C H A P T E R
/
By the end of this section, you should be able to:
/
1
Describe the relations between the enzyme catalysis of
a
reaction, the
thermodynamics of the reaction, and the formation of the transition
state.
/ 2 Explain the relation between the transition state
andHhe
active site of
an enzyme, and list the characteristics of active sites.
/ 3 Explain what reaction velocity is.
/ 4 Explain how reaction velocity is determined and how reaction
velocities are used to characterize enzyme activity.
/ 5 Identify the key properties of allosteric proteins, and describe the
structural basis for these properties.
/ 6 List environmental factors that affect enzyme activity, and describe
how these factors exert their effects on enzymes.
/ 7 Explain how allosteric properties contribute to hemoglobin function.
/ 8 Identify the key regulators of hemoglobin function.
6.1 Enzymes Are Powerful and
Highly Specific Catalysts
6.2 Many Enzymes Require
Cofactors for Activity
6.3 Gibbs Free Energy Is a Useful
Thermodynamic Function for
Understanding Enzymes
6.4 Enzymes Facilitate the
Formation of the Transition
State
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The activity of
an
enzyme is responsible for
the
glow of
the luminescent
jellyfish.
The
enzyme aequorin catalyzes the oxidation of
a
compound
by oxygen
in the presence
of
calcium
to release
C0
2
and
light.
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96
T
he energy and information processing that takes place inside a cell consists of
thousands of individual chemical reactions. For these reactions to take place in
a physiologically useful fashion, they must occur at a rate that meets the cell's
needs, and they must display specificity; that is, a particular reactant should always
yield a particular product. Side reactions leading to the formation of useless or
hazardous by-products must
be
minimized. In this chapter, we consider the key prop-
erties of enzymes, with a special look at the energetics of enzyme-catalyzed reactions.
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6.1
Enzymes Are Powerful and Highly Specific Catalysts
Enzymes
accelerate the rate of reactions by factors of
as
much as a million or more
(Table 6.1). Indeed, most reactions in biological systems do not take place at
perceptible rates in the absence of enzymes. Even a reaction as simple as adding
water to carbon dioxide is catalyzed by an
enzyme—namely,
carbonic anhydrase.
This reaction facilitates the transport of carbon dioxide from the tissues where it is
produced to the lungs where it is exhaled. Carbonic anhydrase is one of the fastest
known enzymes. Each enzyme molecule can hydrate 10
6
molecules of
C0
2
per
second.
This catalyzed reaction is 10
7
times as fast as the uncatalyzed one. The
transfer of
C0
2
