68
HENRY EYRING, RICHARD P. BOYCE AND JOHN D. SPIKES
reactions, one a hydrolysis, the other a dehydration. Thus, in the case of
the hexokinase reaction,
ATP + H 2 0 -* ADP + H3PO4
Δ^
7.8 kcal.
Glucose + Pi -> Glucose-6-phosphate + H 2 0
AF = 3.0 kcal.
ATP + Glucose -> Glucose-6-phosphate + ADP
AF = -4.8 kcal.
Calculating the equilibrium constant gives a value of
K = 2.5 X 10
3
This value assures that the phosphorylation of glucose proceeds essentially to completion.
The possible synthesis of ATP from ADP and inorganic phosphate
(Pi) has been sometimes considered as a function of the free energy
of an oxidation reaction. Thus, if the standard free energy of an oxidation is of the order of magnitude of 10 kcal, per mole, it is concluded
that one ATP molecule may be generated in a coupled reaction. If the
free energy of the oxidation is several times as large, then the total free
energy divided by the "bond energy" gives the maximum number of
ATP molecules that may be formed. Biochemical arithmetic of this type
is a completely invalid application of the free energy concept. Analysis
using rate theory shows that it is not the free energy of a reaction that
determines whether or not a reaction will proceed, but rather the free
energy of activation.
It is important to note that the term "bond energy" in so-called highenergy phosphate bonds was created to provide a convenient means for
expressing the free energy changes of hydrolysis that can be coupled biologically. This coupling is usually effected by enzymes by lowering the
free energy of activation. Absolute rate theory thus provides a deeper
insight into the mechanisms of energy coupling.
2. The Effect of External Forces on Reactions
We are now in a favorable position to discuss one of the central
problems in biology: how does the cell microstructure modify results
derived for ideal systems?
In general, cell microstructure will manifest itself by exerting an
external force that will act on some cellular process in such a way that
it will either tend to aid or hinder the process by increasing or decreasing the free energy of the initial or final configuration with respect to
the activated state. In other words, it alters the free energy of activation. This may be represented by the simple diagram in Fig. 6.
For simplicity of treatment consider a two-component system governed by a single unimolecular rate constant having a symmetrical bar-
HENRY EYRING, RICHARD P. BOYCE AND JOHN D. SPIKES
reactions, one a hydrolysis, the other a dehydration. Thus, in the case of
the hexokinase reaction,
ATP + H 2 0 -* ADP + H3PO4
Δ^
7.8 kcal.
Glucose + Pi -> Glucose-6-phosphate + H 2 0
AF = 3.0 kcal.
ATP + Glucose -> Glucose-6-phosphate + ADP
AF = -4.8 kcal.
Calculating the equilibrium constant gives a value of
K = 2.5 X 10
3
This value assures that the phosphorylation of glucose proceeds essentially to completion.
The possible synthesis of ATP from ADP and inorganic phosphate
(Pi) has been sometimes considered as a function of the free energy
of an oxidation reaction. Thus, if the standard free energy of an oxidation is of the order of magnitude of 10 kcal, per mole, it is concluded
that one ATP molecule may be generated in a coupled reaction. If the
free energy of the oxidation is several times as large, then the total free
energy divided by the "bond energy" gives the maximum number of
ATP molecules that may be formed. Biochemical arithmetic of this type
is a completely invalid application of the free energy concept. Analysis
using rate theory shows that it is not the free energy of a reaction that
determines whether or not a reaction will proceed, but rather the free
energy of activation.
It is important to note that the term "bond energy" in so-called highenergy phosphate bonds was created to provide a convenient means for
expressing the free energy changes of hydrolysis that can be coupled biologically. This coupling is usually effected by enzymes by lowering the
free energy of activation. Absolute rate theory thus provides a deeper
insight into the mechanisms of energy coupling.
2. The Effect of External Forces on Reactions
We are now in a favorable position to discuss one of the central
problems in biology: how does the cell microstructure modify results
derived for ideal systems?
In general, cell microstructure will manifest itself by exerting an
external force that will act on some cellular process in such a way that
it will either tend to aid or hinder the process by increasing or decreasing the free energy of the initial or final configuration with respect to
the activated state. In other words, it alters the free energy of activation. This may be represented by the simple diagram in Fig. 6.
For simplicity of treatment consider a two-component system governed by a single unimolecular rate constant having a symmetrical bar-