Enzymes accelerate chemical reactions by lowering the activation energy needed for the reaction to occur. They do this by providing an alternative reaction pathway through the enzyme-substrate complex. The active site of the enzyme binds specifically to the substrate in a lock-and-key or induced-fit mechanism. This binding forms an enzyme-substrate complex that leads to products. The rate of reaction is increased as more molecules can now surpass the lowered transition state energy. Enzymes display specificity through their active site allowing only certain substrates, reactions, or stereoisomers to bind and undergo catalysis.

1. MODE OF ACTION OF
ENZYMES
Dr.Geeta Jaiswal

2. Catalytic Activity of Enzymes
The main characteristics of
enzymes is their Catalytic
power and Specificity
They can accelerate
reactions at least million
times by reducing their
energy of activation.

3. What is this energy of
activation?
Before a chemical reaction
can occur, the reacting
molecules need to gain some
minimum amount of energy
----- which is called the Energy
of Activation.

4. • This energy of activation can
be decreased by increasing
the temperature.
• Since our human body is
maintained at a normal body
temperature that is 37 Degree
centigrade. This is therefore
achieved by enzymes.

6.  All chemical reactions have an energy
barrier separating the reactants and
products .
This barrier is called the Free Energy of
Activation
It is the energy difference between the
reactants and a high energy intermediate
that occurs during formation pf product.
A T B (conversion of A to B through
transition state T)

7. • All chemical
reactions have an
energy barrier
separating the
reactants and the
products .
• This barrier is
called the Free
Energy of
Activation

8. The role of a catalyst or enzyme is
comparable with a tunnel made in a
mountain to reduce the barrier.
It is the energy difference between
the reactants and a high energy
intermediate that occurs during
formation of product.
A  T  B
 ( Conversion of A to B through transition state T )

9. Free Energy of Activation:
This peak represents the transition;
state in which a high energy
intermediates is formed during the
conversion of reactant to product.
Because of large activation energy,
the rate of uncatalyzed reactions is
often low.

10. The rate of reaction is determined by
such energized molecules.
In general lower the energy of
activation, the more molecules have
sufficient energy to pass over the
transition state, and thus faster is the
rate of reaction.

12. Alternate Reaction Pathway:
An enzyme allows a reaction to proceed
rapidly under conditions prevailing in the
cell by providing an alternate reaction
pathway for a lower energy of activation.
 The enzyme does not change the free
energy of reactants or products ,therefore
does not change the equilibrium of the
reaction

13. Rate of Reaction:
For molecules to react they must contain
sufficient energy to overcome the energy
barrier of the transition state.
In the absence of enzyme only a small
portion of the population of molecules may
possess enough energy to achieve the
transition state between reactants and
products.

14. Measurement of Enzyme Activity.
Enzyme activity can be measured on the basis
of
 Enzyme catalyzed reactions are highly efficient,
proceeding at 103 to 106 times faster than
uncatalyzed reactions.
Each enzyme molecule is capable of
transforming 100 t0 1000 substrate molecules
into products per second

15. Turn over number
The number of molecules of
substrate converted to product per
enzyme molecule per second is
called the turn over number.
Turnover Number, kcat: is the number
of substrate molecules metabolized per
enzyme. Molecules per unit time of
min-1 or sec-1

16. Specific Activity:
• Specific Activity: is usually
expressed as µmol of
substrate transformed to
product per minute per
milligram of enzymes under
optimal conditions of
measurements.

17. Models of Enzyme-Substrate
Complex
The prime requisite for enzyme
catalysis is that the substrate (S)
combines with the enzyme (E) at the
active site to form an enzyme-
substrate (ES) complex which
ultimately results in the formation of
product (P)

18. Lenor Michaelis and Maud
Menten in 1913 put forward this
theory
 E + S = ES (complex)  P + E
 A few theories have been put
forth to explain mechanism of
enzyme substrate complex
formation.

20. Fischer’s Lock and Key Template
model
Put forward in 1894.
This model explains that
enzymes have a rigid pre shaped
configuration like a lock, and
substrate has a shape
complementary to the lock that is
like a key

21. Fischer’s Lock and Key Template
model

23. II Koshlands Induced Fit Model
put forward in 1962
Koshland’s Induced Fit Model
This model explains on the basis that
active site of an enzyme is flexible
It undergoes conformational change
to attain final catalytic shape to suit
the substrate molecule

26. Catalytic or Active Site of
Enzymes
The enzyme proteins are big
large sized molecules as
compared to the substrates which
are relatively smaller.
Only a portion of the enzyme
molecule is involved in the
binding of the substrate

27.  The portion of the enzyme protein
molecule which actually takes part in
catalysis is called the Active or
Catalytic Site.
 Although enzymes differ widely in
structure specifically there are certain
common features about the Active site
such as:-
1)Active site is a small portion of three
dimensional enzyme proteins.

28. 2)It is situated in the crevice of the
enzyme molecule
3)To the active site a specific substrate
binds. This binding of substrate
depends on the specific groups or
atoms at the active site.
4) Specific groups come out from the
linear amino acid chain. The residues
may be far apart in a linear sequence,
but may come together to bring about
catalysis.

29. Active Site

32. 5). During binding these groups may
realign themselves to provide the
unique conformational orientation, so
as to promote exact fitting of
substrate to the active site.
6). The substrate binds to the Enzyme
at the active site by – Non –Covalent
Bonds. These forces are hydrophobic
in nature.

33. 7). The amino acids or groups that
directly participate in making or
breaking the bonds are called
Catalytic residues or groups.
8). The active site contains a
substrate binding site and a
catalytic site. Sometimes they
may be separate.

34. S.No Enzyme Important amino acids at Active site
1 Chymotrypsin His(57), Asp(102), Ser(195)
2 Trypsin Serine ,Histidine
3 Thrombin Serine ,Histidine
4 Phosphoglucomutase Serine
5 Alkaline Pase Serine
6 Acetyl-Cholinesterase Serine
7 Carbonic Anhydrase Cysteine
8 Hexokinase Histidine
9 Carboxy-peptidase Histidine, Arginine, Tyrosine
10 Aldolase Lysine

35. Enzyme Specificity
Enzyme specificity is determined by how
well the reactants fit into the enzyme
surface.
Some enzymes are very specific and
show activity with only one substrate.
However other enzymes are much less
particular and will catalyze reactions with
similar compound
Eg: Hexokinase, Glucokinase.

36. Types of Enzyme Specificity.
Specificity can be of three
types
Stereo specificity
Substrate Specificity
Reaction Specificity

37. Stereo Specificity:
 Stereo Specificity: acts on isomers holding
relevant groups in a particular configuration.
 Isomer Specificity:  Succinic Dehydrogenase
while acting on Succinic acid will give only Fumaric
acid and not Malic acid its isomer.
 Optical Specificity:D and L amino acid oxidases
 Geometric Specificity Specific for cis and Trans
bonds

38. Substrate Specificity:
It is of 2 types.
(a) Absolute Specificity (this is rare. )
eg:Urease will only catalyze the hydrolysis of
Urea
(b) Relative Specificity
Group dependent
Bond dependent

39. Group Dependent:
eg: Trypsin, Chymotrypsin
Trypsin hydrolyzes residues
of Lysine --- Arginine
Chymotrypsin ------ residue of
Aromatic a.a

40. Bond Specificity:
eg Proteolytic enzyme 
Peptide Bond
Glycosidases  glycosidic
bond
Lipases  ester bonds

41. Reaction Specificity
A substrate can undergo many reactions
In reaction specificity one enzyme can
catalyze only one reaction.
Eg: Conversion A D ,
E1 E2 E3
A B C D
( eg : Conversion 0f oxaloacetic acid to
Malic acid )

42. Thank - You