96
Chapter 4
processes spontaneity is indeed an important issue, but disordering of the universe by
spontaneous processes is an impractical criterion to use for assessing spontaneity, as it is
impossible to determine changes in the entropy of the entire universe. Furthermore, the
spontaneity of a process cannot be decided from the entropy change of the system in question
alone, because an exothermic process (A//3ystem
< 0,
i.e.,
heat is evolved from the system) may be
spontaneous even if it is accompanied by a decrease in the entropy of the system, A5syitem < 0 (of
course, the
total
entropy change is positive in this case too, due to an increase in the entropy of
the environment that more than counterbalances the decrease in the entropy of the system). An
example is the spontaneous folding of denatured proteins to their highly ordered
(i.e.,
ASsystem
< 0)
native conformation.
Due to these difficulties with the use of entropy, spontaneity is determined by using another state
function, the
Gibbs free energy
:
AG = AH - TAS
(4.1)
The meaning of the free energy is that, for a process at constant temperature and pressure, the
maximum work that can be done by the system (but not including the work o f displacement) is
equal to the decrease in the free energy of the system. For constant temperature and pressure
processes, which describe the vast majority of biological systems, the criterion for spontaneity is
A G ^O .
Spontaneous processes,
i.e.,
those with AG - 0, are said to be
exogenic,
and they can be utilized
to do work. Processes that are not spontaneous,
i.e.,
those that have positive ^ G values, are
termed
endergonic
, and they must be driven by the input of free energy. Processes at equilibrium,
i.e.
the forward and backward processes are exactly balanced, are characterized by AG = 0.
Notice that the Gibbs free energy varies with temperature, which must therefore always be
specified. This dependence on temperature explains the spontaneous denaturation of proteins
above a certain temperature. Formation of a native protein from its denatured form has both a
negative A
H
and a negative
AS.
Above the temperature where
AH
equals
TAS,
the Gibbs free
energy of the folding process is positive and the reverse reaction
(i.e,,
denaturation) is a
spontaneous process,
i.e.
the native protein will tend to denature.
It is important to note that a large negative value of AG does not necessarily imply that a
chemical reaction will proceed at a measurable rate. Thus, the free energy change of the
phosphorylation of glucose to glucose-6-phosphate by ATP is large and negative, but this
reaction does not occur just by mixing glucose and ATP. Only when the enzyme hexokinase is
added does the reaction proceed. Similarly, most biological molecules, including proteins,
nucleic acids, carbohydrates, and lipids, are thermodynamically unstable to hydrolysis, but their
spontaneous hydrolysis is insignificant. Only when hydrolytic enzymes are added do the
hydrolysis reactions proceed at a measurable rate. Despite their importance in accelerating a
reaction, enzymes do not change the AG for the reaction. As catalysts they can only speed up the
attainment of thermodynamic equilibrium, but they do not allow a reaction with a positive AG to
proceed.
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