Apparent Equilibrium Constants for Enzyme-Catalyzed Reactions

Apparent Equilibrium Constants for Enzyme-Catalyzed Reactions... 

APPARENT EQUILIBRIUM CONSTANTS FOR ENZYME-CATALYZED REACTIONS... 

The procedure for calculating standard formation properties of species at zero ionic strength from measurements of apparent equilibrium constants is discussed in the next chapter. The future of the thermodynamics of species in aqueous solutions depends largely on the use of enzyme-catalyzed reactions. The reason that more complicated ions in aqueous solutions were not included in the NBS Tables (1992) is that it is difficult to determine equilibrium constants in systems where a number of reactions occur simultaneously. Since many enzymes catalyze clean-cut reactions, they make it possible to determine apparent equilibrium constants and heats of reaction between very complicated organic reactants that could not have been studied classically. 

When enzyme-catalyzed reactions produce or eonsume hydrogen ions, their apparent equilibrium constants depend on the pH. The term apparent equilibrium constant and the symbol K are used to indicate that a biochemical reaction and the expression for its apparent equilibrium constant are written in terms of sums of species. Hydrogen ions are omitted in writing a biochemical reaction, and [H ] is omitted in writing the expression for the apparent equilibrium constant of a biochemical reaction because [H ] is specified. 

These standard transformed entropies of formation determine the contributions these reactants make to the apparent equilibrium constant for an enzyme-catalyzed reaction, but the more fundamental property, the standard molar entropies of species, are discussed in Chapter 15. 

If the apparent equilibrium constant K for an enzyme-catalyzed reaction has been determined at 298.15K and AfG ° values can be calculated at the experimental pH and ionic strength using known functions of pH and ionic strength for all the reactants but one, the AfG ° of that reactant under the experimental conditions can be calculated using equation 4.4-2. So far functions of pH and ionic strength that yield AfG ° are have been published for 131 reactants at 298.15 K (Alberty, 2001f). 

This chapter has been about calculating species properties from apparent equilibrium constants and transformed enthalpies of reaction, but there is a prior question. Where is the experimental data Fortunately, Goldberg, Tewari, and coworkers have searched the literature for these data, have evaluated it, and have published a series of review articles (10-15). These review articles provide thermodynamic data on about 500 enzyme-catalyzed reactions involving about KXX) reactants. In principle all these reactants can be put into thermodynamic tables. Goldberg, Tewari, and Bhat (16) have produced a web site to assist in the acquisition of data from the review articles. 

When the apparent equilibrium constant K for an enzyme-catalyzed reaction depends on pH and pMg, the calorimetric enthalpy of reaction Af//(cal) is given by (2)... 

This package provides data on the species of 131 reactants at 298.15 K and programs for calculating various transformed thermodynamic properties. Programs are given for the calculation of apparent equilibrium constants and other transformed thermodynamic properties of enzyme-catalyzed reactions by simply typing in the reaction. 

This table contains values of apparent equilibrium constants K for selected enzyme-catalyzed reactions at specified temperatures T and pHs. In those cases where the ionic strength I and/or the pMg (pMg = -logjj,[Mg ]) have been reported, the values of these quantities are given. The Enzyme Commission numbers [Webb (1992)] of the enzymes that were used to catalyze the reactions are also given. 

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