The document discusses enzymes and their classification. It defines enzymes as biological catalysts that are usually proteins and increase the rate of chemical reactions. It describes the six main classes of enzymes based on their catalytic activity as well as the Enzyme Commission (EC) numbering system. The key points are that enzymes have unique active sites that substrates fit into, they are most active at optimal temperatures and pH levels, and their reaction rates depend on enzyme and substrate concentrations.

1. 4th SEMESTER – BOTANY
KARNATAKA UNIVERSITY, DHARWAD
Modified from various internet resources by
Dr. Jayakara Bhandary
Associate Professor of Botany
Government Arts & Science College
Karwar, Uttara Kannada
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2. Introduction & History
Enzyme, in Greek means in living (en= in, zyme =
living).
Biocatalysts or Organic catalysts, usually high
molecular weight proteins (exception- Ribozymes
or RNA enzymes).
Coined by Kuhne in 1878.
First enzyme extract from Yeast cells by Buchner
(1897).
First purified enzyme is urease, by James B.
Summer (1926).
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3. Enzymes are Biological Catalysts
Enzymes are proteins that:
Increase the rate of
reaction by lowering the
energy of activation.
Catalyze nearly all the
chemical reactions taking
place in the cells of the
body.
Have unique three-
dimensional shapes that
fit the shapes of reactants.
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4. Activation Energy
Think of activation energy as the
BARRIER
required to make a product.
Most stable product is the one with
the lowest
energy.
Most reactions require a “push” to
get them
started! “Push” is called “energy
of activation” for
reaction - Also represented by EA
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5. Trivial Names of Enzymes
The name of an enzyme:
Usually ends in –ase.
Identifies the reacting substance. For
example,
sucrase catalyzes the reaction of sucrose.
Describes the function of the enzyme. For
example, oxidases catalyze oxidation.
Could be a common name, particularly for
the digestion enzymes such as pepsin and
trypsin.
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6. IUB Classification of Enzymes
Enzymes are classified according to the reaction
they catalyze.
Class Reactions catalyzed
Oxidoreductases Oxidation-reduction
Transferases Transfer groups of atoms
Hydrolases Hydrolysis
Lyases Add atoms/remove atoms
to/from a double bond
Isomerases Rearrange atoms
Ligases Use ATP to combine
molecules
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7. Systematic Name
According to the International union Of Biochemistry
an enzyme name has two parts:
-First part is the name of the substrates for the
enzyme.
-Second part is the type of reaction catalyzed by
the enzyme.This part ends with the suffix “ase”.
Example: Lactate dehydrogenase

8. EC number
Enzymes are classified into six different groups
according to the reaction being catalyzed. The
nomenclature was determined by the Enzyme
Commission in 1961 (with the latest update having
occurred in 1992), hence all enzymes are assigned an
“EC” number. The classification does not take into
account amino acid sequence (ie, homology), protein
structure, or chemical mechanism.

9. EC numbers
EC numbers are four digits, for example a.b.c.d, where
“a” is the class, “b” is the subclass, “c” is the sub-
subclass, and “d” is the sub-sub-subclass. The “b” and
“c” digits describe the reaction, while the “d” digit is
used to distinguish between different enzymes of the
same function based on the actual substrate in the
reaction.
Example: for Alcohol:NAD+oxidoreductase EC
number is 1.1.1.1

10. The Six Classes
EC 1. Oxidoreductases
EC 2. Transferases
EC 3. Hydrolases
EC 4. Lyases
EC 5. Isomerases
EC 6. Ligases

11. EC 1. Oxidoreductases
Catalyze the transfer of hydrogen or oxygen atoms
or electrons from one substrate to another.
Since these are ‘redox’ reactions, an electron
donor/acceptor is also required to complete the
reaction.
A H2 +B → A+ BH2
Ex. Oxidases, Dehydrogenases,
Reductases.

12. EC 2. Transferases
Catalyze group transfer reactions, excluding
oxidoreductases (which transfer hydrogen or
oxygen and are EC 1). These are of the general
form:
A-X + B ↔ BX + A
Ex: Transaminases (transfer amino group),
Kinases (transfer Phosphate group)

13. EC 3. Hydrolases
Catalyze hydrolytic reactions.
Includes.
A-X + H2O ↔ X-OH + A-H
Ex: lipases, esterases, Amylases,
peptidases/proteases, etc.

14. EC 4. Lyases
Catalyze non-hydrolytic (covered in EC 3) removal
of functional groups from substrates, often
creating a double bond in the product; or the
reverse reaction, ie, addition of function groups
across a double bond.
A- X +B-Y → A=B + X-Y
Ex: Decarboxylases, Aldolases, Dehydrases,
Deaminases, Synthases, etc.

15. EC 5. Isomerases
Catalyzes isomerization reactions, including
epimerizations and cis-trans
isomerizations.
A →A’
Ex: Isomerases (Cis-Trans),
Epimerases (D—L)

16. EC 6. Ligases
Catalyzes the synthesis of various (mostly C-X)
bonds, coupled with the breakdown of energy-
containing substrates, usually ATP .
A+B → A-B
ATP → ADP+iP
Ex: Synthetases, Carboxylases

17. Classification of Enzymes: Oxidoreductases
and Transferases
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18. Classification: Hydrolases and Lyases
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19. Classification: Isomerases and Ligases
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20. Active Site
The active site:
Is a region within an
enzyme that fits the
shape of molecules
called substrates.
Contains amino acid R
groups that align and
bind the substrate.
Releases products
when the reaction is
complete.
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21. Enzyme Specificity
Enzymes may recognize and catalyze:
A single substrate.
A group of similar substrates.
A particular type of bond.
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22. Mechanism of
Enzyme Catalyzed Reactions
The proper fit of a substrate (S) in an active site
forms an enzyme-substrate (ES) complex.
E+S ES
Within the ES complex, the reaction occurs to
convert substrate to product (P).
ES E+P
The products, which are no longer attracted to
the active site, are released.
Overall, substrate is convert to product.
E+S ES E+P
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23. Mechanism of Enzyme Action
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24. 24

25. Example of An Enzyme Catalyzed Reaction
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26. Mechanism of Enzyme Action:
1.Lock-and-Key Model
In the lock-and-key model of enzyme action:
The active site has a rigid shape.
Only substrates with the matching shape can
fit.
The substrate is a key that fits the lock of the
active site.
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27. 2. Induced-fit Model
In the induced-fit model of enzyme action:
The active site is flexible, not rigid.
The shapes of the enzyme, active site, and
substrate adjust to maximum the fit, which
improves catalysis.
There is a greater range of substrate specificity.
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28. Isoenzymes
Isoenzymes
catalyze the same
reaction in
different tissues in
the body.
Lactate
dehydrogenase,
which converts
lactate to pyruvate,
(LDH) consists of
five isoenzymes.
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29. Temperature and Enzyme Action
Enzymes:
Are most active at an
optimum
temperature (usually
37°C in humans).
Show little activity at
low temperatures.
Lose activity at high
temperatures as
denaturation occurs.
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30. pH and Enzyme Action
Enzymes:
Are most active at
optimum pH.
Contain R groups of
amino acids with
proper charges at
optimum pH.
Lose activity in low or
high pH as tertiary
structure is
disrupted. 30

31. Optimum pH Values
Most enzymes of the body have an optimum pH
of about 7.4.
In certain organs, enzymes operate at lower and
higher optimum pH values.
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32. Enzyme Concentration
The rate of
reaction increases
as enzyme
concentration
increases (at
constant substrate
concentration).
At higher enzyme
concentrations,
more substrate
binds with
enzyme. 32

33. Substrate Concentration
The rate of reaction
increases as
substrate
concentration
increases (at
constant enzyme
concentration).
Maximum activity
occurs when the
enzyme is
saturated.
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