EFRAIM RACKER 



4, (a) and (b), e.g., phosphorylation with ATP and dephos- 

 phorylation by hydrolysis. 



This brings us to the general problem of enzymes participat- 

 ing as reactants in specific physiological functions. Interaction of 

 aldehydes with proteins in the process of vision (51), interaction 



b 



-> AA+A 



Figure 4. 



of nucleotides with the proteins involved in muscular contrac- 

 tion (39), and the role of cholinesterase in the process of trans- 

 mission of nerve impulses (31) may represent examples of the 

 participation of enzymes as reactants in physiological processes. 



Concluding Remarks 



We can look upon enzymes as active centers embedded in a 

 protein matrix. The function of the protein molecule is to pro- 

 vide activating groups which increase the rate of catalysis 

 and permit specificity of interaction between the enzyme and its 

 substrate. Perhaps the most striking illustration for this differ- 

 entiation between the active center and the activating group of 

 the protein is the example of the "little enzymes" obtained from 

 hydrolyzed pepsin. These dialyzable fragments catalyze effi- 

 ciently the hydrolysis of a small molecular substrate, but have 

 lost most of their activity with proteins as substrate. Thus, the 

 activating groups of the native enzyme are required for the spe- 

 cific and efficient hydrolysis of the large molecular substrate. 



Perhaps another function of the protein is to provide the cata- 

 lytic center with the anchors and the biochemical stability which 

 is needed for intracellular localization. 



Kinetic investigations have received a new impetus during 

 the past ten years. The concept of the polyaffinity of substrates, 



236 



