Specificity and Inhibition of Fnniarase 



Robert A. Albertv 



University of Wisconsin, Madison, Wis. 



THERE ARE TWO ASPECTS of the Specificity of an enzyme, the specificity 

 with respect to the substrate and the specificity with respect to inhibi- 

 tors. Some enzymes, such as chymotrypsin, have many substrates. For 

 others, Hke fumarase, no additional substrates have been found. In the case of 

 enzymes such as the latter there is some question as to how much information 

 is forthcoming from the fact that a large number of other compounds are not 

 substrates, and this increases the interest in the inhibition constants for com- 

 petitive inhibitors. 



Competitive inhibition constants are of special interest because they are be- 

 lieved to apply to the site at which the substrate combines. However, there are 

 a number of difficulties involved in the interpretation of competitive inhibition 

 constants. Today I want to talk primarily about one of them, the complication 

 that is introduced by the fact that the kinetics of most enzymatic reactions, 

 like protein reactions in general, depend upon the pH. 



The effect of pH upon the binding of oxygen by hemoglobin (Wyman, 1948), 

 the binding of antigen by antibody (Singer, Eggman and Campbell, 1955) and 

 the binding of competitive inhibitors by cholinesterase (Wilson and Bergmann, 

 1950) have been studied. Such effects may result from the ionization of vicinal 

 groups for which the ionization constants are altered by the presence of the 

 inhibitor or the substrate. The effect of pH may be more subtle, but this simple 

 interpretation has been quite satisfactory as a theoretical basis in a number of 

 instances. 



While I am going to use this sort of picture in discussing the data for fuma- 

 rase, the discussion of Professor Kirkwood (1956) of an alternative interpreta- 

 tion of the effect of pH changes should be kept in mind. We prefer the interpre- 

 tation which I am going to present because it brings together several aspects 

 of the behavior of fumarase. However, we must keep in mind the fact that the 

 force due to charge fluctuation may have an important effect. Perhaps experi- 

 ments can be devised which will give decisive answers to the relative importance 

 of these two effects. 



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