Essential Cell Biology 5th edition

Macromolecules in Cells
59
Figure 2–30 Polysaccharides, proteins, and nucleic acids are made
from monomeric subunits. Each macromolecule is a polymer formed
from small molecules (called monomers or subunits) that are linked
together by covalent bonds.
SUBUNIT
sugar
MACROMOLECULE
polysaccharide
organic monomers, or subunits, into long chains, or polymers (Figure
2–30 and How We Know, pp. 60–61). Yet they have many unexpected
properties that could not have been predicted from their simple constituents.
For example, it took a long time to determine that the nucleic acids,
DNA and RNA, store and transmit hereditary information (see How We
Know, Chapter 5, pp. 193–195).
Proteins are especially versatile and perform thousands of distinct functions.
Many proteins act as highly specific enzymes that catalyze the
chemical reactions that take place in cells. For example, one enzyme in
plants, called ribulose bisphosphate carboxylase, converts CO 2 to sugars,
thereby creating most of the organic matter used by the rest of the living
world. Other proteins are used to build structural components: tubulin, for
example, self-assembles to make the cell’s long, stiff microtubules (see
Figure 1−27B), and histone proteins assemble into disc-like structures
that help wrap up the cell’s DNA in chromosomes. Yet other proteins, such
as myosin, act as molecular motors to produce force and movement. We
examine the molecular basis for many of these wide-ranging functions in
later chapters. Here, we consider some of the general principles of macromolecular
chemistry that make all of these activities possible.
amino
acid
nucleotide
protein
nucleic acid
ECB5 e2.28/2.30
Each Macromolecule Contains a Specific Sequence of
Subunits
Although the chemical reactions for adding subunits to each polymer are
different in detail for proteins, nucleic acids, and polysaccharides, they
share important features. Each polymer grows by the addition of a monomer
onto one end of the polymer chain via a condensation reaction, in
which a molecule of water is lost for each subunit that is added (Figure
2–31). In all cases, the reactions are catalyzed by specific enzymes, which
ensure that only the appropriate monomer is incorporated.
The stepwise polymerization of monomers into a long chain is a simple
way to manufacture a large, complex molecule, because the subunits are
added by the same reaction performed over and over again by the same
set of enzymes. In a sense, the process resembles the repetitive operation
of a machine in a factory—with some important differences. First,
apart from some of the polysaccharides, most macromolecules are made
from a set of monomers that are slightly different from one another; for
example, proteins are constructed from 20 different amino acids (see
Panel 2–6, pp. 76–77). Second, and most important, the polymer chain is
not assembled at random from these subunits; instead, the subunits are
added in a particular order, or sequence.
The biological functions of proteins, nucleic acids, and many polysaccharides
are absolutely dependent on the particular sequence of subunits
in the linear chains. By varying the sequence of subunits, the cell could
in principle make an enormous diversity of the polymeric molecules.
Thus, for a protein chain 200 amino acids long, there are 20 200 possible
combinations (20 × 20 × 20 × 20... multiplied 200 times), while for a
DNA molecule 10,000 nucleotides long (small by DNA standards), with
its four different nucleotides, there are 4 10,000 different possibilities—an
unimaginably large number. Thus the machinery of polymerization must
QUESTION 2–7
What is meant by “polarity” of a
polypeptide chain and by “polarity”
of a chemical bond? How do the
meanings differ?
subunit
H OH + H
H
growing polymer
H 2 O
Figure 2–31 Macromolecules are formed
by adding subunits to one end of a chain.
In a condensation reaction, a molecule
of water ECB5 is lost E2.29/2.31
with the addition of each
monomer to one end of the growing chain.
The reverse reaction—the breakdown of the
polymer—occurs by the addition of water
(hydrolysis). See also Figure 2–19.