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Enzymes
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An enzyme is a biological catalyst that
accelerates a chemical reaction by lowering
the energy needed to start the reaction without actually being permanently changed
itself.
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So what is a catalyst ?
A catalyst is a substance that increases the
rate at which a reaction takes place.
An enzyme is therefore an organic molecule
that helps a reaction proceed without needing
high amounts of energy. It causes reactions
to happen much faster than they would if it
wasn't present.
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Most enzymes are proteins and therefore
have the same basic structure as proteins.
Enzymes are actually globular proteins,
they have an almost spherical shape.
Many enzymes also contain or require nonprotein parts and only work properly when
these parts are present. These can be lipids,
sugars, metallic ions (e.g Ca2+) vitamins or
substances derived from vitamins. These
components are called cofactors.
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A cofactor is a substance that is essential for
some enzymes to function efficiently.
There are three types of cofactor:
activators
coenzymes
prosthetic groups
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The first two cofactors are not permanently
bound to the enzyme activators
Many enzymes are not active in the sites
where they are manufactured. They are
purposely left incomplete. This is because
they might otherwise catalyse reactions there
that should not take place. This is particularly
true of digestive enzymes and enzymes
involved in blood clotting. If blood-clotting
enzymes were always active, they would clot
blood in the blood vessels all the time.
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Coenzymes are non-protein molecules.
They are only present when the enzyme is
busy catalysing a reaction and do not always
bond to the enzyme.
Many vitamins are either precursors to
coenzymes (they will be turned into
coenzymes) or are already coenzymes. An
example of this is ascorbic acid (Vitamin C).
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All enzymes have a small area on their
surface where the amino acid chains making
up the enzyme form an indented area. This is
where the catalytic action of the enzyme is (ie
where the enzymes do their work). This is
called the active site and is formed by the
specific folding of the amino acid chains
making up the protein
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Three main types of reaction occur in your
body - catabolic reactions, anabolic reactions
and reactions involving the turning of one
substance into another (called conformational
changes).
Enzymes are able to catalyse all of these
reactions.
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The substances that are undergoing the
reaction are called the substrates.
Active sites are places that the substrate
molecules fit into.
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Most enzymes are very specific and able to
accept only certain substrates.
Enzymes are usually highly substrate specific.
We need many thousands of different
enzymes in order to catalyse the many
thousands of reactions in the body.
Some substances are able to "cheat" the
enzyme into "thinking" that they are the
correct substrates.
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The ability of an enzyme to work efficiently
depends on the presence and position of
amino acid residues in the active site and on
the structure of the whole enzyme.
The shape of the active site is also
determined by the structure of the whole
enzyme
The shape of the active site is important in
ensuring enzyme specificity.
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There are two theories concerning how the
active site is able to accept only certain
substrates:
the lock and key mechanism
the induced fit theory
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This theory states that the substrate (the key)
fits exactly into the active site (the lock) of
the enzyme. As there is usually only one key
that fits a lock, there is usually only one
substrate or group of substrates that fit the
active site of a certain enzyme. This theory
supposes that enzymes have active sites that
do not change shape easily. So the way in
which the substrate fits is purely a result of
the shape of the substrate and the active site.
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Many enzymes have been found to have active sites
that are not so rigid. The induced fit model is an
extension of the previous model. It states that the
active site only fits the shape of the substrate after
the substrate has bound to the enzyme. Only the
correct substrate can cause the active site to change
shape to fit it. The way in which the active site
changes its shape is through interactions with
cofactors such as coenzymes, activators or even
pieces of the substrate. These cause the breaking and
reforming of certain hydrogen bonds in the enzyme
structure and change the shape of the active site.
Only the correct substrate will trigger the correct
changes to the active site.
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When an enzyme is in solution with substrate molecules,
the enzyme's active sites are able to accept substrate
molecules only if it is the correct substrate.
In anabolic reactions, the active site is formed specifically
so that the substrates will be correctly aligned to bond
together.
In catabolic reactions and reactions involving internal
molecular changes, the active site acts by breaking
existing bonds and forming new ones. This is done
through the action of the amino acid residues in the active
site pulling the molecule apart.
Substrates bound with the active site are called an
enzyme-substrate complex. When the new product has
formed, it is no longer attractive to the active site and
leaves it. The new molecule is called the product.
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Enzymes can function both inside cells
(intracellular) or outside cells (extracellular).
For example, the enzymes that function in
our digestive systems are manufactured in
cells - but work extracellularly.
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Enzymes that act inside cells are responsible
for catalysing the millions of reactions that
occur in metabolic pathways such as
glycolysis in the mitochondria
The lysosomes contain many enzymes that
are mainly responsible for destroying old
cells
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Temperature – enzymes do not function at
low or high temperatures
PH- some have an optimum ph range at
which they work.
Enzyme Concentration
Substrate Concentration
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An inhibitor is a substance that inhibits
(restrains) the functioning of an enzyme.
There are two main types:
reversible inhibitors
irreversible inhibitors
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Irreversible inhibitors are substances that bond to the
enzyme covalently. They are not displaced by the
substrate that usually binds to the enzyme (because
the substrate binds with hydrogen bonds which are
much weaker than covalent bonds). By bonding to the
enzyme, either at the active site (so the substrate
cannot bind there) or at any other part of the enzyme,
the inhibitor may be able to change the conformation
(shape) of the enzyme. It does this by either breaking
hydrogen bonds or making others form in the wrong
place. This changes the enzyme structure. These
inhibitors are called irreversible because they do not
easily leave the enzyme. The enzyme is no longer
able to function.
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There are two types of reversible inhibitors:
competitive
non-competitive
In both cases reversible inhibitors can be
removed from the enzymes - but only under
certain conditions.
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Competitive inhibitors are inhibitors that compete
with substrates for the active site. They resemble the
substrate in that they can fit into the active site,
fooling the enzyme into thinking that they are
substrates. They differ from the substrate in that they
are unreactive. They therefore reduce the number of
enzymes available to catalyse a reaction. If there is
enough substrate available though, the chances of an
enzyme attracting an inhibitor is smaller - most
enzymes will still attract the correct substrates and a
reaction can still occur. To help overcome the effect
of a competitive inhibitor, the substrate
concentration should be increased. The inhibitors
usually leave the active site when the substrate
concentration is high enough.
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These substances do not bind to the active
site of an enzyme, but to other parts of the
enzyme. In doing so, they may change the
conformation of the active site and possibly
inactivate it