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Enzymes
How the confectioners make the runny yolklike inside to chocolate eggs?
The answer is : use of an enzyme
What are enzymes?
 Enzymes are complex chemicals that control reactions in living cells. They are
biochemical catalysts speeding up reactions that would otherwise happen too
slowly.
 The chemical which an enzyme works on is called its substrate.
 An enzyme combines with its substrate to form a short-lived enzyme/substrate
complex. Once a reaction has occurred, the complex breaks up into products and
enzyme.
E+S
ES
EP
E+P
 The enzyme remains unchanged at the end of reaction and is free to interact
again with more substrate.
The role of enzymes in an organism
 Many of the complex chemicals that living organisms need cannot be made in a
single reaction. Instead a series of simpler reactions occur, one after another,
forming a metabolic pathway. A single pathway may have many steps in which
each chemical is converted to the next. A specific enzyme controls each reaction.
 Enzymes control cell metabolism by regulating how and when reactions occur.
Using this very simple pathway as an example:
A
B
C
D
The final product is substance D, the chemical needed by the living organism. The
pathway needs three different enzymes and when D is no longer needed or if too
much has been produced, one of the three enzymes is ‘switched off’.
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.
The specificity of enzymes
Two models that may explain how
enzymes work are:
1) The lock and key hypothesis
2) The induced fit hypothesis
1) The lock and key hypothesis
 Enzyme has a particular
shape into which the
substrate or substrates fit
exactly. This is often referred
to as the ‘lock and key’
hypothesis where the
substrate is imagined being
like a key whose shape is
complementary to the
enzyme or lock. The site
where the substrate bonds in
the enzyme is known as the
active site and it has a
specific shape.
Fig: The lock and key hypothesis
2) The induced fit hypothesis
The active site in many enzymes is not exactly the same shape as the
substrate, but moulds itself around the substrate as the enzyme
substrate complex is formed.
Only when the substrate binds to the enzyme is the active site, the correct
shape to catalyze the reaction. As the products of the reaction from they fit
the active site less well and fall away from it. Without the substrate, the
enzyme reverts to its ‘relaxed’ state, until the next substrate comes along.
Fig: Diagrams to show the induced fit hypothesis of enzyme action.
Naming and classifying enzymes
Although there are many different enzymes, they can be put into one of
six main categories according to the type of reaction they catalyze:
 Oxidoreductases: These catalyze oxidation and reduction reactions.
 Transferases: These catalyze the transfer of a chemical group from
one compound to another.
 Hydrolases: These catalyse hydrolysis (splitting by use of water)
reactions. Most digestive enzymes are hydrolases.
 Lyases: these catalyze the break down of molecules by reactions that
do not involved hydrolysis.
 Isomerases: These catalyze the transformation of one isomer into
another,
 Ligases: These catalyze the formation of bonds between compounds,
often using the free energy made available from ATP hydrolysis.
Factors affecting enzyme activity
The factors that affect enzyme activity also affect the functions of the cell and
ultimately the organism. Enzymes are proteins and their function is therefore
affected by:
 Temperature
 pH
 Substrate concentration
 Enzyme concentration
 Cofactors
 Inhibitors
Temperature
 For a non-enzymatic chemical
reaction, the general rule is: the
higher the temperature, the
faster the reaction. This same
rule holds true for a reaction
catalyzed by an enzyme, but
only up to about 40-450C.
Above this temperature, enzyme
molecules begin to vibrate so
violently that the delicate bonds
that maintain tertiary and
quaternary structure are broken,
irreversibly changing the
shape of the molecule. When
this happens, the enzyme can
no longer function and it is said
to be denatured.
pH
 Like other proteins, enzymes are
stable over a limited range of pH.
Outside this range, at the extremes
of pH, enzymes are denatured.
Free hydrogen ions (H+) or hydroxyl
ions (OH-) affect the changes on
amino acid residues, distorting the
3D shape and causing an
irreversible change in the proteins
tertiary structure.
Enzymes are particularly
sensitive to changes in pH because
of the great sensitivity of their
active site. Even if a slight change in
pH is not enough to denature the
molecule, it may upset the delicate
chemical arrangement at the active
site and so stop the enzyme
working.
Substrate concentration
 The rate of an enzyme-controlled
reaction increases as the
substrate concentration
increases, until the enzyme is
working at full capacity. At this
point, the enzyme molecules reach
their turnover number and
assuming that all other conditions
such as temperature are ideal, the
only way to increase the speed of
the reaction even more is to add
more enzyme.
Enzyme concentration
In any reaction catalyzed by an
enzyme, the number of
enzyme molecules present is
very much smaller than the
number of substrate
molecules. When an
abundant supply of
substrate is available, the
rate of reaction is limited by
the number of enzyme
molecules present. In this
situation, increasing the
enzyme concentration
increases the rate of
reaction.
Cofactors
Some enzymes cannot work on their own, they need a molecule called a
cofactor in order to work properly. Cofactors modify the enzyme complex so
that it has the chemical properties necessary to catalyze a reaction.
There are three kinds of cofactors:
a) Prosthetic group: Organic molecule that is permanently attached
to an enzyme.
b) Coenzymes: Relatively small organic molecules are not
permanently attached to the enzyme molecule.
c) Metal cofactors: Inorganic metal ions that are also known as
enzyme activators.
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.
Fig: Competitive inhibitors bind reversibly to the enzyme, preventing the binding of
substrate. On the other hand, binding of substrate prevents binding of the inhibitor.
Substrate and inhibitor compete for the enzyme.
 Non-competitive inhibitors
Non-competitive inhibitors bind to the enzyme away from the active
site but change the overall shape of the molecule, modifying the active site
so that it can no longer turn substrate molecules into product. Noncompetitive inhibition has this name because there is no competition for the
active site.
Fig: Non-competitive inhibition
Irreversible inhibitors
Irreversible inhibitors bind permanently to the enzyme, rendering it
useless. For example, cyanide is an irreversible inhibitor.