Page 124 - Thermodynamics of Biochemical Reactions

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

Binding of Oxygen by Hemoglobin Tetramers
Further Transformed Gibbs Energy at Specified
Oxygen Concentration
Partial Dissociation of Tetramers into Dimers
Experimental Determination of Seven Apparent
Equilibrium Constants
Dissociation of a Diprotic Acid
Effect of pH on Protein-Ligand Equilibria
Calculation of Standard Transformed Gibbs
Energies of Formation of the Catalytic Site of
Fumarase

The binding of oxygen by hemoglobin is an important example of the binding of
a ligand by a protein, and so it is of interest to consider this series of reactions
from the point of view of the transformed Gibbs energy at a specified pH. The
experimental determination of the oxygen binding by the tetramer is complicated
by the partial dissociation of the tetramer into dimers. In view of the fact that it
is not possible to connect either the tetramer or dimer to its elements in the
standard state, the standard transformed Gibbs energy of the tetramer can be set
equal to zero. This convention has already been used for other reactants that
cannot be connected to the elements by reactions with known equilibrium
constants. This chapter shows how all seven apparent equilibrium constants for
the binding of oxygen by hemoglobin at specified pH can be determined by
measuring the fractional saturation of heme as a function of the concentration of
molecular oxygen and the concentration of heme at a specified pH.
A number of biochemical reactions involve proteins as reactants, and so it is
important to be able to determine the standard transformed Gibbs energies of
formation of their reactive sites at specified pH. The standard transformed Gibbs
energies of formation of the active sites of ferredoxin, cytochrome c, and
thioredoxin are given in tables discussed earlier in Chapter 4.
The effect of pH on protein-ligand equilibria is discussed and the equations
are applied to the binding of succinate, D-tartrate, L-tartrate, and meso-tartrate
by the catalytic site of fumarase.

121