ENZYME KINETICS 



site spends a large fraction of the time without a proton on the 

 group which functions as a proton donor, and during this time 

 the reaction cannot occur. Protons are transferred very gener- 

 ally in enzymatic reactions, and it may be that the observed pH. 

 dependence of the kinetics may be a consequence largely of the 

 ionization of groups which are involved in providing or accepting 

 hydrogen ions in the reactions. 



The dependence of the kinetic constants on />H for a re- 

 action following mechanism (14) is expected to be somewhat 

 more complicated than indicated by equations (15) and (16) 

 because of the progressive change in ionization constants of 

 individual groups in a protein as the net charge on the molecule 

 changes from large positive values below the isoelectric point to 

 large negative values above the isoelectric point. This effect is 

 well known from studies of acid-base equilibria of proteins of 

 known amino acid composition. From a knowledge of the 

 titration curve of an enzyme and its isoelectric point the electro- 

 static effect on the free energy of ionization of individual groups 

 may be calculated. Thus it is to be expected that knowledge of 

 classical protein chemistry will be found to be increasingly useful 

 in the interpretation of enzyme kinetic data. 



INHIBITION OF ENZYMATIC REACTIONS 



Inhibition studies may provide information about the 

 nature of the enzymatic site. Although this approach is rather 

 indirect, still it is one of the few experimental approaches avail- 

 able. An inhibitor may combine at a site other than the enzy- 

 matic site, and this is indeed required in the case of noncompetitive 

 inhibition. An inhibitor which is competitive with the substrate 

 at low concentrations may also combine at other sites at higher 

 concentrations. If the binding of inhibitor at other sites causes a 

 partial inhibition or even an activation, the inhibition may be 

 neither competitive nor noncompetitive. At low substrate con- 

 centrations the inhibitory effect may predominate so that there 

 will be a net inhibition while at high substrate concentrations 



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