section
7.2
Allosteric Enzyme Regulation
111
receptor that produces an intracellular signal (i.e., a
sec-
ond messenger
); one or more modifying enzymes whose
activity is affected by the intracellular signal; a target en-
zyme, which is the substrate for the modifying enzyme
and subjected to covalent modification with consequent
metabolic alteration; and an enzyme that reverses the mod-
ification of the target enzyme. The transducer of the sys-
tem is adenylate cyclase, and it catalyzes the cyclization
of ATP to cyclic AMP (cAMP), the second messenger.
The modifying enzyme is a protein kinase activated by
cAMP. Other protein kinases are activated by non-cAMP-
dependent or Ca2+-dependent systems (Chapter 30). Phos-
phorylation of a target enzyme may be either stimulatory or
inhibitory. For example, phosphorylation converts glyco-
gen phosphorylase to an activated form and glycogen syn-
thase to an inactivated form, thus preventing the simulta-
neous occurrence of glycogen breakdown and synthesis
(Chapter 15).
7.2 Allosteric Enzyme Regulation
Those enzymes in metabolic pathways whose activities
can be regulated by noncovalent interactions of certain
compounds at sites other than the catalytic are known as
allosteric enzymes.
They are usually rate-determining en-
zymes and play a critical role in the control and integration
of metabolic processes. The term “allosteric” is of Greek
origin, the root word “alios” meaning “other.” Thus, an
allosteric site
is a unique region of an enzyme
other
than
the substrate binding site that leads to catalysis. At the
allosteric site, the enzyme is regulated by noncovalent in-
teraction with specific ligands known as
effectors, modu-
lators,
or
modifiers.
The properties of allosteric enzymes differ significantly
from those of nonregulatory enzymes. Ligands (in some
instances even the substrate) can bind at such sites by
a cooperative binding process. Cooperativity describes
the process by which binding of a ligand to a regula-
tory site affects binding of the same or of another lig-
and to the enzyme. Allosteric enzymes have a more com-
plex structure than nonallosteric enzymes and do not
follow Michaelis-Menten kinetics. An allosteric site is
specific for its ligand, just as the active site is specific for
its substrate. Binding of an allosteric modulator causes a
change in the conformation of the enzyme (see below)
that leads to a change in the binding affinity of the en-
zyme for the substrate. The effect of a modulator may
be positive (activatory) or negative (inhibitory). The for-
mer leads to increased affinity of the enzyme for its sub-
strate, whereas the reverse is true for the latter. Activatory
sites and inhibitory sites are separate and specific for their
respective modulators. Thus, if an end product of a
metabolic pathway accumulates in excess of its steady-
state level, it can slow down or turn off the metabolic
pathway by binding to the inhibitory site of the regulatory
enzyme of the pathway. As the concentration of the end
product (inhibitor) decreases below the steady-state level,
the number of enzymes having bound inhibitor decreases
and they revert to their active form. In this instance, the
substrate and the negative modulator bear no structural re-
semblance. An allosteric enzyme may be positively mod-
ulated by the substrate itself or by a metabolite of another
pathway that depends on production of the end product
in question for its eventual utilization (e.g., pathways of
synthesis of purine and pyrimidine nucleotides in the for-
mation of nucleic acids; see Chapter 27).
Most allosteric enzymes are
oligomers
(i.e., they con-
sist of two or more polypeptide chains or subunits). The
subunits are known as
protomers.
Two types of interaction
occur in allosteric enzymes:
homotropic
and
heterotropic.
In a homotropic interaction, the same ligand influences
positively the cooperativity between different modulator
sites. An example is a regulatory enzyme modulated by
its own substrate. Thus, this class of enzyme has at least
two substrate binding sites which respond to situations that
lead to substrate excess by increasing its rate of removal.
Heterotropic interaction refers to the effect of one ligand
on the binding of a
different
ligand. For example, a regula-
tory enzyme modulated by a ligand other than its substrate
constitutes a heterotropic system, in which the cooperativ-
ity can be positive or negative. Some allosteric enzymes
exhibit mixed homotropic and heterotropic interactions.
Kinetics of Allosteric Proteins
The kinetic properties of allosteric enzymes vary signif-
icantly from those of nonallosteric enzymes, exhibiting
cooperative interactions between the substrate, the activa-
tor, and the inhibitor sites. These properties are respon-
sible for deviations from the classic Michaelis-Menten
kinetics that apply to nonallosteric enzymes. Nonallosteric
enzymes yield a rectangular hyperbolic curve when the
initial velocity (v) is plotted against the substrate con-
centration [S], For allosteric enzymes, a plot of v versus
[S] yields curves of different shapes, including sigmoid-
shaped curves in some cases. (A sigmoidal curve can result
from other mechanisms.)
The v versus [S] plot for a homotropic enzyme is shown
in Figure 7-1. The following features should be noted:
1. The substrate functions as a positive modulator; i.e.,
there is positive cooperativity between the substrate
binding sites so that binding of the substrate at one
