15 



of the molecular structure of every enzyme and deter- 

 mine the specificity thereof. Side-chains of one sort or 

 another are probably attached to all sides of the protein 

 portion of the enzyme molecule, but those which are at 

 the amino end are the ones which are responsible for 

 proteolytic activity if the enzyme is one which attacks 

 the carboxyl ends of the food molecules. In the pepsin 

 molecule it appears to be the tyrosine group which con- 

 tributes such activity. (Science, Nov. 26, 1937, p. 482.) 

 Although the specificity of an enzyme depends primarily 

 on the chemical structure of some active side-chain, it 

 is the normal protein portion which determines the de- 

 gree of activity. 



Those who wish to make a further study of the mecha- 

 nism of proteolytic enzymes should read the article by 

 Max Bergmann in the May 18, 1934 issue of Science. 

 Experiments with peptide-splitting enzymes have shown 

 that the presence of alpha hydrogen atoms in the amino 

 acid residues, and their arrangement in the cis-position 

 in diketopiperazine rings, are essential for the action of 

 those enzymes which attack the carboxyl ends of pohqDcp- 

 tides, although not for those which attack the amino ends. 

 This seems to indicate that in the protein molecules the 

 hydrocarbon side-chains (attached to the alpha carbon 

 atoms) are slanted towards the amino ends and not to- 

 wards the carboxyl ends of the spirals. Since these hy- 

 drocarbon side-chains are hydrophobic, they will be 

 pressed back towards the other end of the protein mole- 

 cule by repulsion from the water clinging to the hydro- 

 philic carboxyl groups. 



The subject of protein structure should not be con- 

 cluded without a brief mention of monomolecular films, 

 a preliminary account of which will be found in the Jan. 

 15, 1937 issue of Science, and a more detailed account 

 in the June 3, 1938 issue. Most of the experiments were 

 conducted with egg albumin, although it may be ex- 



