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11
 Enzyme

Enzyme = En + Zyme

In Yeast

Almost all enzymes are proteinaceous in nature except some nucleic acids
that behave like enzymes, called ribozyme.
 Enzyme

Buchner discovered & isolated the enzyme zymase from yeast cells.

Kuhne coined the term enzyme.

J.B. Sumner purified and crystalized urease enzyme from the

Canavalia/Jack bean/Lobia plant.
Question Enzymes were discovered for the first time in:
(1) Bacteria
(2) Yeast
(3) Algae
(4) Spinach

Ans. (2)
Question The first enzyme isolated in crystalline form was:
(1) Catalase
(2) Urease
(3) Peroxidase
(4) Amylase

Ans. (2)
Question Which enzyme is not a protein?

(1) Hexokinase
(2) Synthetase
(3) Endonuclease
(4) Ribozyme

Ans. (4)
Characteristics of Enzymes

Enzymes are colloidal substances, which are macromolecules of amino

acids and are synthesised by ribosomes under genetic control.

An enzyme like any protein has:

Characteristics of Enzymes
An enzyme like any protein has:

A. Primary structure

It is linear chain of amino acid sequence of the protein which are joined
to each other by peptide bond.
Amino Acids
N-terminal
C-terminal

Primary structure
Characteristics of Enzymes

B. Secondary structure

A protein thread does not exist throughout as an extended rigid rod. The
thread is folded in the form of a helix (similar to a revolving staircase).

It is a helical structure of proteins.

N-terminal

Beta Sheets
Alpha Helix
C- terminal

Secondary structure
Characteristics of Enzymes

C. Tertiary structure

In this structure, the backbone of the protein chain folds upon itself, the
chain criss-crosses itself, and hence many crevices or pockets are made;
such pockets represent active sites.
The catalytic structure of most of the enzymes is tertiary and globular.
N- terminal

Tertiary structure

C- terminal
Characteristics of Enzymes

Active site:
An active site of enzyme is a crevice or pocket into which the substrate
fits. Thus, enzyme through their active site, catalyse reactions at high rate.

Substrate
Active site
Characteristics of Enzymes

D. Quaternary structure

Some proteins are an assembly of more than one polypeptide or subunits.

These individual folded polypeptides or subunits are arranged with

respect to each other in different manner

e.g. linear string of spheres, spheres arranged one upon another in the
form of a cube or plate, etc.
Polypeptide chains
 NOTE

Enzymes are very specific for substrate or reactions.

They are required in very small amounts to catalyse a reaction.

Enzyme type 1 Enzyme type 2 Enzyme type 3

Specific substrate Specific substrate Specific substrate

of enzyme 1 of enzyme 2 of enzyme 3
 Characteristics of Enzymes

Turn Over Number:

It is the number of substrate molecules converted into products per unit
time by a molecule of enzyme.

Thus, catalytic power is directly proportional to the turnover number.

Carbonic anhydrase
CO2 + H2O H2CO3
Carbon dioxide Water Carbonic acid

Turn over number of Carbonic anhydrase =

600,000 molecules / second
 Characteristics of Enzymes

Turn Over Number:

Catalytic power of an enzyme remains the same even outside the living
system.

Enzyme Turnover Number

Carbonic anhydrase 360 lakhs/minute
Catalase 50 lakhs/minute
Flavoprotein 50/minute
Lysozyme 30/minute
Question The fastest-acting enzyme, in the biological kingdom among the
following is:
(1) Lipase
(2) Amylase
(3) Peptidase
(4) Carbonic anhydrase

Ans. (4)
 Characteristics of Enzymes

Km constant:
This was coined by Michaelis and Menten.

It is the concentration of substrate at which the rate of reaction attains

half of its maximum velocity.
Vmax
Velocity of
reaction
𝟏
Km = 𝑽𝒎𝒂𝒙
𝟐
Km

Substrate conc.

High Km value Low affinity of enzyme for substrate

Low Km value High affinity of enzyme for substrate
Question Substrate concentration at which an enzyme attains half of its
maximum velocity is:
(1) Half-life of enzyme
(2) Km-constant of enzyme
(3) Concentration ratio
(4) Turn over number (TON) of enzyme E

Ans. (2)
NOTE

Enzymes when not in use, represent inactive form, called zymogen or

pro-enzyme.

e.g. Pepsinogen is inactive form of pepsin,

Trypsinogen is inactive form of trypsin.

Zymogen Activation

Activating
Protease Inactive State

Repressing Subunit Active State

Biocatalyst (Enzyme) v/s Inorganic Catalyst

Enzyme (Biocatalyst) Inorganic Catalyst

Enzymes are thermo-sensitive They work efficiently at high
and get damaged at high temperatures and high
temperatures (say above 40°C) pressures.
 Biocatalyst (Enzyme) v/s Inorganic Catalyst

However, enzymes isolated from organisms who normally live under

extremely high temperatures (e.g. hot vents and sulphur springs), are
stable and retain their catalytic power even at high temperature.

Thermal stability, is thus an important quality of such enzymes isolated

from thermophilic organisms. e.g. Taq polymerase.
Question Which of the following statements is incorrect with respect to
inorganic catalysts?
(1) They do not occur in living cells
(2) They are not specific for any reaction
(3) They get damaged at high temperature
(4) They work efficiently at high pressure

Ans. (3)
Almost all enzymes are proteinaceous in nature except some nucleic
acids that behave like enzymes, called ribozyme.

Active site:- a crevice or pocket into which the substrate fits.

Km:- is the concentration of substrate at which the rate of reaction attains

half of its maximum velocity.

Enzymes are thermo-sensitive and get damaged at high temperatures

(say above 40°C).
 Enzyme-II
Nature of
Enzyme Action
 Uncatalysed Reaction versus Catalysed Reaction

Carbonic anhydrase
CO2 + H2O H2CO3
Carbon dioxide Water Carbonic acid

Absence of Carbonic Presence of Carbonic

Anhydrase Anhydrase

200 molecules of 600,000 molecules

H2CO3 per hour every second

Therefore, the enzyme has accelerated the reaction rate by about

10 million times.
Question In the absence of any enzyme, the reaction CO2 + H2O → H2CO3
is very slow, formation of 200 molecules of H2CO3 per hour.
However, by the use of carbonic anhydrase enzyme reaction
speeds up with about:
(1) Formation of 6 × 105 molecules of H2CO3 every second
(2) Formation of 36 × 105 molecules of H2CO3 every minute
(3) Formation of 36 × 106 molecules of H2CO3 every second
(4) Formation of 6 × 106 molecules of H2CO3 every minute

Ans. (1)
 How do Enzymes bring about such
High Rates of Chemical Reactions

Old bonds New bonds

break form

Substrate Product

Transition state
Altered structural state
(High Energy/Unstable)
 Concept of Activation Energy

S→P
Transition state

Activation energy
without enzyme
Potential
energy Activation energy
with enzyme
Substrate (s)

Product (P)
Progress of reaction

The difference in average energy content of substrate from that

of the transition state is called ‘activation energy’.
Question Enzymes catalyse biochemical reactions by-
(1) Lowering the activation energy
(2) Increasing the activation energy
(3) Establishing stable bonds with substrate
(4) Increasing temperature

Ans. (1)
 Nature of Enzyme Action

Enzyme + Substrate ⇌ ESC → EPC → Enzyme + Product

The catalytic cycle of an enzyme action can be described in the

following steps:
1. First, the substrate binds 2. The binding of the substrate
to the active site of the induces the enzyme to alter
enzyme, fitting into the its shape, fitting more tightly
active site. around the substrate.

Substrate
Active site
 Nature of Enzyme Action

Enzyme + Substrate ⇌ ESC → EPC → Enzyme + Product

The catalytic cycle of an enzyme action can be described in the

following steps:
1. First, the substrate binds 2. The binding of the substrate
to the active site of the induces the enzyme to alter
enzyme, fitting into the its shape, fitting more tightly
active site. around the substrate.
Enzyme changes
Substrate shape slightly as
substrate binds
Active site

Enzyme substrate complex

 Nature of Enzyme Action

3. The active site of the enzyme, 4. The enzyme releases the products
now in close proximity of the of the reaction & the free enzyme
substrate breaks the chemical is ready to bind to another
bonds of the substrate & the molecule of the substrate & run
new enzyme-product complex through the catalytic cycle once
is formed. again.

Enzyme products complex

 Nature of Enzyme Action

3. The active site of the enzyme, 4. The enzyme releases the products
now in close proximity of the of the reaction & the free enzyme
substrate breaks the chemical is ready to bind to another
bonds of the substrate & the molecule of the substrate & run
new enzyme-product complex through the catalytic cycle once
is formed. again.

Products

Enzyme products complex Products leaving Active site of enzyme

Question In plants, enzymes are present:
(1) Only in flowers
(2) Only in leaves
(3) In all the living cells
(4) Only in parenchyma

Ans. (3)
Question Which of the following statements is true?
(1) The rates of uncatalyzed and catalyzed reactions are the same.
(2) The rate of uncatalyzed reaction is higher than the rate of the
catalyzed reaction.
(3) The rate of the catalyzed reaction is higher than the rate of the
uncatalyzed reaction.
(4) None of them is true.

Ans. (3)
Question Which of the following statements is not true regarding the
activity of an enzyme?
(1) The substrate binds to the active site of the enzyme.
(2) Enzyme changes shape slightly as substrate binds.
(3) The binding of the substrate induces the enzyme to alter its
shape.
(4) The enzyme molecules are permanently destroyed when the
reaction is complete.

Ans. (4)
Question Which of the following statements is true?
(1) The energy of the transition state is higher than the energy of
the product.
(2) The energy of the transition state is lower than the energy of
the product.
(3) The energy of the transition state is lower than the energy of
the substrate.
(4) The energy of the transition state is equal to that of the
substrate and product.

Ans. (1)
Concept of Activation Energy

S→P

Transition state

Activation energy
without enzyme
Potential
energy Activation energy
with enzyme
Substrate (s)

Product (P)
Progress of Reaction
Nature of Enzyme Reaction:

Products
Substrate
Active site

Enzyme + E-S complex E-P Enzyme +

Substrate complex Product
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