Effects of pH in Biochemical Thermodynamics and Enzyme Kinetics

In biochemical thermodynamics, the apparent equilibrium constants of enzyme-catalyzed reactions have been represented by K’=Kref10 f(pH), where Kref is a reference chemical reaction, n is the number of hydrogen ions in the reference reaction, and f(pH) is a function of pH that brings in the pKs of the substrates. This equation suggests that hydrogen ions are involved in two different ways in biochemical thermodynamics. If this is true in thermodynamics, it has to be true in kinetics. However, the choice of reference reaction in thermodynamics is arbitrary, and so n cannot be determined from equilibrium measurements. However, when hydrogen ions are consumed in the rate-determining reaction, the experimental limiting velocity of the forward reaction is given by Vfexp = 10 npHVf. Vf is the limiting velocity in the forward direction when n = 0, or Vf can be calculated from experimental data using Vf = 10 npHVfexp. Vf brings in the pKs of the enzyme-substrate complex that reacts in the rate-determining reaction. When hydrogen ions are consumed in the rate-determining reaction, the Haldane equation yields K’=Kref10 f(pH). Since n can be -8 (EC 1.7.7.1), the effects of pH on kinetic and thermodynamic properties can be very large. webMathematica can provide the thermodynamic properties of enzyme-catalyzed reactions that are difficult to calculate and require a database without having Mathematica in a personal computer or knowing how to use it. 63 http://www.beilstein-institut.de/escec2007/proceedings/Alberty/Alberty.pdf ESCEC, September 23 – 26, 2007, Rüdesheim/Rhein, Germany