132 



PROTEINS 



suggest that the antibiotic activity of gramicidin S is related largely to 

 its cyclic structure. 



A very extensive piece of work has been done recently by Sanger and 

 associates on insulin. This protein consists of two types of polypeptide 

 chains. One type has glycine as the N-terminal (free NHo group) residue, 

 and the other chain is headed by phenylalanine. Each molecule of 

 insulin contains two glycine chains and two phenylalanine chains held 

 together by — S— S— bridges of cystine residues (p. 117). The bridges 

 can be broken by oxidation with performic acid to — SO3H groups, thus 

 setting free the chains. The two types- of chains can then be separated 

 and the sequence of the amino acids in them determined. This was done 

 by partial hydrolysis with hydrocliloi'ic acid and enzymes and isolation 

 of the peptides split off from the chain. More than 60 peptides ranging 

 from dipoptides to hexapeptides were isolated from the hydrolyzate of 

 the phenylalanine chain, and their structures determined by chromatog- 

 raphy. From all these fractions the sequence of the amino acids in 

 the whole chain was deduced to be: ■^' 



Phe.Val.Asp.Glu.His.Leu.CySO3H.Gly.Ser.His.Leu.Val.Glu.Ala.Leu.Tyr.Leu.VaL 

 CySO3H.Gly.Glu.Arg.Gly.Phe.Phe.Tyr.Thr.Pro.Lys.Ala. 



The total number of amino acid residues in the cliain is 30. 



The sequence of the amino acids in the glycine chain was worked out 

 in the same way. The chain consists of 21 amino acid residues arranged 

 as follows: 



Gly.Ileu.Val.Glu.CySO3H.CySO3H.Ala.Ser.Val.CySO3H.Ser.Leu.Tyr.Glu.Leu. 

 Glu.Asp.Ty r.CySOsH .Asp . 



In the phenylalanine chain the last amino acid residue in the chain, 

 that is, the residue with a free carboxyl group, is alanine. In the glycine 

 chain the carboxyl terminal residue is designated as Asp., i.e., an aspartic 

 acid residue. In the intact insulin it is an asparagine residue. During 

 acid hydrolysis the amide group is split to give aspartic acid. It is be- 

 lieved that two other aspartic acid residues in the chain are in reality 

 asparagine residues, and six of the glutamic acid residues are actually 

 glutamine residues. The exact location of these amide groups is not 

 known. 



The relation of the four chains to one another is still to be determined. 

 Several arrangements are possible, but the presence of four cysteic acid 

 residues in the glycine chain and two in the phenylalanine chain suggests 

 that the two glycine chains lie between the phenylalanine chains, A 

 diagram of such an arrangement for the intact insulin follows: 



