Essential Cell Biology 5th edition

How Proteins Are Controlled
151
Feedback inhibition is a form of negative regulation: it prevents an
enzyme from acting. Enzymes can also be subject to positive regulation,
in which the enzyme’s activity is stimulated by a regulatory molecule
rather than being suppressed. Positive regulation occurs when a product
in one branch of the metabolic maze stimulates the activity of an enzyme
in another pathway. But how do these regulatory molecules change an
enzyme’s activity?
Allosteric Enzymes Have Two or More Binding Sites That
Influence One Another
Feedback inhibition was initially puzzling to those who discovered it, in
part because the regulatory molecule often has a shape that is totally different
from the shape of the enzyme’s preferred substrate. Indeed, when
this form of regulation was discovered in the 1960s, it was termed allostery
(from the Greek allo, “other,” and stere, “solid” or “shape”). Given the
numerous, specific, noncovalent interactions that allow enzymes to interact
with their substrates within the active site, it seemed likely that these
regulatory molecules were binding somewhere else on the surface of
the protein. As more was learned about feedback inhibition, researchers
realized that many enzymes must contain at least two different binding
sites: an active site that recognizes the substrates and one or more sites
that recognize regulatory molecules. These sites must somehow “communicate”
to allow the catalytic events at the active site to be influenced
by the binding of the regulatory molecule at a separate location.
The interaction between sites that are located in different regions on a
protein molecule is now known to depend on a conformational change
in the protein. The binding of a ligand to one of the sites causes a shift
in the protein’s structure from one folded shape to a slightly different
folded shape, and this alters the shape of a second binding site that can
be far away. Many enzymes have two conformations that differ in activity,
each of which can be stabilized by the binding of a different ligand.
During feedback inhibition, for example, the binding of an inhibitor at a
regulatory site on a protein causes the protein to spend more time in a
conformation in which its active site—located elsewhere in the protein—
becomes less accommodating to the substrate molecule (Figure 4−44).
As schematically illustrated in Figure 4–45A, many—if not most—protein
molecules are allosteric: they can adopt two or more slightly different
conformations, and their activity can be regulated by a shift from one
to another. This is true not only for enzymes, but also for many other
proteins as well. The chemistry involved here is extremely simple in concept.
Because each protein conformation will have somewhat different
contours on its surface, the protein’s binding sites for ligands will be
active site
ON
ACTIVE ENZYME
5 nm
CTP
regulatory
sites
OFF
INACTIVE ENZYME
bound CTP
molecule
Figure 4−44 Feedback inhibition triggers
a conformational change in an enzyme.
Aspartate transcarbamoylase from E. coli,
a large multisubunit enzyme used in early
studies of allosteric regulation, catalyzes
an important reaction that begins the
synthesis of the pyrimidine ring of
C, U, and T nucleotides (see Panel 2–7,
pp. 78–79). One of the final products of
this pathway, cytidine triphosphate (CTP),
binds to the enzyme to turn it off whenever
CTP is plentiful. This diagram shows the
conformational change that occurs when
the enzyme is turned off by CTP binding to
its four regulatory sites, which are distinct
from the active site where the substrate
binds. Figure 4−10 shows the structure of
aspartate transcarbamoylase as seen from
the top. This figure depicts the enzyme as
seen from the side.