Reversible enzyme inhibitors can reduce or inhibit enzyme catalytic activity through reversible, non-covalent interactions with the enzyme. There are three main types of reversible inhibitors: competitive inhibitors, which compete with the substrate for the active site; uncompetitive inhibitors, which only bind to the enzyme-substrate complex; and noncompetitive inhibitors, which bind randomly to the free enzyme or enzyme-substrate complex, inhibiting both. Reversible inhibitors form equilibrium complexes with the enzyme and do not permanently modify it, allowing activity to be restored once the inhibitor is removed. Examples of reversible inhibitors discussed include sulfanilamide as a competitive inhibitor of PABA and methotrexate as a competitive inhibitor of dihydrofolate reductase.

1. REVERSIBLE ENZYME INHIBITORS
Prepared by
MAHENDRA.G.S
1 M pharm
Department of Pharmaceutical
chemistry
J S S College of Pharmacy
Mysuru

2.  Introduction
 Importance of enzyme inhibition
 Types of enzyme inhibitors
 Reference
2
CONTENTS

3. Introduction
• Enzyme is a protein molecule acting as catalyst in enzyme
reaction.
• Enzyme inhibition is a science of enzyme-substrate reaction
influenced by the presence of any organic chemical or
inorganic metal or biosynthetic compound due to their
covalent or non-covalent interactions with enzyme active
site.
• It is well known that all these inhibitors follow same rule to
interplay in enzyme reaction.

4. What are enzyme inhibitors?
 The enzyme inhibitors are low molecular weight chemical
compounds.
 They can reduce or completely inhibit the enzyme catalytic
activity either reversibly or permanently (irreversibly).
 Inhibitor can modify one amino acid, or several side chain(s)
required in enzyme catalytic activity.
 In drug discovery, several drug analogues are chosen or
designed to inhibit specific enzymes.

5. Importance of enzyme inhibition
• To understanding the regulation of enzyme activity
within the living cells.
• To elucidate the kinetic mechanism of an enzyme
catalyzing in a multi substrate reaction.
•Identification of the catalytic groups at the active site.
• Provide information about substrate specificity of
the enzyme.

6. Michaelis-menten equation
The michaelis-menten equation arises from the
general equation for an enzymatic reaction.
E+S ES E+P
The michaelis menten equation is:
V˳=
Where=
V˳= velocity of the reaction
Vmax= maximal rate of the reaction
[substrate]= conc. Of the substrate
Km= michaelis-menten constant
Vmax [S]
km+[S]
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8.  Inhibitor binds to Enzyme reversibly through non covalent
interactions.
 An Equilibrium is established between the free inhibitor &
EI Complex and is defined by an equilibrium constant (Ki).
 The activity of Enzyme Is fully restored on removing the
Inhibitor by dialysis.
1) Reversible inhibitor:

9. Competitive Inhibitors
• A competitive inhibitor often has structural features similar to the
substrates whose reactions they inhibit.
• A I and S are in direct competition for the same binding site on
the enzyme.
• The enzyme-bound inhibitor may either lack an appropriate
functional group for further reaction/signalling or may be bound
in the wrong position with respect to the catalytic residues.
• In any event, the enzyme- inhibitor complex (EI) is unreactive /
dead-end complex.

10. Typically, I is a substrate
analog.

11. 11
Example:
• The antibiotic sulfanilamide is similar in structure to para-
aminobenzoic acid (PABA), an intermediate in the biosynthetic
pathway for folic acid.
• Sulfanilamide can competitively inhibit the enzyme that has
PABA as it's normal substrate by competitively occupying the
active site of the enzyme.

12. The p-aminobenzoic acid substrate and sulfanilamide as
a competitive inhibitor
during the bacterial folate synthesis.

13. Example:
• competitive inhibitory effect of malonic acid on succinate
dehydrogenase (SD) of the Krebs' cycle that reversibly
dehydrogenates succinate into fumarate.
The substrate and different competitive inhibitors of
succinate dehydrogenase (SD).

14. Methotrexate - competitive inhibitor of dihydrofolate reductase
(DHFR). The drug is used as anticancer antimetabolite
chemotherapy particularly for pediatric leukemia.
Example:
The substrate and methotrexate as a competitive inhibitor for
dihydrofolate reductase.

15. Peptide-based
The substrate mimics competitive inhibitors are the peptide-based
protease inhibitors, a very successful class of antiretroviral drugs
used to treat HIV, e.g.,ritonavir that contains three peptide bonds.
Example:
The peptide-based competitive protease inhibitor
ritonavir.

16. Uncompetitive Inhibitors
 Uncompetitive inhibitors do not bind to the free enzyme.
They bind only to the enzyme-substrate complex to yield an
inactive ESI complex.
 Uncompetitive inhibition is rarely observed in single-
substrate reactions but is frequently observed in multisubstrate
reactions.
 An uncompetitive inhibitor can provide information about the
order of binding of the different substrates.
E
S
E.S E + P
I
E.S.I

18. Noncompetitive Inhibitor
 Classical noncompetitive inhibitors have no effect on substrate
binding, rather they bind randomly and reversibly to different
sites on the enzyme. (Allosteric site)
 They also bind with the same affinity to the free enzyme and to
the enzyme substrate complex. Both the enzyme- inhibitor
complex (E.I) and the enzyme substrate inhibitor complex
(E.S.I) are inactive.

19.  These inhibitors do not affect substrate binding.
 Again, this type of inhibition is rarely seen in single-substrate
reactions. The affinity of the noncompetitive inhibitor for the
free enzyme, and the enzyme-substrate complex, are different.
 These non-ideally behaving noncompetitive inhibitors are
called mixed-type inhibitors.

22. Example:
 Silver ions (heavy metal) react with -SH groups in the side groups
of cysteine residues in the protein chain:
 If the cysteine residue is somewhere on the protein chain which
affects the way it folds into its tertiary structure, then altering this
group could have an effect on the shape of the active site, and so
stop the enzyme from working.
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25. References
•Ashutosh Kar. Medicinal Chemistry (Fifth revised &
Expandded edition), New Age International Publishers. 2010:
917-972
•Burger’s. Medicinal Chemistry & Drug Discovery. Sixth
Edition (Volume 1). A john wiley and sond, Inc., Publiction,
new jersey. 2003: 715-774
•Albert L. Lehninger. Biochemistry (Second edition), Worth
Publishers, Inc. New York. 1979: 189-195