ENZYME KINETICS 



watch to keep time can be determined by making quantitative 

 measurements of the dimensions and weight of the balance 

 wheel, the strength of the hair spring, the number of teeth on the 

 gears, etc. Without a properly constructed balance wheel a 

 watch is a machine, but it is not a timepiece. The study of 

 enzyme kinetics is analogous to the quantitative study of a watch 

 and may be required in identifying some of the gears and balance 

 wheels of the enzymatic mechanism. 



Thinking in terms of mechanism is useful at all stages in the 

 investigation of enzyme kinetics because a mechanism is a con- 

 venient device for the integration of information and a stimulus 

 for further experiments. As more facts are obtained it is possible 

 to build, step by step, a framework into which the final detailed 

 mechanism must fit. In an absolute sense the determination of 

 the mechanism is never complete. Like absolute zero the 

 complete elucidation of the mechanism may be approached, but 

 never reached New types of experiments will require further 

 elaboration of the mechanism, which in this manner becomes 

 more detailed. The mechanism of an enzymatic reaction is 

 necessarily incomplete as long as the chemical nature of the 

 enzyme is unknown. 



This article will be concerned only with the information on 

 mechanism which comes directly from kinetic data — that is, the 

 stoichiometry and rate constants of steps which are rate-deter- 

 mining and the nature of the equilibria preceding the rate- 

 determining step. Another facet of mechanism which lies out- 

 side the scope of the present treatment has to do with the manner 

 in which the transfer of electrons and atoms is actually accom- 

 plished and with the correlation between structure and kinetic 

 constants. Thus we will refer only in passing to the valuable 

 information concerning enzymatic mechanisms which may be 

 obtained by use of isotopic tracers. Isotopic tracer experiments 

 are useful in interpreting enzyme kinetic experiments because 

 they may be used to determine which bonds are broken, which 

 atoms exchange with the solvent, and which steps are rate- 

 determining. For example, in the case of the alcohol and lactic 



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