Essential Cell Biology 5th edition

156 CHAPTER 4 Protein Structure and Function
Figure 4−51 “Protein machines” can
carry out complex functions. These
machines are made of individual proteins
that collaborate to perform a specific task
(Movie 4.11). The movement of proteins is
often coordinated and made unidirectional
by the hydrolysis of a bound nucleotide
such as ATP. Conformational changes of
this type are especially useful to the cell
if they occur in a large protein assembly
in which the activities of several different
protein molecules can be coordinated by
the movements within the complex, as
schematically illustrated here.
ADP P
ADP + P
ATP
ATP
P
ADP
ATP
ADP
+
P
ATP
QUESTION 4–8
Explain why the hypothetical
enzymes in Figure 4−51 have a
great advantage in opening the
safe if they work together in a
protein complex, as opposed to
working individually in an unlinked,
sequential manner.
the ensemble of proteins to move coordinately (Figure 4−51). In these
machine-like complexes, the appropriate enzymes can be positioned to
carry out successive reactions in a series—as during the synthesis of proteins
on a ribosome, for example (discussed in Chapter 7). And during cell
division, a large protein machine moves rapidly along DNA to replicate
the DNA double helix (discussed in Chapter 6 and shown in Movie 6.3
and Movie 6.4).
A large number of different protein machines have evolved to perform
many critical biological tasks. Cells make wide use of protein machines
for the same reason that humans have invented mechanical and electronic
machines: for almost any job, manipulations that are spatially and
temporally coordinated through linked processes are much more efficient
than is the sequential use of individual tools.
Many Interacting Proteins Are Brought Together by
Scaffolds
We have seen that proteins rely on interactions with other molecules to
carry out their biological functions. Enzymes bind substrates and regulatory
ligands—many of which are generated by other enzymes in the
same reaction pathway. Receptor proteins in the plasma membrane,
when activated by extracellular ligands, can recruit a set of intracellular
signaling proteins that interact with and activate one another, propagating
the signal to the cell interior. In addition, the proteins involved in DNA
replication, gene transcription, DNA repair, and protein synthesis form
protein machines that carry out these complex and crucial tasks with
great efficiency.
ECB5 04.51
But how do proteins find the appropriate partners—and the sites where
they are needed—within the crowded conditions inside the cell (see
Figure 3−22)? Many protein complexes are brought together by scaffold
proteins, large molecules that contain binding sites recognized by
multiple proteins. By binding a specific set of interacting proteins, a scaffold
can greatly enhance the rate of a particular chemical reaction or cell
process, while also confining this chemistry to a particular area of the
cell—for example, drawing signaling proteins to the plasma membrane.
Although some scaffolds are rigid, the most abundant scaffolds in cells
are very elastic. Because they contain long unstructured regions that
allow them to bend and sway, these scaffolds serve as flexible tethers
that greatly enhance the collisions between the proteins that are bound