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

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CATALYSIS AND THE USE OF ENERGY BY CELLS
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amount of disorder created in the universe when a reaction takes place. Energetically
favorable reactions, by definition, are those that decrease free energy; in
other words, they have a negative ∆G and disorder the universe (Figure 2–28).
An example of an energetically favorable reaction on a macroscopic scale is
the “reaction” by which a compressed spring relaxes to an expanded state, releasing
its stored elastic energy as heat to its surroundings; an example on a microscopic
scale is salt dissolving in water. Conversely, energetically unfavorable reactions
with a positive ∆G—such as the joining of two amino acids to form a peptide
bond—by themselves create order in the universe. Therefore, these reactions can
take place only if they are coupled to a second reaction with a negative ∆G so large
that the ∆G of the overall process is negative (Figure 2–29).
The Concentration of Reactants Influences the Free-Energy
Change and a Reaction’s Direction
As we have just described, a reaction Y ↔ X will go in the direction Y → X when
the associated free-energy change, ∆G, is negative, just as a tensed spring left to
itself will relax and lose its stored energy to its surroundings as heat. For a chemical
reaction, however, ∆G depends not only on the energy stored in each individual
molecule, but also on the concentrations of the molecules in the reaction
mixture. Remember that ∆G reflects the degree to which a reaction creates a more
disordered—in other words, a more probable—state of the universe. Recalling our
coin analogy, it is very likely that a coin will flip from a head to a tail orientation if
a jiggling box contains 90 heads and 10 tails, but this is a less probable event if the
box has 10 heads and 90 tails.
The same is true for a chemical reaction. For a reversible reaction Y ↔ X, a
large excess of Y over X will tend to drive the reaction in the direction Y → X.
Therefore, as the ratio of Y to X increases, the ∆G becomes more negative for the
transition Y → X (and more positive for the transition X → Y).
The amount of concentration difference that is needed to compensate for a
given decrease in chemical-bond energy (and accompanying heat release) is not
intuitively obvious. In the late nineteenth century, the relationship was determined
through a thermodynamic analysis that makes it possible to separate
the concentration-dependent and the concentration-independent parts of the
free-energy change, as we describe next.
The Standard Free-Energy Change, ∆G°, Makes It Possible to
Compare the Energetics of Different Reactions
Because ∆G depends on the concentrations of the molecules in the reaction mixture
at any given time, it is not a particularly useful value for comparing the relative
energies of different types of reactions. To place reactions on a comparable
basis, we need to turn to the standard free-energy change of a reaction, ∆G°.
The ∆G° is the change in free energy under a standard condition, defined as that
where the concentrations of all the reactants are set to the same fixed value of 1
mole/liter. Defined in this way, ∆G° depends only on the intrinsic characters of
the reacting molecules.
For the simple reaction Y → X at 37°C, ∆G° is related to ∆G as follows:
∆G = ∆G° + RT ln
[X]
[Y]
where ∆G is in kilojoules per mole, [Y] and [X] denote the concentrations of Y and
X in moles/liter, ln is the natural logarithm, and RT is the product of the gas constant,
R, and the absolute temperature, T. At 37°C, RT = 2.58 J mole –1 . (A mole is
6 × 10 23 molecules of a substance.)
A large body of thermodynamic data has been collected that has made it possible
to determine the standard free-energy change, ∆G°, for the important metabolic
reactions of a cell. Given these ∆G° values, combined with additional information
about metabolite concentrations and reaction pathways, it is possible to
quantitatively predict the course of most biological reactions.
ENERGETICALLY
FAVORABLE
REACTION
Y
X
The free energy of Y
is greater than the free
energy of X. Therefore
ΔG < 0, and the disorder
of the universe increases
during the reaction
Y X.
this reaction can occur spontaneously
ENERGETICALLY
UNFAVORABLE
REACTION
Y
X
If the reaction X Y
occurred, ΔG would
be > 0, and the
universe would
become more
ordered.
this reaction can occur only if
it is coupled to a second,
energetically favorable reaction
Figure 2–28 The distinction between
energetically favorable and energetically
unfavorable reactions.
C
negative
ΔG
D
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Y
X
positive
ΔG
the energetically unfavorable
reaction X Y is driven by the
energetically favorable
reaction C D, because the net
free-energy change for the
pair of coupled reactions is less
than zero
Figure 2–29 How reaction coupling is
used to drive energetically unfavorable
reactions.