Page 8 - Thermodynamics of Biochemical Reactions

Thernwdyanamics of Biochemical Reactions. Robert A. Alberty
Copyright 0 2003 John Wiley & Sons, Inc.
ISBN 0-471-22851-6

Brief History of the Thermodynamics of
Biochemical Reactions
Acid Dissociation Constants and Dissociation
Constants of Complex Ions
Binding of Hydrogen Ions and Magnesium Ions
by Adenosine Triphosphate
Apparent Equilibrium Constants of Biochemical
Reactions
Production of Hydrogen Ions and Magnesium
Ions in the Hydrolysis of Adenosine
Triphosphate
1.6 pKs of Weak Acids

Two types of equilibrium constant expressions are needed in biochemistry. The
thermodynamics of biochemical reactions can be discussed in terms of species like
ATP4-, HATP3-, and MgATP2- or in terms of reactants (sums of species) like
ATP. The use of species corresponds with writing chemical reactions that balance
atoms of elements and electric charges; the corresponding equilibrium constants
are represented by K. This approach is required when chemical details are being
discussed, as in considering the mechanism of enzymatic catalysis. But discussion
in terms of metabolism must involve, in great deal detail, acid dissociation
constants and dissociation constants of complexes with metal ions. Therefore
metabolism is discussed by writing biochemical reactions in terms of reactants -
that is, sums of species, like ATP-at a specified pH and perhaps specified
concentrations of free metal ions that are bound reversibly by reactants. Bio-
chemical reactions do not balance hydrogen ions because the pH is held constant,
and they do not balance metal ions for which free concentrations are held
constant. When the pH is held constant, there is the implication that acid or alkali
will be added to the system to hold the pH constant if the reaction produces or
consumes hydrogen ions. In actual practice a buffer is used to hold the pH nearly
constant, and the pH is measured at equilibrium. The corresponding equilibrium
constants are represented by K', which are referred to as apparent equilibrium
constants because they are functions of pH and perhaps the free concentrations

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