Chapter 5
Enzymes
Enzymes as organic catalysts
• Enzyme is for
lowering of
activation energy.
No. of substrate
particles
Energy level of substrate particles
No. of substrate
particles
E: Activation
energy when
enzyme is not added.
E’:Activation
energy when
enzyme is added.
E
E’
Energy level of substrate particles
Lock and key hypothesis
Active site
substrate
Enzyme
Enzyme-substrate
complex
Products
Induced fit model
The active site is induced by the
substrate to change its shape to fit
the shape of the substrate.
Enzyme can catalyze both forward and
backward reactions. It does not alter the
equilibrium position of a biochemical
reaction. The normal progress of a
biochemical reaction is due to the
immediate removal of the end product.
Specificity
The degree of specificity varies from
one enzyme to another.
Intracellular enzyme works on one
particular substrate.
Extracellular enzyme works on a
range of related substrates.
Factors affecting enzymatic
activity
temperature
pH value
Substrate
concentration
enzyme
concentration
cofactors
inhibitors
Temperature affects the energy content and
hence the mobility of the enzyme and substrate
molecules. Higher temperature enables enzyme
and substrate molecules to collide more
frequently and therefore facilitates the substrate
molecule to bind with the active site of the
enzyme.
If the temperature is too high (above optimum),
the polypeptide chains made up the enzyme will
vibrate too much so that the structure will be
disrupted, and the enzyme is said to be
denatured.
Temperature
Temperature coefficient Q10
Q10 = Reaction rate at (X+10)oC
Reaction rate at XoC
When the temperature is below the optimum
temperature, Q10 usually is 2.
When the temperature is above the optimum
temperature, Q10 usually is less than 1.
No. of substrate
particles
T2>T1
T1
T2
Energy level of substrate particles
pH
Changes in pH alter the ionic charge
on the enzyme surface, thus causing
change in shape of its active site, thus
diminishing its catalytic activity. If
extremes of pH is encountered, the
enzyme is denatured and loses its
catalytic function.
Inhibitor
Reversible
inhibitor
Competitive
inhibitors
irreversible
inhibitor
Non-competitive
inhibitors
Reversible inhibitors: The effect is
temporary. The inhibiting effect can be
eliminated if the reversible inhibitor is
removed.
Irreversible inhibitors: This kind of
inhibitor disrupts the bonds of the
polypeptides which form the enzyme.
Therefore the shape of the active site is
altered permanently and loses its catalytic
properties.
Reversible inhibitor(I):
Competitive Inhibitor
Normal
substrate
Competitive
Inhibitor
Enzyme
Usually the end product of a
biochemical reaction acts as
competitive inhibitor. This is known
as end-product inhibition.
Immediate removal of the end
product facilitates enzyme reaction.
Non-competitive Inhibitor
Allosteric
site
Noncompetitive
Inhibitor
Normal
substrate
The shape of the
enzyme changes so
that the normal
substrate cannot bind
to it.
Example of competitive inhibitor
Sulphonamides have very similar molecular shape to
aminobenzoic acid, a compound essential for growth of
many pathogenic bacteria. Therefore, sulphonamide acts as
competitive inhibitor to check the growth of many bacteria
in treatment of many diseases caused by bacteria.
Example of non-competitive inhibitor
Diisopropylflurophosphate (DFP), a nerve gas used in
warfare is a non-competitive inhibitor. DFP acts on enzyme
cholinesterase which is involved in synaptic transmission.
DFP causes prolonged muscle contraction and death is the
end result.
Enzyme concentration and
substrate concentration
affects the concentration of
the enzyme-substrate
complex and hence the rate
of enzyme-catalysed
biochemical reaction.
Enzyme cofactor
It is non-protein substance which makes some
enzyme to function more efficiently.
Cofactors
Enzyme
activator
Coenzyme
Prosthetic
group
Enzyme activator
• It is an inorganic ion which loosely bound
to an enzyme or an substrate to increase the
chance to an enzyme-substrate complex
forming. Some times, the inorganic ions are
free and does not bind to the enzyme or
substrate,
• e.g. copper, iron, calcium, Cl- etc.
Coenzyme
• It is an organic non-protein molecule
firmly associated with the enzyme and is
essential for enzyme activity.
• It acts a carrier for transferring chemical
groups or atoms from one enzyme to
another.
• Many coenzymes are synthesized from
vitamins.
• e.g. NAD
Prosthetic group
• It is an organic non-protein group which
is tightly bound to the enzyme or as an
integral part with the enzyme.
• It can act as carriers of atoms or electrons
while transferring one compound to another
in an overall metabolic pathway.
• e.g. FAD, haemoglobin etc.
Application of enzymes
Some washing powders are biologically active. They
contain enzyme protease to digest the protein dirt.
Meat tenderizer contains enzyme protease which break
down and soften the texture of meat.