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Enzyme Kinetics of Hydrolytic Reactions /I7 : 3 



complex E-S. The complex E-S was assumed to be in quasi-equilibrium 

 with E and S, but also to break down to form the products. The reaction 

 then would follow the scheme 



e-p x k p 



E + S^±E-S 



E-S + H 2 -^ E + Products 



where the letters above the reactants will be used to denote concen- 

 trations, thereby avoiding need for the square brackets. Note that e is 



Figure 3. The rate at which a 

 substrate S is hydrolyzed as a 

 function of substrate concentra- 

 tion. Curves are plotted for three 

 concentrations of the enzyme E. 

 Note that all three curves reach 

 half maximum velocity at [S] = 



Km- 



</[S] 

 dt 



Figure 4. The rate at which a 

 substrate S is hydrolyzed as a 

 function of the concentration of 

 the enzyme E. In general, curves 

 of this type are more difficult to 

 obtain than those as in Figure 3. 



the total enzyme concentration added at time t = 0. This situation 

 is illustrated diagrammatically in Figure 5. Although the shapes have 

 no significance, the diagram illustrates the essentially cyclic nature of the 

 enzyme process and also implies our belief that enzyme activity is 

 dependent on the shapes of the reacting molecules. Sometimes enzymes 

 have been compared to a key fitting into a lock. This is an over- 

 simplification unless shapes are discussed in terms of the atomic con- 

 figurations and of the electron orbitals in the substrate, intermediate 

 complex, and products. 



