PROTEINS l-^l 



Glutamic acid is also linked through its y-carboxyl in several peptides 

 of bacteriological origin. In the vitamin, pteroyltriglutamic acid (one 

 of the several forms of folic acid), the second and third glutamic acid 

 residues are linked to the preceding by a y-linkage. Thus, 



P— NH-CH-(CHj).-CO.NH-CH(CH2)2-CO-NHCH(CHj),-COOH 



' * I 1 I 



COOH COOH 



COOH 



7-Linkage 7-Linkage 



P stands for the pteroyl group. For the complete formula see p. 247. 

 The corresponding triglutamate with a-glutamic acid linkages has also 

 been synthesized, but it has no vitamin potency. 



A still more unusual polypeptide is the capsular substance produced 

 by Bacillus mithrncis. This substance is a long chain polypeptide con- 

 taining only D-glutamic acid residues, 50 to 100 in number, joined together 

 mainly through y-linkages. There is some indication that small numbers 

 of y-glutamic acid linkages occur in such common proteins as casein, 

 edestin, hemoglobin, etc. To 'date, the authors have found no reports 

 regarding the occurrence of second or /?-carboxyl linkages for aspartic 

 acid in naturally occurring polypeptides. 



Sequence of the amino acids in the protein molecule 



Data are now available for the number of amino acid residues present 

 in many proteins, and information concerning the order in which the 

 amino acids occur in the chain is gradually accumulating. Data have 

 been obtained by partially hydrolyzing the protein and isolating some 

 of the peptides that have been formed, then determining the order of 

 the amino acids in the peptides. Examples of such peptides are as 

 follows: glycyl-alanine, alanyl-glycine, and glycyl-tyrosine from silk 

 fibroin; prolyl-phenylalanine and leucyl-glutamic acid from ghadin; 

 phosphoseryl-glutamic acid from casein. 



Applying this technique to the antibiotic gramicidin S, Consden et al. 

 isolated four dipeptides and one tripeptide from the partial hydrolysis 

 products. These pieces :ontained all of the five amino acids found in 

 gramicidin S, and, hence, the order of the amino acids was worked 

 out to be: 



(-a- (L-valyl) -L-ornithyl-L-leucyl-D-phenylalanyl-L-proline-) 1 or 2 



Gramicidin S then appears to be a cyclic penta- or decapeptide. It is 

 obviously a very simple compound in comparison to the common pro- 

 teins. An open chain peptide having the same sequence of amino acids 

 has been synthesized by Harris and Work, but its antibiotic activity 

 was small as compared to that of the natural compound. These results 



