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chapter7
Enzymes 11: Regulation
FIGURE 7-1
Relationship between the initial velocity (v) and the substrate
concentration [S] for an allosteric enzyme that shows a homotropic effect.
The substrate functions as a positive modulator. The profile is sigmoidal,
and during the steep part of the profile, small changes in [S] can cause
large changes in v. A'o.s represents the substrate concentration
corresponding to half-maximal velocity.
binding site greatly enhances binding of the substrate
at the other sites. As the substrate concentration
increases, there is a large increase in the velocity of
the reaction.
2. Owing to the above effect, the shape of the curve is
sigmoidal.
3. The value of the substrate concentration
corresponding to half-maximal velocity is designated
as
K0.5
and not
Km
since the allosteric kinetics do not
follow the hyperbolic Michaelis-Menten relationship.
4. Maximum velocity (Vmax) is attainable at a rather high
substrate concentration, implying saturation of the
catalytic site of the enzyme.
The v versus [S] plot for heterotropic enzymes is more
complex. The kinetic profiles can be divided into two
classes, depending upon whether the allosteric effector
alters
Kq,s
and maintains a constant Vmax or alters Vmax
and maintains a nearly constant A'o.s. The v versus [S]
profile of an allosteric enzyme that follows the former
set of properties is shown in Figure 7-2. In the absence
of any modulators, the profile is sigmoidal (curve a). In
the presence of a positive modulator (curve b), the value
for
K05
is decreased; i.e., a lower substrate concentra-
tion is required to attain half-maximal velocity. Curve
b is more hyperbolic than sigmoidal. Curve c obtained
with a negative modulator is more sigmoidal than curve
a, and the ATo.s value is increased, reflecting a decreased
affinity for the substrate, i.e., a higher substrate concen-
tration is required to attain half-maximal velocity. Regula-
tion of the enzyme is achieved through positive and nega-
tive modulators. Thus, at a given substrate concentration
(e.g., steady-state level), the activity of the enzyme
FIGURE 7-2
Relationship between the initial velocity (v) and the substrate
concentration [S] for an allosteric enzyme that shows a heterotropic effect
with constant Vmax but with varying A'o.s. Curve
a
is obtained in the
absence of any modulators, curve
b
in the presence of a positive modulator,
and curve
c
in the presence of a negative modulator. Regulation is achieved
by modulation of A'o.s without change in Vmax.
can be turned on or off with appropriate modulators.
Figure 7-3 shows v versus [S] plots for allosteric enzymes
modulated by changes in Vmax but retaining an essentially
constant
Ko.5.
This type of modulation is less common than
the two previous cases considered. The positive modulator
increases Vmax (curve b), whereas the negative modulator
decreases Vmax (curve c).
Examples of Allosteric Proteins
We will consider two specific examples of allosteric
proteins—one an enzyme and the other an oxygen
transport protein.
FIGURE 7-3
Relationship between the initial velocity (v) and the substrate
concentration [S] for an allosteric enzyme that shows a heterotropic effect
with constant A'o.s but with varying VmaX. Curve a is obtained in the
absence of any modulator, curve b in the presence of a positive modulator,
and curve c in the presence of a negative modulator. Regulation is achieved
by modulation of Vmax without change in A'o.s.
