1. Compiled & Edited by Dr Syed Ismail, MKV Parbhani
Enzymes
Compiled & Edited by
Dr. Syed Ismail
VN Marathwada Agril. University,
Parbhani, India
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Enzymes are complex chemicals that
control reactions in living cells. They are
biochemical catalysts speeding up
reactions that would otherwise happen too
slowly.
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• Biological catalysts – a molecule which speeds up a
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chemical reaction but remains unchanged at the end
of the reaction
Generally names end in –ase
Globular proteins (soluble)
Active site – region (cleft or depression) to which
another molecule or molecules (substrate) can bind
Substrate fit perfectly and is held in place by temporary
bonds which form between the substrate and some of the
R groups of the enzyme’s amino acids (enzymesubstrate complex)
Specific - shape of active site will only allow one shape
of molecule to fit
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The Chemical nature of enzymes
Enzymes are globular proteins. They have a complex tertiary and quaternary
structure in which polypeptides are folded around each other to form a roughly
spherical or globular shape. The overall 3D shape of an enzyme molecule is very
important: if it is altered, the enzyme cannot bind to its substrate and so cannot
function. Enzyme shape is maintained by hydrogen bonds and ionic forces.
Enzymes have several important properties:
Enzymes are specific: each enzyme usually catalyses only one reaction.
Enzymes combine with their substrates to form temporary enzyme-substrate
complex.
Enzymes are not altered or used up by the reactions they catalyze, so can be
used again and again.
Enzymes are sensitive to temperature and pH.
Many enzymes need cofactors in order to function.
Enzyme function may be slowed down or stopped by inhibitors.
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TRANSFERASES
TRANSFER OF FUNCTIONAL GROUPS
SOME EXAMPLES:
– TRANSFERASES
– PHOSPHORYLASES
TRANSFER OF Pi OR PPi
– KINASES
A PHOSPHATE TRANSFER
UTILIZE OR GENERATE ATP
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LYASES
CATALYZES REACTIONS THAT
– GENERATE A DOUBLE BOND
– ADDS A SUBSTRATE MOLECULE TO
DOUBLE BOND OF A SECOND SUBSTRATE
EXAMPLES:
– DECARBOXYLASES
– DEHYDRATASES
– ALDOLASE
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LIGASES
TWO MOLECULES ARE JOINED
ANABOLIC REACTIONS
REQUIRE NUCLEOTIDES (ATP, GTP) TO
DRIVE THEM
– A PYROPHOSPHATE BOND MUST BE BROKEN
EXAMPLES:
– CARBOXYLASE
– SYNTHETASES
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Activation energy
• Enzymes increase the rate at
which chemical reactions occur
• Activation energy – extra
energy temporarily given to
substrate to convert it to
product
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Like all catalysts, enzymes work by lowering the activation
energy for a reaction, thus dramatically accelerating the rate of
the reaction.
Most importantly;Enzymes are not used up by the reaction. After
they have done their work they release the products and are not
changed.
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• Vmax occurs when
enzyme active sites are
saturated with substrate
• Km (Michaelis-Menten
constant) reflects affinity
of enzyme for its
substrate
• smaller the Km, the
greater the affinity an
enzyme has for its
substrate
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FACTORS AFFECTING ENZYME ACTIVITY
Following are the important factors
affecting the enzyme activity.
Temperature
pH
Salt & Inorganic ions
Enzyme concentration
Inhibitors
Cofactors and coenzymes
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Inhibitors
Inhibitors slow down or stop enzyme reaction. Usually, enzyme
inhibition is a natural process, a means of switching enzymes on or off
when necessary.
Inhibition can be reversible and the enzyme returns to full activity once
the inhibitor is removed. Drugs and poisons can inhibit particular
enzymes, this type of inhibition is often non-reversible.
Reversible inhibitors are either competitive or non-competitive.
Competitive inhibitors
Compete with normal substrate molecules to occupy the active
site. A competitive inhibitor fits into the active site of the enzyme
preventing the real substrate from gaining access. The inhibitor cannot be
converted to the products of the reaction and so the overall rate of
reaction is slowed down.
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Competitive Inhibition
This is where the inhibitor is a molecule which has a similar shape
to the molecule which is supposed to be binding to the active site.
In the case of enzymes, a competitive inhibitor may have the same
shape as that of the substrate, but it doesn't react in the same way.
Rather than turning into the product, it simply uses up time and
prevents the substrate from getting to the active site. It blocks the
way.
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Non-competitive Inhibition
• This is the kind that exists when a molecule binds to a
different site on the enzyme, rendering it inactive.
Sometimes it does this before the substrate reaches the
active site, sometimes afterwards, but in either case it
stops the enzyme doing its job, and prevents a product
being formed.
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• In both competitive and non-competitive inhibition,
• it is possible to have both reversible and irreversible
inhibitors.
• As the name suggests, a reversible inhibitor does not have a
permanent affect - it will stop the enzyme doing what it is
supposed to do,
• but it will move off again and allow the enzyme to function later
on;
• an irreversible inhibitor, on the other hand, permanently renders
the enzyme inactive, so it will have to be replaced by a brand
new one .
ALBIO9700/2006JK
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End-product inhibition
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Cofactors and Coenzymes
• Inorganic substances (zinc, iron) and vitamins
(respectively) are sometimes need for proper
enzymatic activity.
• Example:
Iron must be present in the quaternary
hemoglobin in order for it to pick up oxygen.
structure -
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APOENZYMES and
COENZYMES
Coenzymes
are not proteins and so are not inactivated by
heat. Examples of coenzymes are the vitamins or
compounds derived from vitamins. The reaction
involving a coenzyme can be written as follows:
coenzyme + apoenzyme = enzyme
Coenzyme A is essential in the metabolism of
carbohydrates, lipids, and proteins in the body.
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•Coenzymes are bound at the active site in order to interact
with the substrate and play an essential role in the catalysed
reaction.
•They act as carriers of a variety of chemical groups.
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Most water-soluble vitamins are components of coenzymes
Vitamin
Coenzyme
Deficiency
Thiamine (B1)
Thiamine
pyrophosphate
Beriberi (weight
loss,other problems
Riboflavin (B2)
FAD+
Mouth lesions, dermatitis
Nicotinic acid
(niacine)
NAD+
Pellagra (dermatitis,
depression)
Pantohtinic acid
Coenzyme A
Hypertension
Biotin
Biotin
Rash, muscle pain
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Zymogen
Active Enzyme
Function
Pepsinogen
Chymotrypsinogen
Trypsinogen
Procarboxypeptidase
Proelastase
Prothrombin
Fibrinogen
Factor VII
Factor X
Proinsulin
Procollagen
Procollagenase
Pepsin
Chymotrypsin
Trypsin
Carboxypeptidase
Elastase
Thrombin
Fibrin
Factor VIIa
Factor Xa
Insulin
Collagen
Collagenase
protein digestion
protein digestion
protein digestion
protein digestion
protein digestion
blood clot formation
blood clot formation
blood clot formation
blood clot formation
plasma glucose
homeostasis
component of skin and
bone remodeling
processes during
metamorphosis, etc.
