78
Chapter 3
a. The binding of trace metal cofactors maintains the
conformation of the enzyme’s binding site so that it is able
to bind substrate.
b. Coenzymes, derived from vitamins, transfer small groups
of atoms from one substrate to another. The coenzyme is
regenerated in the course of these reactions and can do its
work over and over again.
Regulation of Enzyme-Mediated Reactions
I. The rates of enzyme-mediated reactions can be altered by
changes in temperature, substrate concentration, enzyme
concentration, and enzyme activity. Enzyme activity is altered
by allosteric or covalent modulation.
Multienzyme Reactions
I. The rate of product formation in a metabolic pathway can be
controlled by allosteric or covalent modulation of the enzyme
mediating the rate-limiting reaction in the pathway. The end
product often acts as a modulator molecule, inhibiting the
rate-limiting enzyme’s activity.
II. An “irreversible” step in a metabolic pathway can be reversed by
the use of two enzymes, one for the forward reaction and one
for the reverse direction via another, energy-yielding reaction.
SECTION D KEY TERMS
SECTION D REVIEW QUESTIONS
1. How do molecules acquire the activation energy required for a
chemical reaction?
2. List the four factors that infl uence the rate of a chemical
reaction and state whether increasing the factor will increase or
decrease the rate of the reaction.
3. What characteristics of a chemical reaction make it reversible or
irreversible?
4. List fi ve characteristics of enzymes.
5. What is the difference between a cofactor and a coenzyme?
6. From what class of nutrients are coenzymes derived?
7. Why are small concentrations of coenzymes suffi cient to
maintain enzyme activity?
8. List three ways to alter the rate of an enzyme-mediated
reaction.
9. How can an “irreversible step” in a metabolic pathway be
reversed?
activation energy
72
active site
74
anabolism
71
calorie
72
carbonic anhydrase
74
catabolism
71
SECTION E
Metabolic Pathways
Enzymes are involved in many important physiological reac-
tions that together promote a homeostatic state. In addition,
enzymes are vital for the regulated production of cellular
energy (ATP), which, in turn, is needed for such widespread
events as muscle contraction, nerve cell function, and chemical
signal transduction.
Cells use three distinct but linked metabolic pathways to
transfer the energy released from the breakdown of fuel mol-
ecules to ATP. They are known as (1) glycolysis, (2) the Krebs
cycle, and (3) oxidative phosphorylation (
Figure 3–40
). In
the following section, we will describe the major character-
istics of these three pathways, including the location of the
pathway enzymes in a cell, the relative contribution of each
pathway to ATP production, the sites of carbon dioxide for-
mation and oxygen utilization, and the key molecules that
enter and leave each pathway. Later, in Chapter 16, we will
refer to these pathways when we describe the physiology of
energy balance in the human body.
Several facts should be noted in Figure 3–40. First,
glycolysis operates only on carbohydrates. Second, all the
categories of nutrients—carbohydrates, fats, and proteins—
contribute to ATP production via the Krebs cycle and oxida-
tive phosphorylation. Third, mitochondria are the sites of the
Krebs cycle and oxidative phosphorylation. Finally, one impor-
tant generalization to keep in mind is that glycolysis can occur
in either the presence or absence of oxygen, whereas both the
Krebs cycle and oxidative phosphorylation require oxygen.
Cellular Energy Transfer
Glycolysis
Glycolysis
(from the Greek
glycos,
sugar, and
lysis,
breakdown)
is a pathway that partially catabolizes carbohydrates, primar-
ily glucose. It consists of 10 enzymatic reactions that convert
a six-carbon molecule of glucose into two three-carbon mol-
ecules of
pyruvate,
the ionized form of pyruvic acid (
Figure
3–41
). The reactions produce a net gain of two molecules of
ATP and four atoms of hydrogen, two transferred to NAD
+
and two released as hydrogen ions:
Glucose + 2 ADP + 2 P
i
+ 2 NAD
+
⎯→
2 Pyruvate + 2 ATP + 2 NADH + 2 H
+
+ 2 H
2
O
These 10 reactions,
none of which utilizes molecular oxygen,
take place in the cytosol. Note (see Figure 3–41) that all the
intermediates between glucose and the end product pyru-
vate contain one or more ionized phosphate groups. Plasma
membranes are impermeable to such highly ionized mole-
cules, and thus these molecules remain trapped within the
cell.
Note that the early steps in glycolysis (reactions 1 and
3) each
use,
rather than produce, one molecule of ATP, to
form phosphorylated intermediates. In addition, note that
reaction 4 splits a six-carbon intermediate into two three-
carbon molecules, and reaction 5 converts one of these
catalyst
72
chemical equilibrium
72
coenzyme
74
cofactor
74
end-product inhibition
77
enzyme
73
enzyme activity
75
FAD
74
irreversible reaction
73
kilocalorie
72
law of mass action
73
metabolic pathway
76
metabolism
71
NAD
+
74
rate-limiting reaction
77
reversible reaction
72
substrate
74
vitamin
74
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