144 GENERAL BIOCHEMISTRY 



different pH values because no account is taken of changes in the 

 concentration of the active form of the substrate. Knowledge of the 

 appropriate equilibrium constants might be incorporated into this 

 equation, and this aspect of pH effects taken into account. Usually 

 such a modified equation is not employed because it does not recognize 

 the other effects of pH and because the identification and measure- 

 ment of the proper equilibrium constants are difficult. 



The third type of pH effect arises from participation of hydrogen 

 ions in enzymatic reactions. In the reaction of page 140, as in a 

 great many others, hydrogen ions are formed. In this reversible system 

 they disappear, of course, when the reaction proceeds from right to 

 left. As either a reactant or a product, the concentration of the hydro- 

 gen ion affects the extent of reaction through application of the law 

 of mass action. Using another system as an example, 



CH3CHOHCOO- + DPN+ + H2O ± 



lactate dehydrogenase 



CH3COCOO- + DPNH + H3O+ 



pyruvate 



the equilibrium constant becomes 



^ [CH3COCOO-] [DPNH] [H3O+] 

 [CH3CHOHCOO-][DPN+][H20] 



As usual the concentration of water is relatively great and little altered 

 by the reaction, so this factor is incorporated into the constant. There 

 is also a relationship between the equilibrium constant K and the 

 velocity constants. When k^ is the velocity (rate) constant for the 

 reaction from left to right and kn the constant for the reverse reaction, 



K = T 



A 2 



The actual rates of the reactions are proportional to these rate con- 

 stants and are given by 



Ri = Ai[GH3CHOHCOO-][DPN^][H2()] 

 /?2 = ^-2[CH3COCOO-][DPNH][H30+] 



with the concentration of water usually ignored. The equations show 

 the interdependence of the rate of either reaction on the pH. The 

 oxidation of lactate ion is initially independent of pH in the absence 

 of pyruvate and DPNH as far as this kind of consideration goes 

 because there can be no reverse reaction without both pyruvate and 

 DPNH. However, as soon as a trace of these materials forms, the 



