Page 5 - Thermodynamics of Biochemical Reactions

Preface

This book is about the thermodynamics of enzyme-catalyzed reactions that make
up the metabolism of living organisms. It is not an introductory text, but the
fundamental principles of thermodynamics are reviewed. The reader does need
some background in thermodynamics, such as that provided by a first course in
physical chemistry. The book uses a generalized approach to thermodynamics
that makes it possible to calculate the effects of changing pH, free concenrations
of metal ions that are bound by reactants, and steady-state concentrations of
coenzymes. This approach can be extended to other types of work that may be
involved in a living organism.
The concepts involved in this approach are simple, but the equations become
rather complicated. Biochemical reactions are written in terms of reactants like
ATP that are made up of sums of species, and they are referred to as biochemical
reactions to differentiate them from the underlying chemical reactions that are
written in terms of species. The thermodynamics of biochemical reactions is
independent of the properties of the enzymes that catalyze them. However, the
fact that enzymes may couple reactions that might otherwise occur separately
increases the number of constraints that have to be considered in ther-
modynamics.
Biochemical thermodynamics is complicated for several reasons: (1) Bio-
chemical reactants consist of sums of species whenever a reactant has a pK within
about two units of the pH of interest or binds metal ions reversibly. (2) Species
of a biochemical reactant are often ions, and the activity coefficients of ions are
functions of ionic strength. (3) Enzyme catalysis may introduce constraints in
biochemical reactions in addition to balances of atoms of elements. (4) Metab-
olism is sufficiently complicated that it is important to find ways to obtain a more
global view. (5) In biochemistry other kinds of work, such as electric work,
elongation work, and surface work may be involved. It is remarkable that the
same basic reactions are found in all living systems. The most important thing
about these reactions is that they provide the means to carry out the oxidation
of organic matter in a sequence of steps that store energy that is needed for the
synthesis of organic molecules, mechanical work, and other functions required for
life.
The theme of this book is that Legendre transforms make the application of
thermodynamics more convenient for the users. The logic used here is a continu-
ation of the process described by Gibbs that introduced the enthalpy H,