Enzymes
An enzyme is a specific protein produced within the organism that is
capable of catalysing specific chemical reactions. As they are of
biological origin and catalyse various reactions, they are also called
biocatalysts.
The term ‘Enzyme’ was coined by Kuhne (1878) for catalytically active
substances previously called ferments. Protein nature of the enzyme
was first found out by Sumner (1926). Like catalysts, the enzymes do
not start a chemical reaction or change its equilibrium, but enhance
the rate of reaction.
Chemical Nature of Enzymes
All enzymes are globular proteins with the exception of recently
discovered RNA enzymes. Some enzymes may additionally contain a
non-protein group.
There are two types of enzymes on the basis of composition
1. Simple enzyme The enzyme which completely made up of
protein, e.g., pepsin, trypsin, urease, etc.
2. Conjugate enzyme It is the enzyme formed by two parts
Classification of Enzymes
On the basis of reaction they performed, enzymes are classified into
six categories
(i) Oxidoreductases Oxidase, reductase and dehydrogenases
are included in this class of enzymes.
(ii) Transferases These enzymes perform group transfer reaction.
(iii) Hydrolases These enzymes induce hydrolysis, e.g., amylase,
lactase, etc.
(iv) Lyases They induce the cleavage without hydrolysis and
addition of double bond takes place, e.g., aldolase.
(v) Isomerases Rearrangement of molecular structure,
e.g., isomerase, epimerase, mutase, etc.
(vi) Ligases/Synthetases These enzymes induced the bonding of
two molecules after taking energy from ATP.
Nomenclature of Enzymes
Enzymes are named by adding the suffix-ase after the substrate
(e.g., lipase, amylase, maltase, etc.) or chemical reaction (e.g., succinate
dehydrogenase). Some old names also persist as pepsin, trypsin, etc.
Mechanism of Enzyme Action
The general mechanism of enzyme action has two steps
1. Formation of Enzyme-Substrate Complex
When an enzyme acts upon a substrate, it forms an enzyme-substrate
complex. Subsquently, this complex decomposes the substrate,
undergoes chemical change and the enzyme is regenerated afterwards.
E + S ES →
ES E P → +
Following two models have been put forth to explain the formation of
ES complex
(i) Lock and key model Proposed by Emil Fisher in 1894. He
states that both the components (i.e., enzyme and substrate)
have strictly complementary structure.
(ii) Induced fit model Proposed by D Koshland in 1958.
According to this, when enzyme binds to substrate, the change
in the shape of active sites of enzyme takes place.
2. Lowering of Activation Energy
All chemical reactions have a potential energy barrier that must be
overcome before the reactants can be converted into products.
The energy required to break this barrier is equivalent to activation
energy.
The enzyme lowers the energy of activation during its complexing with
substrate. After the combination of enzyme and substrate, the energy
level of substrate gets raised, and it reacts faster.
Turnover Number
Being large sized protein molecule, enzyme exists as colloid. Substrate
molecule changed per minute into product is called turn over
number, e.g., 36 millions for carbonic anhydrase, 5 millions for
catalase, etc.
Factors Affecting Enzyme Activity
The activity of an enzyme can be affected by a change in the conditions
which can alter the tertiary structure of the protein.
1. Substrate concentration Enzyme activity increases with
increase in concentration of the substrate to a maximum and
then it levels off.
2. Enzyme concentration In general, the rate of reaction will
increase with increasing enzyme concentration, due to
availability of more active sites for reaction.
3. Temperature and pH In most of the enzymatic reactions, rise
of 10°C in the temperature doubles the rate of reaction
between 5-40°C. Enzymes are denatured (secondary and above
level of structures degraded) at higher temperature due to
proteinaceous nature and rate of reaction drops.
4. Redox potential Enzymes are sensitive to redox potential of
the cell also. Many enzymes are affected by redox potential due
to the presence of oxidisable SH-group.
Enzyme Inhibition
Reduction or stoppage of enzyme activity due to certain adverse
conditions or chemicals is called enzyme inhibition.
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