ENZYMES AS REAGENTS 



EFRAIM RACKER, Division of Nutrition and Physiology, The Public 

 Health Research Institute of the City of New York, Inc., New York 



end to which our currents tend 



Inevitable sea 

 To which we flow, what do we know 



What shall we guess of thee? 



Arthur Hugh Clough 



During the past ten years our concept of enzymes has 

 undergone a noteworthy change. Enzymes are no longer the 

 mysterious catalysts of a decade ago. They have become the 

 mysterious reactants of today. Fifty years ago, kinetic experi- 

 ments were performed which suggested the formation of enzyme- 

 substrate intermediates during catalysis, but only in recent years 

 has a direct demonstration of their existence been achieved. 

 Now, an extensive search for these enzyme-substrate compounds 

 is under way in various laboratories. The enzymologist of 

 yesterday who isolated a crystalline enzyme and recorded a few 

 of its properties considered his work a task well done. The 

 enzymologist of today lacks this satisfaction. He realizes that 

 with the isolation of the enzyme his task has just begun, and, 

 depending on his own background and direction of interest, he 

 will approach the purified enzyme as a protein of unknown 

 chemical structure, or as a catalyst with specific kinetic proper- 

 ties, or as a reactant which combines with the substrate. As a 

 protein chemist, he will determine the amino acid composition; 

 he will attempt a study of the amino acid sequence; he will 

 modify the protein and search for a relationship between its 



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