Definition of enzyme, it's properties, classification, factors affecting enzyme activity, coenzyme, cofactor, enzyme specificity, enzyme inhibition

1. Chemistry ofChemistry of
EnzymeEnzyme
Dr Ifat Ara Begum
Associate Professor
Dept of Biochemistry
Dhaka Medical College
Dhaka
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2. What is Enzyme?What is Enzyme?
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 Enzymes are heat-labile, reaction specific
biocatalysts, mostly protein in nature.
Exception:
Ribozyme (Catalytic RNA molecules)
 Being produced by the living cells, enzymes
accelerate the rate of biochemical reactions.

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4. Enzyme vs. CatalystEnzyme vs. Catalyst
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Points Enzyme Catalyst
Chemica
l nature
Protein Organic /
Inorganic
Rate of
chemical
reaction
Always
accelerates
Accelerates /
Retards

5. Enzyme vs. HormoneEnzyme vs. Hormone
5
Points Enzyme Hormone
Chemical
nature
Protein Not always protein
. e.g. Steroid in
nature
Site of
action
At the site of
production
Distant to the site
of production
Transport
via blood
to the site
of action
No Yes
Nature of
activity
Shows
catalytic
activity
 Shows
regulatory
activity.
 Controls enzyme
activity

6. Properties of EnzymeProperties of Enzyme
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 Protein in nature [Exception: Ribozyme]
 Colloidal substance, Nondialyzable
 Water soluble
 Heat labile
 Catalyst
 Have definite isoelectric pH

7. Characteristics of Enzyme activityCharacteristics of Enzyme activity
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 Accelerate the rate of reaction after initiation
but remain unchanged chemically in the
overall process
 Do not :
 Initiate the reaction
 Alter the extent of reaction

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 Highly reaction specific
 Effective in extremely small quantity
 Functionally independent of parent cell
 Activity can be regulated by activation /
inhibition as per cellular need

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12. Active & Allosteric sites on enzymeActive & Allosteric sites on enzyme
moleculemolecule
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13. Classification of EnzymesClassification of Enzymes
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Class of
Enzymes
Catalyzes Example
Oxidoreductas
es
Redox
reaction
Oxidases,
catalases,
dehydrogenas
es
Transferases Transfer of
functional
group from 1
substrate to
other
substrate
ALT, AST
Hydrolases Hydrolysis of
substrate
Lactase,
lipase,
amylase

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Class of
Enzymes
Catalyzes Example
Lyases
(Synthases)
 Splitting of a large
substrate with
removal of small
group by
non hydrolytic
cleavage
 Linking of 2
molecules without
ATP expenditure
Aldolase,
decarboxylas
e,
dehydratase
Isomerases Isomerization Isomerase,
mutase,
epimerase
Ligases
(Synthetase
s)
Linkage of 2
molecules by covalent
bond with
expenditure of ATP
DNA ligase,
carboxylase

15. CoenzymeCoenzyme
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 Non protein organic
substances that help in
enzyme activity
 Heat stable
 LMW
 Usually remain tightly
bound to enzyme (so, is
called prosthetic group/
cosubstrate) of enzyme
Example :
 Coenzyme derived
from vitamin B
complex: NAD, FAD,
CoA SH etc
 Non vitamin
coenzyme: Coenzyme
Q, Lipoic acid etc

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17. CofactorsCofactors
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 Inorganic metallic ions
(Trace elements) that
help in enzyme activity
 Most of cofactors remain
loosely bound to enzyme,
these enzymes are ‘Metal
activated enzymes’
 Less frequently,
cofactors remain tightly
bound to enzyme, these
enzymes are
‘Metalloenzyme’
Example :
Zn as cofactor:
DNAP, RNAP, ALP
etc
 Fe as cofactor:
Catalase, Peroxidase
 Cu as cofactor:
Cytochrome oxidase
Mg as cofactor: CPK

18. Enzyme SpecificityEnzyme Specificity
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Reaction specificity:
A substrate can
undergo different
types of reaction but
an enzyme catalyzes
only one specific type
of reaction
Substrate specificity:
An enzyme may act on
only one specific
substrate
(Absolute substrate
specificity)
Or
structurally related
substrate
(Relative substrate
specificity)

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Importance of enzyme specificity:
Lots of enzymes may coexist in the
cell without interfering in each other’s
action

20. Factors affecting Enzyme activityFactors affecting Enzyme activity
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Other factors affecting enzyme
activity:
 Concentration of product
 Activators / Inhibitors molecules
 Cofactors & coenzymes

25. Enzyme InhibitionEnzyme Inhibition
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 It is the process by which some chemical
substances inactivate the enzyme &
decrease the velocity of enzyme catalyzed
reaction
Enzyme inhibitors :
The chemical substances which are involved
in the process of enzyme inhibition .
e.g. Toxins, poisons, pesticides and
antibiotics.

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 Normally,
E + S ↔ ES → E + P
 In presence of enzyme inhibitor
E + I ↔ EI → no P formed
[Here, E = Enzyme, S= Substrate, I=
Inhibitors, P = Product]

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29. Irreversible inhibitors usuallyIrreversible inhibitors usually
covalently modify an enzymecovalently modify an enzyme
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 Irreversible Inhibitors can be
removed from enzyme only with the
loss of enzyme activity

31. 05/02/18Footer Text 31
Enzyme Related
Terminology

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Activation energy:
Minimum amount of
energy needed to initiate
a chemical reaction.
Substrate:
Molecules on which
enzymes operate.
Product:
Resultant molecule of
enzyme catalyzed reaction

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Autocatalysis:
When a product of a chemical
reaction itself acts as a
catalyst for that reaction.
Velocity / Rate of chemical reaction:
The change of molar concentration
of reactant in a given period of time

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1 I/U of enzyme:
 Amount of enzyme
required to convert
1 μmol substrate to
1 μmol product per
minute under
optimum condition
 Expressed as 1 IU /
ml
 1 IU = 1 μmol / min
1 Kat of enzyme:
 Amount of enzyme
required to convert 1
mol substrate to 1 mol
product per second
under optimum
condition
 1 Kat = 1 mol / sec
Expression of enzyme
activity:
IU
Katal unit (Kat)

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Enzyme turnover:
The overall process
of enzyme
synthesis and
degradation.
Q10/ temperature co-
efficient:
The change of
relative reaction
rates of enzyme at
two temperatures
differing by 10 degree
C
Turnover number:
No. of mole of
substrate
transformed or no. of
mole of product
formed per mole of
enzyme per unit of
time

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Km / Michaelis constant of an enzyme:
 It is the substrate concentration at
which an enzyme shows half
maximum activity .
 It indicates the enzyme-substrate
affinity
 It is inversely proportional to the
enzyme affinity for substrate

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