• Unit 1: Introduction to Enzymes Introduction &
Definition of Enzymes, Coenzymes, Cofactors,
Apoenzymes, Holoenzyme, Abzymes, Synzyme,
Ribozyme, Extremozyme, Historical Development
in Enzymology, Characteristics of enzymes,
Enzyme Vs Chemical Catalysts, Enzyme mechanism
& Activation Energy, Enzyme Specificity, Fischer
and Koshland Models Role of B complex Vitamins
in Enzyme catalyzed reactions Multienzyme
Complex: Properties, Examples with Reactions
(PDH Complex, Fatty Acyl Synthase Complex,
Tryptophan Synthase), Physiological Importance,
Advantages of MEC Zymogens, Properties with
examples and Activation of Chymotrypsinogen
What are Enzymes?
Lecture 2
Objective
To understand the nature of enzymes
• The living cell: site of tremendous biochemical activity
called METABOLISM
• Activities:
–
–
–
–
–
Build up new tissues,
Replacement of old tissues,
Conversion food to energy,
Disposal of waste materials,
Reproduction
all the activities we characterize as “life”
• Activities require catalysts
• Catalyst is defined as the acceleration of a chemical
reaction by some substance which itself undergoes no
permanent chemical change
• The catalysts of biochemical reactions are enzymes
• Enzymes are responsible for bringing about almost all the
chemical reactions in living organisms
• Proteins have a vast capacity to bind to a wide range of
molecules
• Using various intermolecular forces, enzymes bring
substrates together in an optimal orientation for reaction
• Chemical reactions need an initial input of energy = THE
ACTIVATION ENERGY
• They catalyse reaction by stabilizing transition state (highest
energy species in reaction pathways)
• By stabilizing transition state, an enzyme determines which
one of several potential chemical reaction actually takes
place
• Without enzymes, these reactions take place at the rate far
too slow for the pace of metabolism
Rate enhancement by selected enzymes
Enzyme
Nonenzymatic
half life
Orotidine
Monophosphate
decarboxylase
78,000,000 2.8 x 10-16
years
69,000
1.0 x 10-11
years
1.9 years 4.3 x 10-6
AMP
Nucleosidase
Triose
Phosphate
Isomerase
Carbonic
anhydrase
5 seconds
Uncatalysed rate
(Kun/s)
1.3 x 10-1
Catalysed rate
(Kcat/s)
Rate
enhancement
(kcat/se
kun/se)
39
1.4 x 1017
60
6.0 x 1012
4300
1.0 x 109
1 x 106
7.7 x 106
A. Radzicka and R Wolenden, Science 267: 90-93, 1995
3 Characteristics of enzymes
1. The most efficient catalysts known
2. Specificity of action
3. Many enzymes are regulated
The individuality of a living cell is due in large part to the
unique set of enzymes that it is genetically programmed to
produce.
If even one is missing or defective, the result can be
disastrous
Robert Bohinsky, Fundamentals of Biochemistry, 5th ed, 1987
Nature of Enzymes
1. Nearly all known Enzymes
are proteins
They have a globular shape
A complex 3-D structure
Human pancreatic amylase
© Dr. Anjuman Begum
German scientist Richard Willstatter (Nobel Prize
in 1915) reported in 1920 that he could not
detect protein in purified enzyme preparation.
This is corrected by Sumner
Evidence providing the protein nature of enzymes
1. In their elementary composition, enzymes show the
usual proportion of C,H,N and S as found in proteins.
2. Enzymes are subjected to the action of those enzymes
which are specifically meant for the breakdown of
peptide bond of proteins (proteases)
3. Like other proteins, enzymes behave as ampholytes in an
electric field. The isoelectric point for various enzymes
are determined.
4. Enzymes like other proteins undergo denaturation and
lose enzymatic activity
5. Many purified enzymes, on injection into animal body
produce specific antibodies.
Nature of Enzymes.. Contd…
2. They are high molecular weight compounds
(10 to 2,000 KDa)
They are huge in comparison to substrate
Due to large size, posses low rate of diffusion
and form colloidal systems in water.
3. They can be precipitated with salts, solvents
and other reagents
4. Have high catalytic power. No side products
are produced.
Do not undergo any change. So small
quantity of enzyme can catalyze large
quantity of substrate
• Catalytic power of an enzyme is measured by the
‘turn over number’
• It is defined as: the number of substrate
molecules converted into products per unit time,
when the enzyme is fully saturated with
substrate
• A single molecule of catalase can convert 50
lakhs H2O2 molecules to H2O in a minute
• 36 million carbonic acid molecules are produced
per minute (60000/sec) by carbonic anhydrase
Turn over number of some enzymes
Enzyme
Turn over number per
second
Lysozyme
0.5
Tryptophan synthetase
2
DNA Polymerase-1
15
Phosphoglucomutase
20.5
B-Galactosidase
208
Lactate Dehydrogenase
1000
B-amylase
18,333
5. Highly specific. They may act
– On one specific type of substrate molecules
• Carbonic anhydrase acts only on CO2 and H2O
– On a group of structurally related compounds
• LDH converts pyr to lactate and vice versa
– On only one of the two optical isomers of a compound
• L-amino acid oxidases
– On only one of the two geometrical isomers (cis and
trans)
• Fumarase converts fumaric (trans) to L-malic acid . It does not
act on cis form of fumaric acid
6. Regulated by naturally occurring compounds
7. Enzymes are sensitive to heat.
Rate of reaction increased to 23 times for every 10°C:
temperature quotient (Q10)
Above 60°C it denatures
8. Enzymes show pH sensitivity
(pepsin 1.5, sucrase 6.2,
arginase 10)
Enzyme Vs Inorganic Catalysts
Enzymes
Inorganic Catalysts
Protein in nature
High catalytic power, very efficient
High specificity for substrate and reaction
Often 1 Enz, 1 Substrate
Regulation by naturally occurring
compounds
Do not require extreme temp and
pressure for reaction
Often works at body temp
Most enzymes accelerates reaction by a
million times
Carbonic anhydrase accelerates 107 times
faster than uncatalysed reaction
Require extreme temp and pressure
Eg: Haber Process for Ammonia synthesis
from N and H: t= 700-900K; p= 100-900
atm
Enzyme Vs Inorganic Catalysts
Enzymes
Inorganic Catalysts
They can bind so many substrates in short
period of time. So small amount of enzyme
is needed compared to quantity of
substrate.
Require coenzyme/ cofactors
Activity of enzymes are regulated in vivo
Activity of enzyme can be increased by
activators (mg2+ for kinases)
Activity of enzyme can be decreased by
inhibitors
Factors like temp, pH, substrate
concentration and enzyme concentration
can affect enzyme activity
Enzyme produce only the expected
products from a reaction; no side reactions
No such requirements
Similarities: Enzyme & Inorganic Catalysts
• They are neither consumed nor produced
during the course of the reaction
• They do not cause reaction to take place; they
speed up reaction