 



called peptides. This linking occurs between the amino group of one 

 amino arid and the carboxy] group of the other. When alanine and gl; 

 coll, with which we are familiar, are thus linked together, thi stiou 

 takes place according to the equation: 



II CB II 



CB 



/ Il+lin OC C Ml HooC-C-Ml 00 C Ml ■ J: 

 BOOC C N '- 



,, \h 11 II II 



(alanini (gly (.11 1 (alanyl - glycocoll) 



In this manner, then, a Bo-called dipeptide is formed, in which it will 

 be observed there still remain free carboxyl and amino groups, thus per- 

 mitting the linking on of other aniino-acid groups to form tripeptides 

 tetrapeptides or ether polypeptides. Indeed, this process of condensa- 

 tion may go on practically indefinitely, a polypeptide containing eighl 

 amino-acid groups namely, three leucine and fifteen glycocoll groups — hav- 

 ing actually been synthesized. The resulting polypeptides have the proper- 

 ties of derived proteins like the proteoses; thus, they give the biuret 

 and other reactions characteristic of proteins and are precipitated by 

 such reagents as mercuric chloride and phosphotungstic acid. Some 

 also digested by trypsin and erepsin. They have the same optical activities 

 as proteins. < >ne of them has Keen prepared which produces a typical 

 anaphylactic reaction. So far a polypeptide thai can he coagulated by 

 heat or is in other ways identical with Ihe naturally occurring proteins, 

 has not been synthesized; but there is no doubt that it is only a matter 

 of time before this will he accomplished. 



Eighteen distinctly different amino acids have been isolated from pro- 

 tein, and it may assist in the conception of our problem if we place tl 

 amino acids side by side and link them together in the manner described 

 above. This is done in the accompanying chart compiled by D. D. Van 

 Slyke, in which also various other important facts concerning the chem- 

 istry of the amino acids are incidentally .added. 



At the lower pari of each formula will be seen the characteristic ear- 

 boxyl and amino groups of neighboring acids linking 'her tl • 



minal carbon atoms. The upper vertical bond of this carbon atom is con- 

 nected with the characteristic group of the amino acid, which may be very 

 simple and represented only by hydrogen, as in glycocoll, or highly com- 

 plex and including a ring formation, as in tryptophane. It will further 



observed thai there may 1>> other amino groups connected in vari 

 positions in this radicle. This is particularly the case in the first three 



the amino acids in the table namely, the hash- amino acids. In lysine 



the extra amino group reacts with nitrous acid, liberatii g fre< i il 



