Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

CATALYSIS AND THE USE OF ENERGY BY CELLS
57
Enzymes Lower the Activation-Energy Barriers That Block
Chemical Reactions
Consider the reaction
paper + O 2 → smoke + ashes + heat + CO 2 + H 2 O
Once ignited, the paper burns readily, releasing to the atmosphere both energy
as heat and water and carbon dioxide as gases. The reaction is irreversible, since
the smoke and ashes never spontaneously retrieve these entities from the heated
atmosphere and reconstitute themselves into paper. When the paper burns, its
chemical energy is dissipated as heat—not lost from the universe, since energy
can never be created or destroyed, but irretrievably dispersed in the chaotic random
thermal motions of molecules. At the same time, the atoms and molecules of
the paper become dispersed and disordered. In the language of thermodynamics,
there has been a loss of free energy; that is, of energy that can be harnessed to do
work or drive chemical reactions. This loss reflects a reduction of orderliness in
the way the energy and molecules were stored in the paper.
We shall discuss free energy in more detail shortly, but the general principle
is clear enough intuitively: chemical reactions proceed spontaneously only in
the direction that leads to a loss of free energy. In other words, the spontaneous
direction for any reaction is the direction that goes “downhill,” where a “downhill”
reaction is one that is energetically favorable.
Although the most energetically favorable form of carbon under ordinary conditions
is CO 2 , and that of hydrogen is H 2 O, a living organism does not disappear
in a puff of smoke, and the paper book in your hands does not burst into flames.
This is because the molecules both in the living organism and in the book are in a
relatively stable state, and they cannot be changed to a state of lower energy without
an input of energy: in other words, a molecule requires activation energy—a
kick over an energy barrier—before it can undergo a chemical reaction that leaves
it in a more stable state (Figure 2–21). In the case of a burning book, the activation
energy can be provided by the heat of a lighted match. For the molecules in the
watery solution inside a cell, the kick is delivered by an unusually energetic random
collision with surrounding molecules—collisions that become more violent
as the temperature is raised.
The chemistry in a living cell is tightly controlled, because the kick over energy
barriers is greatly aided by a specialized class of proteins—the enzymes. Each
enzyme binds tightly to one or more molecules, called substrates, and holds
them in a way that greatly reduces the activation energy of a particular chemical
reaction that the bound substrates can undergo. A substance that can lower the
activation energy of a reaction is termed a catalyst; catalysts increase the rate of
chemical reactions because they allow a much larger proportion of the random
collisions with surrounding molecules to kick the substrates over the energy barrier,
as illustrated in Figure 2–22. Enzymes are among the most effective catalysts
total energy
Y
reactant
(A)
a
b
c
activation
energy for
reaction
Y X
uncatalyzed
reaction pathway
X
product
total energy
(B)
Y
reactant
d
b
c
enzyme lowers
activation
energy for
catalyzed
reaction
Y X
enzyme-catalyzed
reaction pathway
X
product
Figure 2–21 The important principle
of activation energy. (A) Compound Y
(a reactant) is in a relatively stable state,
and energy is required to convert it to
compound X (a product), even though X is
at a lower overall energy level than Y. This
conversion will not take place, therefore,
unless compound Y can acquire enough
activation energy (energy a minus energy
b) from its surroundings to undergo the
reaction that converts it into compound X.
This energy may be provided by means of
an unusually energetic collision with other
molecules. For the reverse reaction,
X → Y, the activation energy will be
much larger (energy a minus energy c);
this reaction will therefore occur much
more rarely. Activation energies are
always positive; note, however, that the
total energy change for the energetically
favorable reaction Y → X is energy c
minus energy b, a negative number.
(B) Energy barriers for specific reactions
can be lowered by catalysts, as indicated
by the line marked d. Enzymes are
particularly effective catalysts because they
greatly reduce the activation energy for the
reactions they perform.