160 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
Noncompetitive Inhibition
An allosteric site is a specific location on the
enzyme where an allosteric regulator, a regulatory
molecule that does not directly block the active site
of the enzyme, can bind. When the allosteric regula-
tor attaches to the specific site, it causes a change in
the conformation of enzyme active site, and the sub-
strate therefore will not fit into the active site of
enzyme; this results in the inhibition phenomenon.
The enzyme cannot catalyze the reaction, and not
only the amount of final product but also the concen-
tration of the allosteric regulator decrease. Since the
allosteric regulator (the inhibitor) does not compete
with the substrate for binding to the active site of
enzyme, the inhibition mechanism is called non-
competitive inhibition.
Competitive Inhibition
A regulatory molecule is not the substrate of the spe-
cific enzyme but shows affinity for attaching to the
active site. When it occupies the active site, the sub-
strate cannot bind to the enzyme for the catalytic
reaction, and the metabolic pathway is inhibited.
Since the regulatory molecule will compete with the
substrate for binding to the active site on enzyme,
this inhibitory mechanism of the regulatory mole-
cule is called competitive inhibition.
ENZYMEKINETICS
During the course of an enzyme-catalyzed reaction,
the plot of product formation over time (product for-
mation profile) reveals an initial rapid increase, ap-
proximately linear, of product, and then the rate of
increase decreases to zero as time passes (Fig.
7.2A). The slope of initial rate (vior v), also called
the steady state rate, appears to be initially linear in
a plot where the product concentration versus reac-
tion time follows the establishment of the steady
state of a reaction. When the reaction is approaching
equilibrium or the substrate is depleted, the rate of
product formation decreases, and the slope strays
away from linearity. As is known by varying the
conditions of the reaction, the kinetics of a reaction
will appear linear during a time period in the early
phase of the reaction, and the reaction rate is meas-
ured during this phase. When the substrate concen-
trations are varied, the product formation profiles
will display linear substrate dependency at this
phase (Fig. 7.2B). The rates for each substrate con-
centration are measured as the slope of a plot of
product formation over time. A plot of initial rate asFigure 7.2. (A)Plot of progress curve during an
enzyme-catalyzed reaction for product formation.
(B)Plot of progress curves during an enzyme-catalyzed
reaction for product formation with a different starting
concentration of substrate. (C) Plot of reaction rate as a
function of substrate concentration measured from
slopes of lines from (B).