Enzymes are protein molecules that act as biological catalysts, speeding up chemical reactions. They have an active site that binds to specific substrate molecules. The "lock and key" hypothesis states that enzymes and substrates fit together precisely, while the induced fit hypothesis is that they can slightly change shape to better fit. Enzymes lower the activation energy of reactions, allowing substrates to more easily convert to products. The rate of enzyme-catalyzed reactions depends on temperature, enzyme and substrate concentrations, and can be inhibited by other molecules binding to the enzyme. Immobilizing enzymes allows them to be reused for reactions.

1. R E V I S I O N
Enzymes

2. Mode of action of enzymes
 Enzymes are protein molecules which can be defined
as biological catalysts.
 A catalyst is a molecule that speeds up reactions.
 Most enzymes end with –ase
 They are essential for life

3. Intracellular and extracellular enzymes
 Enzymes that operate within cells are known as
intracellular.
 Enzymes that are secreted by cells and work outside
cells are known as extracellular.
 Digestive enzymes in the gut are examples of
extracellular enzymes.

4. Lock and key induced fit hypothesis
 Enzymes are globular proteins which have a precise 3D
shape.
 Enzymes posses an active site which is a cleft or
depression into which the enzymes substrate will bind.
 The idea that each enzyme has a specific substrate that
can fit exactly into it is known as the lock and key
hypothesis.
 The substrate is the key and the enzyme is the lock.
 The substrate is held in place by temporary bonds, this
structure is know as the enzyme-substrate complex.

5. Induced fit hypothesis
 This idea is basically the same as lock and key except
with the slight modification that the enzyme and
sometimes the substrate can change shape slightly to
fit each other.
 An enzyme may catalyse a molecule into 2 molecules
or join 2 together.
 This structure is known as the enzyme-product
complex.
 The result is the product.
 The enzyme is unchanged by this process.

6. Activation Energy
 In order for the chemical reaction with the substrate to
occur; the reaction needs a little extra energy to get
started. This is called activation energy.
 One way of increasing the rate of a reaction is to give it
extra heat.
 But raising the temperature is sometimes not enough to
give the substrate energy they need to change to
products.
 Enzymes avoid this problem because they decrease the
activation energy of the reaction which they catalyse.
 They do this in such a way by holding the substrate so
that their molecules can bind more easily.

7. Course of reaction
 For example the enzyme catalase which breaks down
hydrogen peroxide into water and oxygen.
 The reaction begins swiftly and bubbles of O2 are
released.
 A large volume of O2 is collected in the first minute.
 As the reaction continues the rate of O2 released
gradually decreases untill it eventually stops.

8. Explanation for the course of reaction
 When the enzyme and substrate are first mixed,
there are a large number of substrate molecules.
 At any moment virtually every enzyme molecule has
a substrate in its active site.
 As more and more substrate are converted to
product there are fewer substrate molecules for the
enzymes to bind to so they begin to pile up.
 As fewer and fewer substrate molecules are left the
reaction gets slower and slower until it eventually
stops.

9. Factors that affect the rate of reaction

10. Temperature

11. Enzyme concentration

12. Substrate concentration

13. Enzyme inhibitors
 It is possible for another enzyme to fit into the active site of an
enzyme if it has a very similar shape to that of the substrate..
 The inhibitor molecule binds briefly to the active site and
there is competition between the inhibitor and substrate.
 If there are more substrate molecules that inhibitor the
substrate can then bind to the enzyme and its reaction is
unaffected.
 However if the concentration of the inhibitor rises it is less
likely that a substrate will find an active site. The enzymes
function is therefore inhibited.
 This is known as competitive inhibition.
 It is said to be reversible because it can be reversed if the
concentration of the substrate is increased.

14. Non competitive, reversible inhibition
 This is when a molecule can bind to another part of the
enzyme instead of the active site.
 This distorts the bonds in the enzyme which ripples through
to the active site, this makes the enzyme unsuitable for its
substrate.
 While the inhibitor is attached to the enzyme its active site is
blocked. This is an example of non competitive inhibition.
 This is sometimes essential to balance reactions.
 End product chain of reactions is a non competitive reversible
inhibitor.
 As the enzyme converts substrate to product it is slowed down
because the end product binds to another part of the enzyme.

15. Immobilizing enzymes
 Immobilizing enzymes is trying to re-use them again
instead of having to buy them again.
 For example lactase is placed in beads and milk runs
through them this hydrolyses lactose to glucose and
galactose for lactose intolerant .
 This means the enzymes can be re-used and the
product is enzyme free.