Proteins that work as biological catalysts are
called enzymes. Enzymes speed up specific metabolic reactions. Low
contamination, low temperature and fast metabolism are only possible with
enzymes. Metabolism is fast, with the product made to a high degree of purity.
Enzymes have following general properties:
Protein Nature of Enzymes:
Most of enzymes
are globular proteins. some enzymes like ribozymes are nucleic acid in nature.
Globular structure of proteins is very important for proper works of enzymes.
• Composed of C, H, O and N. Sulphur may also be
• One or more polypeptide chains – large number of
linked amino acids.
• Formed on the ribosomes – translation of mRNA
during protein synthesis.
• Destroyed by high temperature and unfavorable pH.
Properties: Enzymes speed up the chemical reactions. They
are not consumed in chemical reaction. An enzyme cannot start the reaction it
can only increase the speed of a reaction. The efficiency of an enzyme reaction
is expressed form of turner number. The number for sucrose’s is 105
and for catalases is 106.
Folded Shape of Enzymes :•
The polypeptide chains are folded into a
specific three-dimensional shape.
• The correct folded
shape is important for enzyme action ‘tertiary structure’.
• The shape gives the
enzyme special areas known as active sites.
• The compatible
substrate molecules bind to the complement active site.
• Different enzymes
have a differently shaped active site.
• Higher temperatures more
kinetic energy, so more collisions
Role of Enzymes in Living Things:
Enzymes catalyze all
• They lower the activation energy – the
energy input needed to bring about the reaction.
• Regulate the
thousands of different metabolic reactions in a cell and in the organism.
• The activity of a cell is determined by
which enzymes are active in the cell at that time.
• Cell activity is altered by removing
specific enzymes and/or synthesizing new enzymes.
Active Site Theory :
“Lock and Key
Hypothesis and Induced Fit”
• The enzyme’s active
site has a shape complementary to the substrate.
• The substrate locks into the active site of
• The active site alters its shape holding the
substrate more tightly and straining it.
• An enzyme-substrate complex is formed.
• The substrate undergoes a chemical change –
a new substance is formed.
• The product is
released from the active site.
• The free unaltered active site is ready to
receive fresh substrate.
Enzymes as proteins are soluble in water or
dilute salt solution
Molecular weight :
Enzymes have high Mw
(varying from 10000 -several thousands)
Enzymes have buffering capacity:
They are amphoteric molecules i.e. behave
both as acids and bases. At pKa they make
the most efficient buffer.
Each enzyme has a specific isoelectric
the pH at which the net charge on protein equal to zero –so they do not move in
an electric field. It is the pH at which the protein molecule carries equal positive
and negative charges Above PI -negatively charged can move in an electric field Below PI -positive charged
and can move under an electric field.
When proteins are heated , or subjected to
extremes of temperature, high salt, organic solvents etc, the non-covalent
bonds break, changing the native structure to random coil. This unfolding of
protein is due to loss of secondary, tertiary and quaternary structure. It does
not affect primary structure. Effect of Denaturation: Loss of activity due to loss of shape and
•Heat •Change in pH •Radiation •Heavy metals •Detergents •Digestive enzymes •Urea
•Repeated freezing and the wing
phenylalanine end tyrosine—give orange color.
phenolic(-OH) group of tyrosine—give red color
sulfur amino acids (cysteine) —-give black or grey color
•This is a general test
for all proteins because it O is given by peptide linkage ( C -N) H
•The reaction occurs
between protein, sodium hydroxide and copper sulfate giving violet complex.
The specific of enzyme is due to primary
amino acids sequence. e.g. sucrose acts upon sucrose and lactase on milk sugar.
•Enzymes are highly
specific. They are specific for:
they catalyze. •and
in their course of reaction, which are called substrates.
The enzyme can act only
on one specific substrate.
•Enzyme act on a group
of related substrates.
•The substrates have a common group on which
the enzyme acts: e.g. -esterase can act on different esters -proteases can act
on different protein e.g. of proteases: chymotrypsin, trypsin, pepsin.
The specificity is due
to substrate binding site which lies on the enzyme surface -in specification is
due to the specific arrangement of amino acids in the active site that
participate in the bond making and bond breaking.
specification of an enzyme is determined by:
(a)Workal groups of enzymes
(b)Workal groups of
During enzyme action,
there is a temporary combination between enzyme and its substrate forming
enzyme by relatively weak forces. This occur at the active site of the enzyme
(most substrates are bound to the enzyme by relatively weak forces.
E + S à ES complex
is minimum energy that is
required to start a reaction. Enzymes lower the activation energy. The reaction
can take place in the absence of enzyme. But it need higher amount of
The accumulation of products of
the reaction can inhibit the enzyme activity. The effect is very important for
cell. It maintain the concentration of the product in a cell.
Some enzymes have specific
allosteric site for binding of certain effecters. The binding of effecters
change in the structure of enzyme. Allosteric is shown by many enzymes.
Enzymes are sensitive for following factors:
b) Enzymes are sensitive for heat. Activity of
enzymes increase with the increase of temperature of to 50 degree. The enzymes
are denatured at 70 to 100 degree. Such conditions are present in spore and dry
d) Enzymes are also sensitive for low
temperature. They are inactivated but not destroyed at 0 degree.
f) Enzymes are also destroyed by radiation of
lower wavelength such as ultraviolet rays and x-rays.
Each enzyme has optimum pH. Its activity
slows down with increase in pH. Catalases and Amylose show optimum activity in
neutral solution. surcease and Ligase act in acidic medium. Trypsin act in
Reversibility of enzyme:
The majority of reaction catalyzed
by enzymes are reversible. Thus an enzyme can speed up a reaction in both
directions. Thus system state in equilibrium in a short time. In guard cell of
stomata the enzyme starch phosphorylase convert into starch and inorganic
phosphate into glucose. The direction of reaction depend upon several factors.
It depend upon pH and chemical potential of both reactions. Synthesis of starch
protein and fats are irreversible.