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4th lecture
--------------------------------------------Enzyme: protein catalysts that increase the rate of reactions without being
changed in the overall process.
Occurrence: Enzymes are produced by all living organisms including
humans and present only in small amounts.
Medical and biological importance of enzymes
1. Enzymes are the chemical work horses of the body. Enzymes are biological
catalysts that speed up the pace of chemical reactions.
2. A chemical reaction without an enzyme is like a drive over a mountain. The
enzyme bores a tunnel through it so that passage is far quicker and takes much
less energy.
3. Enzymes make life on earth possible, all biology from conception to the
dissolution that follows death depends on enzymes.
4. Enzymes regulates rate of physiological process. So, defects in enzyme
function cause diseases.
5. When cells are injured enzymes leak into plasma. Measurement of activity
of such enzymes in plasma is an integral part of modern day medical
6. Enzymes are used as drugs.
7. Immobilized enzymes, which are enzymes attached to solid supports are
used in clinical chemistry laboratories and in industry. For example glucose in
blood or urine is detected by using immobilized glucose oxidase. In
pharmaceutical industry, glucose isomerase is used to produce fructose from
8. Enzymes are used as biosensors.
9. AIDS detection involves use of enzyme dependent ELISA technique.
10. Enzymes are used as cleansing agents in detergent industry.
Classification of enzymes
1. Oxidoreductases: Transfer of hydrogen or addition of oxygen; e.g. Lactate
dehydrogenase (NAD); Glucose-6-phosphate dehydrogenase (NADP);
Succinate dehydrogenase (FAD); di-oxygenases.
2. Transferases: Transfer of groups other than hydrogen. Example,
Aminotransferase. (Subclass: Kinase, transfer of phosphoryl group from ATP;
e.g. Hexokinase)
3. Hydrolases: Cleave bond and add water; e.g. Acetyl choline esterase;
4. Lyases: Cleave without adding water, e.g. Aldolase; HMG CoA lyase; ATP
Citrate lyase. (Subclass: Hydratase; add water to a double bond)
4th lecture
--------------------------------------------5. Isomerases: Intramolecular transfers. The include racemases and
epimerases. Example, Triose phosphate isomerase.
6. Ligases: ATP dependent condensation of two molecules, e.g. Acetyl CoA
carboxylase; Glutamine synthetase; PRPP synthetase
Enzymes Lowering of Activation Energy
i. Enzymes lower the energy of activation.
ii. Activation energy is defined as the energy required to convert all molecules
of a reacting substance from the ground state to the transition state.
Active site
1. Two parts: (a) Catalytic site. It is the portion (part) of the enzyme that
is responsible for catalysis. It determines reaction specificity.
Occasionally, catalytic site and active site are used synonomously.
(b) Binding site. It is the part of the enzyme that binds with substrate. It
determines substrate specificity. (Fig).
The active site (or active centre) of an enzyme represents as the small
region at which the substrate(s) binds and participates in the catalysis.
E–S Complex E + P
4th lecture
--------------------------------------------2. The active sites of enzyme are clefts within the enzyme molecule. For
example, the active site of ribonuclease lies within cleft
3. Active site consists of few amino acid residues only.
4. Active site is three dimensional.
5. The active site is contributed by amino acid residues that are far apart
in the enzyme molecule. During catalysis, they are brought together.
6. The amino acids at the active site are arranged in a very precise manner
so that only specific substrate can bind at the active site.
7. Usually serine, histidine, cysteine, aspartate or glutamate residues
make up active site. Enzymes are named according to the active site
amino acid. For example, trypsin is a serine protease and papain is
cysteine protease.
Factors effect on enzyme activity
1. concentration of enzyme
As the concentration of the enzyme is increased, the velocity of the
reaction proportionately increases.
2.Concentration of substrate
Increase in the substrate concentration gradually increases the velocity of
enzyme reaction within the limited range of substrate
levels. A rectangular hyperbola is obtained when velocity is plotted
against the substrate concentration.
4th lecture
--------------------------------------------Enzyme kinetics and Km value :
The enzyme(E) and substrate( S) combine with each other to form an
unstable enzyme-substrate complex (ES) for the formation of product (P).
E + S --------------> E--S ---------------> E + P
If concentration of substrate is increased, the forward reaction
K1 is increased, and so K3 as well as total velocity is
correspondingly enhanced. The three different constants may
be made into one equation,
Km = K2 + K3
Km is called as Michaelis Constant.
It is further shown that Mechaelis-Menten equation:
Velocity (v) = Vmax [S]
Km + [S]
When concentration of substrate is made equal to Km, i.e.
When [S] = Km
Velocity (v) = Vmax [S] = Vmax [S] = Vmax
[S] + [S]
2 [S]
or v = ½ Vmax
3. Effect of temperature
Velocity of an enzyme reaction increases with increase in temperature up
to a maximum and then declines. A bell-shaped curve is usually observed
4th lecture
--------------------------------------------4. Effect of pH
increase in the hydrogen ion concentration (pH) considerably influences
the enzyme activity and a bell-shaped curve is normally obtained. Each
enzyme has an optimum pH at which the velocity is maximum. Below
and above this pH, the enzyme activity is much lower and at extreme pH,
the enzyme becomes totally inactive.
Enzyme inhibitors
defined as a substance which binds with the enzyme and brings about
a decrease in catalytic activity of that enzyme. The inhibitor may be
organic or inorganic in nature. There are three broad categories of
enzyme inhibition
1. Reversible inhibition.
2. Irreversible inhibition.
3. Allosteric inhibition
Reversible inhibition
The inhibitor binds non-covalently with enzyme and the enzyme
inhibition can be reversed if the inhibitor is removed. The Reversible
inhibition is further sub-divided into
l. Competitive inhibition (Fig.)
ll. Non-competitive inhibition
4th lecture
4th lecture
i. Enzymes may be simple proteins, or complex enzymes, containing a
non-protein part, called the prosthetic group. The prosthetic group
is called the co-enzyme. It is heat stable.
ii. The protein part of the enzyme is then named the apo-enzyme. It is
heat labile.
iii. These two portions combined together is called the holo-enzyme.
4th lecture
--------------------------------------------v. Co-enzymes may be divided into two groups v-a. Those taking part in
reactions catalyzed by oxidoreductases by donating or accepting
hydrogen atoms or electrons. v-b. Those co-enzymes taking part in
reactions transferring groups other than hydrogen
Examples of co-enzymes
Thiamine pyrophosphate (TPP)
Pyridoxal phosphate (PLP)
Coenzyme-A (Co-A)
Tetra hydrofolate (FH4)
Adenosine triphosphate (ATP)
Group transferred
Hydroxy ethyl
Amino group
Carbon dioxide
Acyl groups
One carbon groups
The term co-factor is used as a collective term to include co-enzymes and
metal ions. Co-enzyme is an organic and co-factors is non-organic.
Enzyme containing the metal
Carbonic anhydrase, carboxy peptidase,
alcohol dehydrogenase
Hexokinase, phospho fructo kinase,
enolase, glucose-6-phosphatase
Phospho gluco mutase, hexokinase,
enolase, glycosyl transferases
Tyrosinase, cytochrome oxidase, lysyl
oxidase, superoxide dismutase
Cytochrome oxidase, catalase,
peroxidase, xanthine oxidase
Lecithinase, lipase