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

58 Chapter 2: Cell Chemistry and Bioenergetics
number of molecules
molecules with
average energy
energy required
to undergo
the enzyme-catalyzed
chemical reaction
energy needed
to undergo an
uncatalyzed
chemical reaction
energy per molecule
known: some are capable of speeding up reactions by factors of 10 14 or more.
Enzymes thereby allow reactions that would not otherwise occur to proceed rapidly
at normal temperatures.
MBoC6 e3.13/2.22
Figure 2–22 Lowering the activation
energy greatly increases the probability
of a reaction. At any given instant, a
population of identical substrate molecules
will have a range of energies, distributed as
shown on the graph. The varying energies
come from collisions with surrounding
molecules, which make the substrate
molecules jiggle, vibrate, and spin. For a
molecule to undergo a chemical reaction,
the energy of the molecule must exceed
the activation-energy barrier for that
reaction (dashed lines). For most biological
reactions, this almost never happens
without enzyme catalysis. Even with
enzyme catalysis, the substrate molecules
must experience a particularly energetic
collision to react (red shaded area). Raising
the temperature will also increase the
number of molecules with sufficient energy
to overcome the activation energy needed
for a reaction; but in marked contrast to
enzyme catalysis, this effect is nonselective,
speeding up all reactions (Movie 2.2).
Enzymes Can Drive Substrate Molecules Along Specific Reaction
Pathways
An enzyme cannot change the equilibrium point for a reaction. The reason is simple:
when an enzyme (or any catalyst) lowers the activation energy for the reaction
Y → X, of necessity it also lowers the activation energy for the reaction X → Y by
exactly the same amount (see Figure 2–21). The forward and backward reactions
will therefore be accelerated by the same factor by an enzyme, and the equilibrium
point for the reaction will be unchanged (Figure 2–23). Thus no matter how
much an enzyme speeds up a reaction, it cannot change its direction.
Despite the above limitation, enzymes steer all of the reactions in cells through
specific reaction paths. This is because enzymes are both highly selective and
very precise, usually catalyzing only one particular reaction. In other words, each
enzyme selectively lowers the activation energy of only one of the several possible
chemical reactions that its bound substrate molecules could undergo. In this way,
sets of enzymes can direct each of the many different molecules in a cell along a
particular reaction pathway (Figure 2–24).
The success of living organisms is attributable to a cell’s ability to make
enzymes of many types, each with precisely specified properties. Each enzyme
X Y X Y
(A) UNCATALYZED REACTION
(B) ENZYME-CATALYZED REACTION
AT EQUILIBRIUM
AT EQUILIBRIUM
Figure 2–23 Enzymes cannot change the equilibrium point for reactions. Enzymes, like all
catalysts, speed up the forward and backward rates of a reaction by the same factor. Therefore, for
both the catalyzed and the uncatalyzed reactions shown here, the number of molecules undergoing
the transition X → Y is equal to the number of molecules undergoing the transition Y → X when the
ratio of Y molecules to X molecules is 3 to 1. In other words, the two reactions reach equilibrium at
exactly the same point.