3. Enzymes
Enzymes are biological catalysts
This means they speed up reactions without
being chemically changed themselves
They are GLOBULAR proteins
See notes on molecules to recall what these are
4. Enzyme Definitions
Active site
This is the region of the enzyme where catalysis
occurs and substrates fit
Specificity
An enzyme may catalyse only one, or else lots of
reactions. Its specificity determines this. An enzyme
that catalyses one reaction is a highly specific
enzyme.
Affinity
This is how strong an enzyme binds its substrates
5. Active Site
The substrate is held in the active site by a
variety of bonds, such as hydrogen bonds and
electrostatic interactions
6. Enzyme Definitions
Cofactors
These are non-protein chemical compounds that
are bound to proteins and that are necessary for the
protein to have biological activity. “Helper
molecules” that are usually inorganic molecules.
Common examples include NAD+, FAD, Coenzmye
A, Mg2+
Coenzymes
A loosely bound cofactor, usually an organic
molecule
Prosthetic group
7. Why do reactions take so long?
For a reaction to occur, say A and B turning into C
The rate will depend on how long A and B take to
associate and then turn into C
This depends on how much energy you can
supply to make this happen – the ACTIVATION
ENERGY
8. Enzymes and Activation energy
Enzymes function by lowering the activation
energy for reactions so that can be performed
easier
9. Enzyme-Substrate complex
The enzyme binds both substrates, and lowers
the reactions activation energy
This is called the ENZYME-SUBSTRATE
COMPLEX
This allows the reaction to occur faster than if the
reactants were floating about free in solution
10. Models of enzyme action
How enzymes work is quite an open area of
debate in biology!
The main theories for how enzymes work are;
The lock and key hypothesis
The induced fit hypothesis
11. The lock-and-key hypothesis
In this model;
The reactants fit exactly into the enzyme active
site like a key fits into a lock. Once the catalysis
has occurred, the products are a different shape
and fit than the reagents and so they fall out of
the active site and the enzyme is free to start a
new reaction
12. The induced fit hypothesis
In this model;
The reagents don’t fit exactly into the enzymes
active site. But because enzymes are globular
proteins with flexible shapes, the enzyme
changes its active site shape to accommodate
the reagents
13. How can we make enzymes work
better?
Because enzymes are proteins they are easily
affected by pH, temperature and concentrations
of starting materials
14. The effect of heat on enzyme
activity
Enzymes are held together by hydrogen bonds and
other weak(ish) bonds
At temperature above body temp (37 oC), these
bonds being to break and enzymes will no longer
function. But from 0 to 37, the enzyme activity will
steadily rise! Not all enzymes however are like this, it
depends on each particular enzymes OPTIMUM
15. As complex, folded proteins, enzymes are susceptible
to changes in pH. Like with temperature, there is an
OPTIMUM pH for every enzyme
For instance the enzymes in your stomach love being
at pH 1 or 2. Yet enzymes that are found in your blood
will only work well at pH 7.4-7.6. On either side of the
optimum pH the enzyme rate will decrease to 0 as the
pH is not optimum.
The effect of pH on enzyme activity
16. Substrate means that thing that will be reacted
with and changed by the enzyme
As you put in more substrate, the enzyme will
work and convert it to product! And this will
happen in a directly proportional way until ALL the
enzyme active sites are filled and the enzymes
The effect of substrate concentration
on enzyme activity
17. Amount of enzyme will also affect a reactions
rate. As you put in more enzyme, the rate will
increase proportionally as there is nothing to slow
down the speed of the enzyme.
Eventually however, the amount of substrate will
begin to limit the reaction and the graph will level
off.
The effect of enzyme concentration on
enzyme activity
19. Enzyme Inhibition
Competitive inhibitors
These bind to the active site of the enzyme and prevent
substrates from binding and reacting.
Non-competitive inhibitors
These bind anywhere on the enzyme EXCEPT the active
site. This changes the shape of the enzyme and the
active site and so stops substrate binding.
20. Enzyme Immobilisation
Enzymes are a multi-million pound business
because they are cheap to make and can be
used for a multitude of purposes
They can make reactions happen that would
otherwise take hours or days to happen without
the help of an enzyme
One way to increase the efficiency of enzyme
catalysed reactions is to immobilise the enzymes
on solid supports in such a way that their active
site is easy to access
22. Why do we bother immobilising?
Immobilising an enzyme allows for cost effective
usage of enzymes
Immobilising can lead to;
Increased enzyme stability
Increased pH tolerance
Increased temperature tolerance
It allows continuous flow operation compared to
time consuming batch operation
24. Problems with immobilising
Reduced enzyme
efficiency in some
cases as the steps to
immobilise the
enzymes can damage
them
Sometimes the
enzymes get attached
the wrong way around
which means that the
active site cannot be
accessed by the
25. Lactose-free milk
An example of
immobilised enzyme
continuous culture is in
the production of lactose
free milk for people who
suffer lactose intolerance
or for cats
26. Clinistix
Medical diagnostic
kits use immobilised
enzymes to text for
diabetes and kidney
failure
Pregnancy tests also
use immobilised
enzymes that detect
human gonadotrophin
in urine
