ENZYMES 



171 



If we compare K = 1 1 Km and K = [X]I[E] [S], we see that Km is the 

 value of [S] for which [X] = [E], that is to say it is the value of [S] at 

 which half the enzyme is combined with the substrate, and at which con- 

 sequently {v = /?2[^]), the reaction velocity is half the maximum velocity 

 for that enzyme concentration (Fig. 35). Km is measured, like S, in moles/1. 



msx 





Molar 



Fig. 35 — Current method of measuring the Michaehs constant, v = reaction velocity in 

 moles per min. Vmax = maximum velocity (the reaction velocity increases with the con- 

 centration of substrate but not proportionately). When all the enzyme molecules have 

 formed the enzyme-substrate complex, a further addition of substrate no longer increases 

 the velocity. The maximum velocity has been reached. [<S] = molar concentration of the 

 substrate (the abscissa scale is logarithmic). Km = 0-02 in this example. 



V. THE MECHANISM OF ACTION OF ENZYMES 

 AND COENZYMES 



A. Enzymes 



At the present time, our views on the manner in which enzymes evade 

 the necessity for molecular activation are very hypothetical. 



Barnard and Laidlaw for exam.ple have attempted to explain the mode 

 of action of hydrolases by a "bifunctional" effect. One can discuss catalysis 

 with respect to amino and carboxyl groups without implying that these 

 groups are necessarily the ones which are responsible for the catalytic 

 action. 



