204 



SPEEDS OF SOME PROCESSES IN BIOLOGICAL SYSTEMS 



Initial Substrate Concentration [S] 



Figure 8-6. Rate of a Catalyzed Reaction as a Function of Substrate 

 Concentration for Two Different Concentrations of Catalyst. 



always proportional to the enzyme concentration if the substrate is much in 

 excess; (2) the order, or the index of the substrate concentration, declines 

 from unity down to zero as substrate concentration is increased. In other 

 words, (note Figure 8-6, region a) if substrate is in great excess, [S] > > K m , 

 and K m + [S] Q ~ [S] , and the rate expression reduces to 



v 2 = k 2 [E] 



with rate independent of substrate concentration; but (note region b) if the 

 substrate is in excess of enzyme, yet [S] << K r 

 rate expression reduces to 



and K m + [S] « K m , the 



v 2 = 



[S] [E] t 



with rate increasing linearly with substrate concentration. 



It is clear then that the nature and the extent of the binding of the enzyme- 

 substrate complex, ES (i.e., the value of K m ) is all-important: the bigger the 

 Michaelis constant the smaller the extent of binding; and the weaker the 

 binding, the slower the rate of hydrolysis. 



It is by good chance* that these E-S complexes generally absorb electro- 

 magnetic radiation in the visible and near ultraviolet regions of the spec- 

 trum. Hence their existence, as well as their K m , can be determined spectro- 

 photometrically (by light absorption), and the value of K m compared with 



*This work was pioneered and developed to a highly specialized art by Britton Chance, of 

 Yale University. 



