Essential Cell Biology 5th edition

62 CHAPTER 2 Chemical Components of Cells
Figure 2–34 Most proteins and
many RNA molecules fold into a
particularly stable three-dimensional
shape, or conformation. This shape is
directed mostly by a multitude of weak,
noncovalent, intramolecular bonds. If the
folded macromolecules are subjected to
conditions that disrupt noncovalent bonds,
the molecule becomes a flexible chain
that loses both its conformation and its
biological activity.
CONDITIONS
THAT DISRUPT
NONCOVALENT
BONDS
a stable folded
conformation
unstructured
polymer chains
ECB5 E2.32/2.34
be subject to a sensitive control that allows it to specify exactly which
subunit should be added next to the growing polymer end. We discuss
the mechanisms that specify the sequence of subunits in DNA, RNA, and
protein molecules in Chapters 6 and 7.
QUESTION 2–8
In principle, there are many
different, chemically diverse
ways in which small molecules
can be joined together to form
polymers. For example, the small
molecule ethene (CH 2 =CH 2 ) is
used commercially to make the
plastic polyethylene (...–CH 2 –CH 2 –
CH 2 –CH 2 –CH 2 –...). The individual
subunits of the three major classes
of biological macromolecules,
however, are all linked by similar
reaction mechanisms—that is,
by condensation reactions that
eliminate water. Can you think of
any benefits that this chemistry
offers and why it might have been
selected in evolution over a linking
chemistry such as that used to
produce polyethylene?
Noncovalent Bonds Specify the Precise Shape of a
Macromolecule
Most of the single covalent bonds that link together the subunits in a
macromolecule allow rotation of the atoms that they join; thus the polymer
chain has great flexibility. In principle, this allows a single-chain
macromolecule to adopt an almost unlimited number of shapes, or conformations,
as the polymer chain writhes and rotates under the influence
of random thermal energy. However, the shapes of most biological macromolecules
are highly constrained because of weaker, noncovalent
bonds that form between different parts of the molecule. These weaker
interactions are the electrostatic attractions, hydrogen bonds, van der
Waals attractions, and hydrophobic force we described earlier (see Panel
2–3). In many cases, noncovalent interactions ensure that the polymer
chain preferentially adopts one particular conformation, determined
by the linear sequence of monomers in the chain. Most protein molecules
and many of the RNA molecules found in cells fold tightly into a
highly preferred conformation in this way (Figure 2–34). These unique
conformations—shaped by billions of years of evolution—determine the
chemistry and activity of these macromolecules and dictate their interactions
with other biological molecules.
Noncovalent Bonds Allow a Macromolecule to Bind
Other Selected Molecules
As we discussed earlier, although noncovalent bonds are individually
weak, they can add up to create a strong attraction between two molecules
when these molecules fit together very closely, like a hand in a
glove, so that many noncovalent bonds can occur between them (see
Panel 2–3). This form of molecular interaction provides for great specificity
in the binding of a macromolecule to other small and large molecules,
because the multipoint contacts required for strong binding make it possible
for a macromolecule to select just one of the many thousands of
different molecules present inside a cell. Moreover, because the strength
of the binding depends on the number of noncovalent bonds that are