Chapter 2
Chemistry Comes Alive
53
2
Te structural units of nucleic acids, called
nucleotides
, are
quite complex. Each nucleotide consists of three components: a
nitrogen-containing base, a pentose sugar, and a phosphate group
(Figure 2.22a)
. Five major varieties of nitrogen-containing bases
can contribute to nucleotide structure:
adenine
, abbreviated
A
(ad
9
ĕ-nēn);
guanine
,
G
(gwan
9
ēn);
cytosine
,
C
(si
9
to-sēn);
thy-
mine
,
T
(thi
9
mēn); and
uracil
,
U
(u
9
rah-sil). Adenine and guanine
are large, two-ring bases (called purines), whereas cytosine, thy-
mine, and uracil are smaller, single-ring bases (called pyrimidines).
Te stepwise synthesis of a nucleotide involves the attach-
ment of a base to the pentose sugar to form first a
nucleoside
,
named for the nitrogenous base it contains. Te nucleotide is
formed when a phosphate group is bonded to the sugar of the
nucleoside.
Although DNA and RNA are both composed of nucleotides,
they differ in many respects, as summarized in
Table 2.4
on p. 55. ±ypically, DNA is found in the nucleus (control center)
of the cell, where it constitutes the
genetic material
, also called
the
genes
, or more recently the
genome
. DNA has two funda-
mental roles: It replicates (reproduces) itself before a cell divides,
ensuring that the genetic information in the descendant cells
is identical, and it provides the basic instructions for building
every protein in the body. Although we have said that enzymes
govern all chemical reactions, remember that enzymes, too, are
proteins formed at the direction of DNA.
By providing the information for protein synthesis, DNA de-
termines what type of organism you will be—frog, human, oak
tree—and directs your growth and development. It also accounts
for your uniqueness. A technique called DNA fingerprinting
can help solve forensic mysteries (for example, verify one’s pres-
ence at a crime scene), identify badly burned or mangled bodies
at a disaster scene, and establish or disprove paternity. DNA
fingerprinting analyzes tiny samples of DNA taken from blood,
semen, or other body tissues and shows the results as a “genetic
barcode” that distinguishes each of us from all others.
forces the enzyme to wiggle into a complementary shape or
whether the enzyme wiggles around trying several different
conformations until one finally fits the substrate is still a
question. Although enzymes are specific for particular sub-
strates, other (nonsubstrate) molecules may act as
enzyme
inhibitors
if their structure is similar enough to occupy or
block the enzyme’s active site.
2
The enzyme-substrate complex undergoes internal rear-
rangements that form the product(s).
Tis step shows the
catalytic role of an enzyme.
3
The enzyme releases the product(s) of the reaction.
If the
enzyme became part of the product, it would be a reactant
and not a catalyst. Te enzyme is not changed and returns
to its original shape, available to catalyze another reaction.
Because enzymes are unchanged by their catalytic role and
can act again and again, cells need only small amounts of each
enzyme. Catalysis occurs with incredible speed. Most enzymes
can catalyze millions of reactions per minute.
Check Your Understanding
27.
What is the main event that molecular chaperones prevent?
28.
How do enzymes reduce the amount of activation energy
needed to make a chemical reaction go?
For answers, see Appendix H.
Nucleic Acids (DNA and RNA)
Compare and contrast DNA and RNA.
Te
nucleic acids
(nu-kle
9
ic), composed of carbon, oxygen, hy-
drogen, nitrogen, and phosphorus, are the largest molecules in
the body. Te nucleic acids include two major classes of mole-
cules,
deoxyribonucleic acid
(DNA)
(de-ok
0
sĭ-ri
0
bo-nu-kle
9
ik)
and
ribonucleic acid
(RNA)
.
Substrates (S)
e.g., amino acids
Enzyme (E)
Enzyme-substrate
complex (E-S)
Enzyme (E)
Product (P)
e.g., dipeptide
Energy is
absorbed;
bond is
formed.
Water is
released.
Peptide bond
1
Substrates bind at active site,
temporarily forming an
enzyme-substrate complex.
2
The E-S complex undergoes
internal rearrangements that
form the product.
3
The enzyme releases the
product of the reaction.
Active site
H
2
O
+
Figure 2.21
Mechanism of enzyme action.
In this example, the enzyme catalyzes the
formation of a dipeptide from specific amino acids.
Summary
: E
1
S
S
E-S
S
P
1
E
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