Enzymes are proteins that act as biological catalysts, increasing the rate of reactions without being used up. The Michaelis-Menten model describes enzyme kinetics, including the maximum reaction rate (Vmax) and substrate concentration when the reaction is at half Vmax (Km). Factors like temperature and pH affect enzyme activity, with optimal temperatures and pH levels before heat denatures the enzyme or pH changes disrupt its structure. Enzyme inhibitors decrease reaction rates by binding to the enzyme's active site (competitive) or altering its structure (non-competitive, uncompetitive).

1. KINETICS OF ENZYME
ACTIVITY
By Sayanta Mitra

2. CONTEXT
 Enzyme
 Classification of enzymes
 Structure of enzymes
 Fischer’s Lock and Key Model
 Michaelis-Menten Model
 Factors affecting enzyme activity
Enzyme inhibition
 Conclusion

3.  What is Enzyme?
 Proteins that act as biological catalysts.
 Increase the rate of a reaction without being used up or changed
themselves.
 Enzymes act upon the molecules are known as substrates.
Coenzyme :
A substance that enhances the action of an enzyme.
They cannot by themselves catalyze a reaction but they can help
enzymes to do so. Coenzymes are organic non-protein molecules
that bind with the protein molecule (apoenzyme) to form the
active enzyme (holoenzyme).

4. Apoenzyme :
A protein that forms an active enzyme system by
combination with a coenzyme and determines the specificity of this
system for a substrate is called apoenzyme.
Holoenzyme :
Holoenzymes are the active forms of enzymes. Enzymes
that require a cofactor but are not bound by one are called
apoenzymes. Holoenzymes represent the apoenzyme bound to its
necessary cofactors or prosthetic groups

5. Classification of enzymes:
International Union of Biochemistry (I.U.B.) in 1964 has adopted
classification of enzymes depending on the type of reactions they catalyze.
According to this commission, all enzymes are classified into 6 major
classes
i. Oxidoreductases- Oxidation-reduction reactions (transfer of
electrons).
ii. Transferases- Transfer of group
iii. Hydrolases- Hydrolytic reactions (transfer of functional groups to
water)
iv. Lyases- Addition or removal of groups to form double bonds
v. Isomerases- Transfer of groups within molecules to yield isomeric
forms
vi. Ligases- Condensation of two molecules coupled through ATP
hydrolysis

6. Enzyme Structure :
 Enzymes are proteins and usually have a globular tertiary structure.
 Enzyme has the active site.
Active site:
 The active site of an enzyme is the region that binds the substrate and
converts it into product.
 Three-dimensional structure.
 Consists of portions of a polypeptide chain.
 Enzyme binds its substrate and form enzyme–substrate complex.

7.  Fischer’s Lock and Key Model:
 Model is proposed by Emil Fischer.
 The substrate can fit into its complementary site on the enzyme as a key
fits into a lock.
 Two shapes are rigid and fixed.
 Perfectly complement each other.

8. Kinetics of Enzyme :
 Michaelis-Menten Model:
 The Michaelis-Menten model is one of the simplest and best-known approaches
to enzyme kinetics.
 Michaelis-Menten kinetics is a model of enzyme kinetics which explains how the
rate of an enzyme-catalysed reaction depends on the concentration of the enzyme
and its substrate.
Let’s consider a reaction in which a substrate (S) binds reversibly to an enzyme
(E) to form an enzyme-substrate complex (ES), which then reacts irreversibly to
form a product (P) and release the enzyme again.
S + E ⇌ ES → P + E
Two important terms within Michaelis-Menten kinetics are:

9.  Vmax – The maximum rate of the reaction, when all the enzyme’s
active sites are saturated with substrate.
 Km – The substrate concentration at which the reaction rate is 50% of
the Vmax . Km is a measure of the affinity, an enzyme has for its substrate.
As the lower the value of Km, the more efficient the enzyme is at
carrying out its function at a lower substrate concentration.
The Michaelis-Menten equation for the reaction is:
v0 = Velocity of reaction
Vmax = Maximum rate achieved by the system
[S] = Concentration of a substrate S
Km = Michaelis constant

10.  Factors affecting enzyme activity :
Temperature:
Temperature affects the rate of enzyme-catalyzed reactions in two ways,-
 First, a rise in temperature, increasing the thermal energy of the substrate molecules.
This raises, proportion of substrate molecules with sufficient energy to overcome
the Gibbs free energy of activation (ΔG‡) . Hence increases the rate of reaction.
 The difference in energy level between the substrates and products is termed
the change in Gibbs free energy (ΔG).
 The difference in free energy between the substrate and the transition state is
termed the Gibbs free energy of activation (ΔG‡).
 Second, Increasing the thermal energy of the molecules make up the protein
structure of the enzyme itself will increase the chances of breaking the multiple
weak, non-covalent interactions (hydrogen bonds, van der Waals forces, etc.) which
hold the three-dimensional structure of the enzyme together Ultimately this will lead
to the denaturation of the enzyme.

11.  pH:
Each enzyme has an optimum pH at which the rate of the reaction that it
catalyzes is at its maximum.
Small deviations in pH from the optimum value lead to decreased activity
due to changes in the ionization of groups at the active site of the enzyme.
 Larger deviations in pH lead to the denaturation of the enzyme protein
itself, due to interference with the many weak non-covalent bonds
maintaining its three-dimensional structure.
The effect of (a) temperature and (b) pH on enzyme activity

12. Enzyme inhibition :
Substances which decrease the rate of an enzyme catalyzed reaction are
called as enzyme inhibitors and the process is known as enzyme inhibition.
There are three types of reversible inhibitions:
 Competitive inhibition-
• In competitive inhibition, there is close resemblance in the structure of
inhibitor I and substrate S, therefore, they both compete for the same active
site on the enzyme.
• Competitive inhibitors decreases the rate of reaction by reducing the
amount of active enzyme molecules bound to a substrate.
• The enzyme can form enzyme-substrate ES complex or it can form enzyme-
inhibitor EI complex but not both ESI.
 Non-Competitive inhibition-
• In this type of inhibition, inhibitor has no structural similarity with substrate
and binds with the enzyme at different site other than the active site.

13. • Competitive inhibitors decreases the rate of reaction by reducing the amount
of active enzyme molecules bound to a substrate.
• At very high substrate concentration, the chances of binding of inhibitor
molecule to the enzyme will be reduced, so Vmax for the reaction will not be
changed.
Uncompetitive inhibition-
• In this type of inhibition, inhibitor does not bind to free enzyme. It binds only
to enzyme substrate (ES) complex, directly or its binding is facilitated by the
conformational change that takes place after substrate binds to enzyme.
•In both the cases, the inhibitor does not compete with substrate for the same
binding site.
•Therefore, the inhibition cannot be overcome by increasing substrate
concentration. Both Km and Vmax values are altered.

14. Conclusion :
 Enzyme kinetics finds its usefulness in various reactions
mediate by enzyme , which include biochemical reactions.
 Enzymes work best in their optimum condition .
 Enzyme are used in Foods and beverages processing,
animal nutrition, textile, etc.