Essential Cell Biology 5th edition

Free Energy and Catalysis
91
lake with
waves
uncatalyzed reaction—waves not large
enough to surmount barrier
(A)
(B)
2 3
1 4
uncatalyzed
dry
river
bed
2 3
1
4
enzyme catalysis
of reaction 1
flowing
stream
catalyzed reaction—waves often surmount barrier
the universe (see Figure 3–5). Disorder increases when useful energy that
could be harnessed to do work is dissipated as heat. The useful energy in
a system is known as its free energy, or G. And because chemical reactions
involve a transition from one molecular state to another, the term
that is of most interest to chemists and cell biologists is the free-energy
change, denoted ΔG (“Delta G”).
ECB5 e3.14/3.14
Let’s consider a collection of molecules. ΔG measures the amount of disorder
created in the universe when a reaction involving these molecules
takes place. Energetically favorable reactions, by definition, are those that
create disorder in the universe by decreasing the free energy of the system
to which they belong; in other words, they have a negative ΔG (Figure
3–16).
A reaction can occur spontaneously only if ΔG is negative. On a macroscopic
scale, an energetically favorable reaction with a negative ΔG
is the relaxation of a compressed spring into an expanded state, which
releases its stored elastic energy as heat to its surroundings. On a microscopic
scale, an energetically favorable reaction—one with a negative
ΔG—occurs when salt (NaCl) dissolves in water. Note that just because
a reaction can occur spontaneously does not mean it will occur quickly.
The decay of diamonds into graphite is a spontaneous process—but it
takes millions of years.
(C)
energy
Figure 3–14 Enzymes catalyze reactions
by lowering the activation-energy barrier.
(A) The dam represents the activation
energy, which is lowered by enzyme
catalysis. Each green ball represents
a potential substrate molecule that is
bouncing up and down in energy level
owing to constant encounters with waves,
an analogy for the thermal bombardment of
substrate molecules by surrounding water
molecules. When the barrier—the activation
energy—is lowered significantly, the balls
(substrate molecules) with sufficient energy
can roll downhill, an energetically favorable
movement. (B) The four walls of the box
represent the activation-energy barriers
for four different chemical reactions that
are all energetically favorable because the
products are at lower energy levels than
the substrates. In the left-hand box, none
of these reactions occurs because even
the largest waves are not large enough to
surmount any of the energy barriers. In the
right-hand box, enzyme catalysis lowers
the activation energy for reaction number
1 only; now the jostling of the waves allows
the substrate molecule to pass over this
energy barrier, allowing reaction 1 to
proceed (Movie 3.1). (C) A branching set
of reactions with a selected set of enzymes
(yellow boxes) serves to illustrate how a
series of enzyme-catalyzed reactions—by
controlling which reaction will take place
at each junction—determines the exact
reaction pathway followed by each molecule
inside the cell.
CATALYSIS
enzyme–
substrate
complex
SUBSTRATE BINDING
active site
enzyme
enzyme–
product
complex
PRODUCT RELEASE
Figure 3–15 Enzymes convert substrates
to products while remaining unchanged
themselves. Catalysis takes place in a cycle
in which a substrate molecule (red) binds
to an enzyme and undergoes a reaction to
form a product molecule (yellow), which
then gets released. Although the enzyme
participates in the reaction, it remains
unchanged.