Essential Cell Biology 5th edition

Answers A:7
X Y
Y
Y
Y Z
enzyme lowers the activation-energy barrier, so that more
CO 2 molecules have sufficient energy to undergo the
reaction. The area under the curve from point A to infinite
energy or from point B to infinite energy indicates the total
number of molecules that will react without or with the
enzyme, respectively. Although not drawn to scale, the ratio
of these two areas should be 10 7 .
X
Figure A3–4
X
∆G o X
Z
∆G o X
Y
∆G o Y
We do not know from the information given in Figure
ECB5 eA3.04/A3.04
3−12 how high the activation-energy barriers are; they are
therefore drawn to an arbitrary height (solid lines). The
activation energies would be lowered by enzymes that
catalyze these reactions, thereby speeding up the reaction
rates (dotted lines), but the enzymes would not change the
ΔG° values.
ANSWER 3–5 The reaction rates might be limited by:
(1) the concentration of the substrate—that is, how often a
molecule of CO 2 collides with the active site on the enzyme;
(2) how many of these collisions are energetic enough to
lead to a reaction; and (3) how fast the enzyme can release
the products of the reaction and therefore be free to bind
more CO 2 . The diagram in Figure A3–5 shows that the
number of molecules
Figure A3–5
activation energy
for enzyme reaction
B
energy per molecule
X Y Z
Y
Z
activation energy
for uncatalyzed reaction
A
Z
Z
ANSWER 3–6 All reactions are reversible. If the
compound AB can dissociate to produce A and B, then it
must also be possible for A and B to associate to form AB.
Which of the two reactions predominates depends on the
equilibrium constant of the reaction and the concentrations
of A, B, and AB (as discussed in Figure 3−19). Presumably,
when this enzyme was isolated, its activity was detected by
supplying A and B in relatively large amounts and measuring
the amount of AB generated. But suppose, however, that
in the cell there is a large concentration of AB, in which
case the enzyme would actually catalyze AB → A + B. (This
question is based on an actual example in which an enzyme
was isolated and named according to the reaction in one
direction, but was later shown to catalyze the reverse
reaction in living cells.)
ANSWER 3–7
A. The rocks in Figure 3−29B provide the energy to lift the
bucket of water. (i) In the reaction X + ATP →
Y + ADP + P i , ATP hydrolysis is driving the reaction;
thus ATP corresponds to the rocks on top of the cliff.
(ii) The broken debris in Figure 3−29B corresponds
to ADP and P i , the products of ATP hydrolysis.
(iii) and (iv) In the reaction, ATP hydrolysis is coupled
to the conversion of X to Y. X, therefore, is the starting
material, the bucket on the ground, which is converted
to Y, the bucket at its highest point.
B. (i) The rocks hitting the ground would be the futile
hydrolysis of ATP—for example, in the absence of
an enzyme that uses the energy released by the ATP
hydrolysis to drive an otherwise unfavorable reaction;
in this case, the energy stored in the phosphoanhydride
bond of ATP would be lost as heat. (ii) The energy
stored in Y could be used to drive another reaction. If
Y represented the activated form of amino acid X, for
example, it could undergo a condensation reaction to
form a peptide bond during protein synthesis.
ANSWER 3–8 The free energy ΔG derived from ATP
hydrolysis depends on both the ΔG° and the concentrations
of the substrate and products. For example, for a particular
set of concentrations, one might have
ΔG = –50 kJ/mole = –30.5 kJ/mole + 2.58 ln [ADP] × [P i ]
[ATP]
ΔG is smaller than ΔG°, largely because the ATP
concentration in cells is high (in the millimolar range) and
the ADP concentration is low (in the 10 μM range). The
concentration term of this equation is therefore smaller than
1 and its logarithm is a negative number.
ΔG° is a constant for the reaction and will not vary with
reaction conditions. ΔG, in contrast, depends on the
concentrations of ATP, ADP, and phosphate, which can be
somewhat different between cells.
ANSWER 3–9 Reactions B, D, and E all require coupling
to other, energetically favorable reactions. In each