Page 9 - Thermodynamics of Biochemical Reactions

2 Introduction to Apparent Equilibrium Constants

of one or more free metal ions. Biochemical thermodynamics is more complicated
than the chemical thermodynamics of reactions in aqueous solutions because
there are more independent variables that have to be specified. This introductory
chapter is primarily concerned with the hydrolysis of ATP at specified 7; P, pH,
pMg, and ionic strength. The thermodynamics of the hydrolysis of ATP and
closely related reactions have received a good deal of attention because of the
importance of these reactions in energy metabolism.

1.1 BRIEF HISTORY OF THE THERMODYNAMICS
OF BIOCHEMICAL REACTIONS

The first major publication on the thermodynamics of biochemical reactions was
by Burton in Krebs and Kornberg, Energy Transformations in Living Matter, 1957.
Before that time, apparent equilibrium constants had been measured for a number
of enzyme-catalyzed reactions, but Burton recognized that these apparent equi-
librium constants together with standard Gibbs energies of formation A,Go of
species determined by chemical methods can yield Af Go for biochemical species
to make a table that can be used to calculate equilibrium constants of biochemical
reactions that have not been studied (Burton and Krebs, 1953). In retrospect it is
easy to see that in 1953 to 1957 there were some problems that were apparently
not clearly recognized or solved. Since Burton was the first, it is worth saying a
little more about his 1957 thermodynamic tables. The first table gives Af Go values
for about 100 species in biochemical reactions. A large number of these values
were taken from chemical thermodynamic tables available in the 1950s, but a
number were new values calculated from measured apparent equilibrium con-
stants for enzyme-catalyzed reactions. Af Go values of species can be readily
calculated when the reactants in the enzyme-catalyzed reaction are all single
species and AfGo values are known for all of the reactants except one. It is
noteworthy that Burton omitted the species of orthophosphate from his table and
that he was not able to include species of ATP, ADP, NAD,,, and NAD,,,. His
second table gives standard Gibbs energy changes at pH 7 for oxidation-reduction
reactions that were calculated using the convention that [H'] = 1 mol L-' at
pH 7; the symbol AC' was used for this quantity. This table also gives the
corresponding standard cell potentials for these reactions. The third table gives
AG' values at pH 7 for a number of reactions in glycolysis and alcoholic
fermentation. The fourth table is on the citric acid cycle, and the fifth table is on
Gibbs energies of hydrolysis. When a biochemical reaction is studied at a pH
where there is a predominant chemical reaction, it is possible to discuss ther-
modynamics in terms of species. But when some reactants are represented by an
equilibrium distribution of several species with different numbers of hydrogen
atoms, this approach is not satisfactory. The quantitative treatment of reactions
involving reactants with pKs in the neighborhood of pH 7 was not possible until
acid dissociation constants of these reactants had been determined. Some
measurements of acid dissociation constants of ATP and related substances
(Alberty, Smith, and Bock, 1951) and dissociation constants of ionic complexes of
these substances with divalent cations (Smith and Alberty, 1956) were made in
this period.
In the 1960s there was a good deal of interest in the thermodynamics of the
hydrolysis of ATP and of other organic phosphates (Alberty, 1968, 1969; Phillips,
George, and Rutman, 1969), but standard Gibbs energies of species were not
calculated. The measurement of apparent equilibrium constants for biochemical
reactions was extended in the 1970s (Guynn and Veech, 1973; Veech et al., 1979)
and 1980s (Tewari and Goldberg, 1988).
In 1969 Wilhoit picked up where Burton had left off and compiled the
standard thermodynamic properties A,Go and A,Ho of species involved in
biochemical reactions. He recognized the problems involved in including species