The Relation of the Secondary Structure of Pepsin to 

 Its Biological Activity 



Gertrude E. Perlmann 



The Rockefeller Institute, 

 New York, U.S.A. 



Two decisive advances in the understanding of protein structure are 

 the elucidation by Sanger of the amino acid sequence of insuhn [i] and 

 the more recently completed investigations on pancreatic ribonuclease by 

 Hirs, Moore, and Stein [2]. The biological activity of a protein, however, 

 depends not only on the amino acid sequence but also on the folding of 

 the peptide chains, their arrangement in space and how they are packed 

 into the protein molecule. The only technique available for obtaining 

 detailed and precise information about the folding is undoubtedly the 

 X-ray method which has been demonstrated so clearly by the work of 

 Kendrew and his collaborators on myoglobin [3]. In the studies on pepsin 

 which I shall report, more indirect methods have been used. Nevertheless, 

 such methods may help in defining the conformation of the peptide chain 

 segment necessary for the biological activity of the enzyme, as well as the 

 type of forces responsible for maintaining the "native" configuration. 



Thus far, amino acid sequence work on pepsin has not been attempted, 

 which may partly be attributed to the size of the molecule. Pepsin is a 

 protein with a molecular weight of 35 000 and has only one peptide chain 

 which is cross-linked by three disulphide bonds. Examination of the 

 amino acid distribution, as given in Table I, reveals that the protein has 

 71 acidic residues (44 aspartic acids and 27 glutamic acids) and only four 

 basic ones (i histidine, i lysine, 2 arginines). Moreover, pepsin has a 

 high content of non-polar and hydroxyamino acids and 15 prolines [4]. It 

 is, therefore, not unlikely that such an unusual amino acid distribution 

 influences the secondary structure of the protein. 



It is well established that in most globular proteins a certain fraction 

 of the amino acid residues is in the a-helical configuration. Whenever a 

 helical structure is present, hydrogen bonds between the oxygen atoms of 

 the carbonyl groups and the imino group of the peptide linkages play an 

 essential role in determining the folding of the polypeptide chain. However, 

 hydrophobic bonds may also be important in maintaining the secondary 

 structure. 



