Essential Cell Biology 5th edition

100 CHAPTER 3 Energy, Catalysis, and Biosynthesis
QUESTION 3–5
The enzyme carbonic anhydrase
is one of the speediest enzymes
known. It catalyzes the rapid
conversion of CO 2 gas into the
much more soluble bicarbonate ion
(HCO 3 – ). The reaction:
CO 2 + H 2 O ↔ HCO 3
–
+ H +
is very important for the efficient
transport of CO 2 from tissues, where
CO 2 is produced by respiration,
to the lungs, where it is exhaled.
Carbonic anhydrase accelerates the
reaction 10 7 -fold, hydrating 10 5 CO 2
molecules per second at its maximal
speed. What do you suppose limits
the speed of the enzyme? Sketch
a diagram analogous to the one
shown in Figure 3−13 and indicate
which portion of your diagram has
been designed to display the
10 7 -fold acceleration.
Noncovalent Interactions Allow Enzymes to Bind Specific
Molecules
The first step in any enzyme-catalyzed chemical reaction is the binding
of the substrate. Once this step has taken place, the substrate must
remain bound to the enzyme long enough for the chemistry to occur.
The association of enzyme and substrate is stabilized by the formation of
multiple, weak bonds between the participating molecules. These weak
interactions—which can include hydrogen bonds, van der Waals attractions,
and electrostatic attractions (discussed in Chapter 2)—persist until
random thermal motion causes the molecules to dissociate again.
When two colliding molecules have poorly matching surfaces, few noncovalent
bonds are formed, and their total energy is negligible compared
with that of thermal motion. In this case, the two molecules dissociate
as rapidly as they come together (see Figure 2–35). As we saw in Figure
3−20, even small changes in the number of noncovalent bonds made
between two interacting molecules can have a dramatic effect on their
ability to form a complex. Poor noncovalent bond formation is what
prevents unwanted associations from forming between mismatched
molecules, such as those between an enzyme and the wrong substrate.
Only when the enzyme and substrate are well matched do they form
many weak interactions. It is these numerous noncovalent bonds that
keep them together long enough for a covalent bond in the substrate
molecule to be formed or broken, converting substrate to product.
Enzymes are remarkable catalysts, capturing substrates and releasing
products in mere milliseconds. But though an enzyme can lower
the activation energy for a reaction, such as Y → X (see Figure 3−12),
it is important to note that the same enzyme will also lower the activation
energy for the reverse reaction X → Y to exactly the same degree.
That’s because the same noncovalent bonds are formed with the enzyme
whether the reaction goes forward or backward. The forward and backward
reactions will therefore be accelerated by the same factor by an
enzyme, and the equilibrium point for the reaction—and thus its ΔG°—
remains unchanged (Figure 3–24).
QUESTION 3–6
In cells, an enzyme catalyzes
the reaction AB → A + B. It was
isolated, however, as an enzyme that
carries out the opposite reaction
A + B → AB. Explain the paradox.
Y X Y X
(A) UNCATALYZED REACTION (B) ENZYME-CATALYZED REACTION
AT EQUILIBRIUM
AT EQUILIBRIUM
Figure 3–24 Enzymes cannot change the equilibrium point for reactions.
Enzymes, like all catalysts, speed up the forward and reverse rates of a reaction by
the same amount. Therefore, for both the (A) uncatalyzed and (B) catalyzed reactions
shown here, the number of molecules undergoing the transition Y → X is equal
ECB5 e3.25/3.24
to the number of molecules undergoing the transition X → Y when the ratio of X
molecules to Y molecules is 7 to 1, as illustrated. In other words, both the catalyzed
and uncatalyzed reactions will eventually reach the same equilibrium point, although
the catalyzed reaction will reach equilibrium much faster.