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Enzyme Inhibition

The Michaelis constant (Km) is a measure of the affinity of an enzyme for its substrate. Km is the substrate concentration at which the reaction velocity is half maximal. The turnover number (kcat) is the number of substrate molecules converted to products per unit time by one enzyme molecule when it is saturated with substrate. The specificity constant (kcat/Km) compares the catalytic efficiencies of different enzymes or substrates, with higher values indicating greater efficiency. Competitive inhibitors bind the enzyme's active site, preventing substrate binding. Uncompetitive inhibitors bind the enzyme-substrate complex. Mixed inhibition involves aspects of both competitive and uncompetitive inhibition.

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57 views17 pages

Enzyme Inhibition

The Michaelis constant (Km) is a measure of the affinity of an enzyme for its substrate. Km is the substrate concentration at which the reaction velocity is half maximal. The turnover number (kcat) is the number of substrate molecules converted to products per unit time by one enzyme molecule when it is saturated with substrate. The specificity constant (kcat/Km) compares the catalytic efficiencies of different enzymes or substrates, with higher values indicating greater efficiency. Competitive inhibitors bind the enzyme's active site, preventing substrate binding. Uncompetitive inhibitors bind the enzyme-substrate complex. Mixed inhibition involves aspects of both competitive and uncompetitive inhibition.

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Subrata Kundu
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Significance of the Michaelis Constant

When k2 is rate-limiting, k2 << k-1 and km reduces to


K-1 /k1, which is defined as the dissociation constant,
kd, of the ES complex. Km is therefore a measure of the
affinity of the enzyme for its substrate.

Km is also the substrate concentration at which the


reaction velocity is half-maximal.
Dr. Subrata Kundu, Dept. of Microbiology, RKMVC College
If the reaction has several steps and one is clearly
rate-limiting, k cat is equivalent to the rate constant for
that limiting step.
The constant k cat is a first-order rate constant and
hence has units of reciprocal time.

It is also called the turnover number.

It is equivalent to the number of substrate


molecules converted to product in a given unit of time
on a single enzyme molecule when the enzyme is
saturated with substrate.
Kinetic efficiency of enzymes

The catalytic efficiencies of different enzymes or the


turnover of different substrates by the same
enzyme is compared by the ratio k cat / K m for the
two reactions. This parameter is called the specificity
constant.
It is the rate constant for the conversion of E + S to
E+ P. When [S] << K m ,
V0 in this case depends on the concentration of two
reactants, [E t] and [S]; therefore this is a second-order
rate equation and the constant kcat / K m is a second-
order rate constant with units of M -1 S-1.

The enymes with kcat / K m in the range of 108 to 109 M-1


S-1 are said to have achieved catalytic perfection.
Enzyme inhibition
Competitive inhibition
A substance that competes directly with a normal substrate for an
enzymatic binding site is known as a competitive inhibitor. Such
an inhibitor usually resembles the substrate to the extent that it
specifically binds to the active site but differs from it so as to be
unreactive.

Dr. Subrata Kundu, Dept. of Microbiology, RKMVC College


When [S] far exceeds [I], the probability that an
inhibitor molecule will bind to the enzyme is
minimized and the reaction exhibits a normal Vmax .
However, the [S] at which V0 = 1/2 Vmax , the apparent
Km , increases in the presence of inhibitor by the factor
.

Dr. Subrata Kundu, Dept. of Microbiology, RKMVC College


Succinate dehydrogenase, a citric acid cycle enzyme
that functions to convert succinate to fumarate is
competitively inhibited by malonate, which
structurally resembles succinate but cannot be
dehydrogenated:

Dr. Subrata Kundu, Dept. of Microbiology, RKMVC College


Ethanol in the body is oxidized to acetaldehyde
(CH3CHO) by liver alcohol dehydrogenase (LADH).
Methanol is oxidized by LADH to the quite toxic
product formaldehyde (HCHO). The toxic effects of
ingesting methanol (a component of many commercial
solvents) can be reduced by administering ethanol. The
ethanol acts as a competitive inhibitor of the methanol
by displacing it from LADH. This provides sufficient
time for the methanol to be harmlessly excreted by the
kidneys.

Dr. Subrata Kundu, Dept. of Microbiology, RKMVC College


Uncompetitive inhibition
In uncompetitive inhibition, the inhibitor binds
directly to the enzyme–substrate complex but not to
the free enzyme.

Dr. Subrata Kundu, Dept. of Microbiology, RKMVC College


At high concentrations of substrate, V0 approaches
Vmax/ʹ. Thus, an uncompetitive inhibitor lowers the
measured Vmax. Apparent Km also decreases, because
the [S] required to reach one-half Vmax decreases by
the factor ʹ.

Dr. Subrata Kundu, Dept. of Microbiology, RKMVC College


Mixed inhibition

Dr. Subrata Kundu, Dept. of Microbiology, RKMVC College


Dr. Subrata Kundu, Dept. of Microbiology, RKMVC College
Effects of Inhibitors on the Parameters of the
Michaelis–Menten Equation

Dr. Subrata Kundu, Dept. of Microbiology, RKMVC College

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