Essential Cell Biology 5th edition

Macromolecules in Cells
63
B
B
A
A
the surfaces of A and B, and A
and C, are a poor match and
are capable of forming only a few
weak bonds; thermal motion rapidly
breaks them apart
A
A
C A
C
macromolecule A randomly
encounters other
macromolecules (B, C, and D)
A
D
A
D
the surfaces of A and D match
well and therefore can form
enough weak bonds to withstand
thermal jolting; they therefore
stay bound to each other
Figure 2–35 Noncovalent bonds mediate interactions between macromolecules. They can also mediate interactions between a
macromolecule and small molecules (see Movie 2.4).
formed, associations of almost any strength are possible. As one example,
binding of this type makes it possible for proteins ECB5 to e2.33/2.35 function as enzymes.
Enzymes recognize their substrates via noncovalent interactions, and an
enzyme that acts on a positively charged substrate will often use a negatively
charged amino acid side chain to guide the substrate to its proper
position. We discuss such interactions in greater detail in Chapter 4.
Noncovalent bonds can also stabilize associations between any two
macromolecules, as long as their surfaces match closely (Figure 2–35).
Such associations allow macromolecules to be used as building blocks
for the formation of much larger structures. For example, proteins often
bind together into multiprotein complexes that function as intricate
machines with multiple moving parts, carrying out such complex tasks
as DNA replication and protein synthesis (Figure 2–36). In fact, noncovalent
bonds account for a great deal of the complex chemistry that makes
life possible.
QUESTION 2–9
Why could covalent bonds not be
used in place of noncovalent bonds
to mediate most of the interactions
of macromolecules?
SUBUNITS
amino acids
covalent
bonds
MACROMOLECULES
noncovalent
bonds
MACROMOLECULAR
ASSEMBLY
RNA molecule
ribosome
nucleotides
globular
protein
30 nm
Figure 2–36 Both covalent bonds and noncovalent bonds are needed to form a
macromolecular assembly such as a ribosome. Covalent bonds allow small organic
molecules to join together to form macromolecules, which can assemble into large
macromolecular complexes via noncovalent bonds. Ribosomes are large macromolecular
machines that synthesize proteins inside cells. Each ribosome is composed of about
90 macromolecules (proteins and RNA molecules), and it is large enough to see in the
electron microscope (see Figure 7−34). The subunits, macromolecules, and ribosome
shown here are drawn roughly to scale.
ECB5 e2.34/2.36