1. E-03: MECHANISM OF ENZYME ACTION
By
Santhosh Kumar. N M.Sc., PhD,(Medical)
Assistant Professor
Department of Biochemistry
SIMS & RH

2. TEMPLATE (OR) LOCK AND KEY
MODEL
 Substrate fits into active site of an enzyme
as key fits into lock hence “lock and key”model
Active site already exists in proper
conformation even in absence of substrate
Active site provides a rigid, pre-shaped
template fitting with the size and shape of the
substrate molecule.

3. INDUCED-FIT (OR) KOSHLAND
MODEL
Flexibility of the region of active site.
Active site does not possess a rigid,
preformed structure.
The substrate during its binding induces
conformational changes in the active site
to attain the final catalytic shape and form.

4. STRAIN INDUCTION
• In some cases, the binding of substrate to the enzyme results in a
change in the structure of the substrate.
• This alteration of the substrate 3D structure can be important in
inducing the desired reaction.

5. ENZYME SPECIFICITY

6. • Enzymes are highly specific in their action, when compared with the
chemical catalysts.
• The specificity is characteristic property of the active site.
• Mainly in 3 types
1) Stereo specificity
2) Reaction specificity
3) Substrate specificity

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1.Stereospecificity (or) Optical specificity
Stereoisomer are the compounds, which have the same molecular formula, but
differ in their structural configuration.
The enzymes act only on one isomer and, therefore, exhibit stereospecificity.
Example.
L-amino acid oxidase act on L- amino acids &
 D-amino acid oxidase act on D-amino acids
 Hexokinase acts on D-hexoses;
 Amylase acts on -glycosidic linkages;
 Cellulase cleaves -glycosidic bonds.

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2. Reaction Specificity
The same substrate can undergo different types of reactions, each catalysed
by a separate enzyme and this is referred to as reaction specificity.
 An amino acid can undergo transamination, oxidative deamination,
decarboxylation etc. The enzymes are different for each of these
reactions.

9. 3) Substrate specificity
• The substrate specificity varies from enzyme to enzymes.
• It may be either
a) Absolute substrate specificity
b) Relative substrate specificity
c) Broad substrate specificity.

10. Absolute substrate specificity
• Certain enzymes act only on one substrate.
Eg: Glucokinase act on Glucose to give Glucose -6-P.
Urease cleaves Urea to Ammonia & CO2

11. Relative substrate specificity
• Some enzymes act on structurally related substances.
• It may be dependent on the specific group or bond present.
Example:
 Trypsin hydrolyses peptides linkage involving Arg – Lys.
 Chymotrypsin cleaves peptide bonds, attached to aromatic a.a’s

12. Broad substrate specificity
• Some enzymes act on closely related substrates,
Example:
Hexokinase acts on Glucose, Fructose, Mannose & Galactose to
gives corresponding phosphates.

13. “Believe In Yourself”.
Know that there is something inside you
That is greater than any obstacles.
- Thank You -