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chapter 5
Thermodynamics, Chemical Kinetics, and Energy Metabolism
both reactions are thermodynamically feasible. Therefore,
in a closed system in which neither reactants nor products
are removed, the enzyme
cannot affect the equilibrium
of the reaction;
it only affects the rate at which equi-
librium is attained. If products are removed as they are
formed (as in a biochemical pathway), the enzyme will
continue to accelerate the reaction in the forward direc-
tion; such a state of affairs occurs frequently in biological
systems.
Chemical reactions in living systems involve orderly re-
lease, storage, or utilization of energy. Knowledge of ther-
modynamics and kinetics is essential to appreciate how
this occurs. Thermodynamics deals with the changes in
energy content between reactants and products, whereas
kinetics is concerned with the reaction rates.
5.2 Thermodynamics
The energy changes that occur in cellular reactions obey
the two laws of thermodynamics. The first law states
that energy can be neither created nor destroyed (i.e., the
energy of the universe is constant) but can be converted
from one form into another. The second law states that in
all processes involving energy changes under a given set of
conditions of temperature and pressure, the entropy of the
system and the surroundings (i.e., the universe) increases
and attains a maximum value at equilibrium.
Entropy
(
S
)
may be considered a measure of disorder or randomness, or
that amount of energy unavailable for useful work. In ap-
plying the laws of thermodynamics to human metabolism,
the primary concern is how the concentrations and energy
levels of reactants and products affect the direction of a
reaction. Less emphasis is placed upon factors such as tem-
perature and pressure, since they are maintained at nearly
constant levels.
In considering thermodynamic parameters, e.g., heat
and work, we do not need to know the exact chemical
pathway taken by the reactants in conversion to products.
Using thermodynamics, we can obtain information about
reactions that cannot be studied directly in living systems.
Thermodynamics predicts, on the basis of the known en-
ergy levels of reactants and products, whether a reaction
can be expected to occur spontaneously or how much en-
ergy must be supplied to drive the reaction in one direc-
tion or another. Such information is crucial in establishing
reaction routes in metabolic pathways. Thermodynamics
explains how equilibrium constants are related to changes
in temperature. Thermodynamics also explains the basis
for enzyme catalysis.
A typical energy profile of a chemical reaction,
A
+
B
C + D,
is shown in Figure 5-1. During the conver-
FIGURE 5-1
Energy diagram for a thermodynamically favorable forward chemical
reaction,
A
+
B
-* C +
D . A G ' =
free energy of activation and AG =
free energy difference between reactants and products that has a negative
value (Gc + d - Ga + b < 0).
sion of reactant molecules
(A
+
B)
to product molecules
(C +
D),
there is a change in the potential energy, called
the
Gibbs free energy (G
), of the molecules involved,
which is shown by plotting the change in free energy
of the participating molecules as a function of the re-
action progress. The phrase “progress of the reaction”
on the abscissa of the graph indicates generality and
includes all changes in bond lengths and angles (i.e.,
in the shapes and atomic interactions) of the reactant
molecules as they are interchangeably converted to prod-
uct molecules. “Interchangeability” implies reversibility;
if both reactants and products are present in a reaction
mixture, molecules of reactant will be converted to prod-
uct and “product” molecules to “reactant” molecules at
any given moment. Thus, the equilibrium state is dynamic,
not static. In the chemical reaction shown in Figure 5-1,
the free energy of reactants has to be raised to that of
the transition state before the products are formed. The
transition state is an activated state in which the proba-
bility is high that appropriate chemical bonds are broken
or re-formed in the conversion of reactants to products.
Thus, the rate of reaction is proportional to the concen-
tration of the activated species that exist at the transi-
tion state. The energy required to raise all of the reactant
molecules to the transition state species is known as the
free energy o f activation, AG'.
Prime designates the ac-
tivated state and A the difference between two energy
states. The free-energy difference between reactants and
products is indicated by
AG.
Thus, a negative value for
AG
for a chemical reaction (as in Figure 5-1) indicates
that energy is released in converting reactants to products;
such a reaction is termed
exergonic.
For reactions with
AG
< 0, we can predict that they are
thermodynamically
