Cambridge AS level Biology - Emzymes

1. 3 ENZYMES

2. LO
• explain that enzymes are globular proteins
that catalyse metabolic reactions;
• explain the mode of action of enzymes in
terms of an active site, enzyme-substrate
complex, lowering of activation energy and
enzyme specificity

3. ENZYMES
• Protein molecules; globular proteins
• Biological catalysts – speed up chemical
reactions but remains unchanged at the end
of the reaction
• Name ends with –ase
• Coiled into precise three dimensional shape
with hydrophilic groups on the outside of the
molecule = soluble

4. Active site
• Region to which another molecules
(substrate) can bind
• Lock and key hypothesis
– Substrate is the key whose shape fits the lock of
the enzyme
– The substrate is held in place by temporary bonds
which form between the substrate and some of
the R groups of the enzyme’s amino acids =
enzyme-substrate complex

6. • Induced fit hypothesis
– Same as lock and key but adds the idea that:
– Substrate and enzyme can change shape slightly
as the substrate molecule enters the enzyme in
order to ensure a perfect fit = more efficient
catalysis

7. 3

8. • Each type of enzyme will act on only one type
of substrate molecule = enzymes are specific
• Enzymes can catalyse the joining of two
substrates (synthesis) or braking down the
substrate (lysis) = products
• Enzyme remains unchanged
• Rate can be very high

10. • Lyzozyme
– Enzyme in saliva, tears and other secretions
– Natural defence against bacteria
– Breaks the polysaccharide chains that form the
cell walls of bacteria

11. Activation energy
• Extra energy given to the substrate so that the
reaction can happen
• Energy of the reactant can be increased by
heating (benedict’s test)
• Enzymes decrease the activation energy to
the reaction

13. Factors effecting the enzyme activity
• The effect of enzyme concentration
– The more enzyme present, the more active sites
will be available for the substrate to slot into
– As long as there is plenty of substrate available,
the initial rate of a reaction increases linearly with
enzyme concentration
– P. 59. fig. 3.6

14. • The effect of substrate concentration
– As substrate concentration increases, the initial
rate of reaction also increases
– The more molecules are around the more often an
enzyme’s active site can bind with one
– If the enzyme reaches its maximum possible rate =
Vmax (maximum velocity)

15. • The effect of temperature
– Low temperatures – reaction takes place very
slowly as the molecules move relatively slowly
– As temperature increases – molecules move faster
and therefore collide with the active site of an
enzyme more frequently; more energy is involved
= easier for bonds to be formed or broken
– At some point – bonds within the enzyme break
and enzyme looses its shape and activity = it is
DENATURATED

16. – Optimum temperature = temperature at which an
enzyme catalyses a reaction at the maximum rate
(mostly 40 degrees)

17. • The effect of pH
– Most enzymes work fastest at a pH of around 7;
some work better in acidic conditions (pepsin)
– Too different pH can cause denaturation
– The lower the pH the higher the hydrogen ion
concentration – hydrogen ions can interact with
the R-groups of amino acids – this affects the ionic
bonding between the groups which affects the 3D
arrangement of the enzyme

18. Enzyme inhibitors
• If a different molecule (very similar in shape to
the enzyme’s substrate) binds with the active
site of an enzyme – this inhibits the enzyme’s
function = this molecule is called INHIBITOR

19. • Competitive inhibition
– If an inhibitor molecule binds only briefly to the
site = competition between it an the substrate for
the site
– If there is much more substrate than the inhibitor
– substrate molecules can easily bind to the active
site in the usual way = no effect on the enzyme’s
function
– If the concentration of inhibitor is rises = less likely
that the substrate will collide with an empty site
– This process is reversible

20. – Example:
– ethylene glycol – used as antifreeze; if drunk by
somebody, it is rapidly converted to oxalic acid
which causes irreversible kidney damage
– The active site of the enzyme will accept ethanol
– If the person is given ethanol – it acts as a
competitive inhibitor = results in slowing down
the reaction for long enough to allow the ethylene
glycol to be excreted

21. • Non-competitive inhibition
– Inhibitor can remain permanently bonded with
the active site and therefore cause an irreversible
block to the substrate
– No competition occurs
– Example:
– Penicillin – permanently occupies the active site of
an enzyme that is essential for the synthesis of
bacterial cell wall

22. • Second type of non-competitive inhibition
– Molecule binds to another part of the enzyme
(not the active site) – this disrupts the normal
arrangement of hydrogen bonds and hydrophobic
interactions holding the enzyme molecule in its 3D
shape = results in unsuitability of the enzyme for
the substrate; the enzyme is blocked
– This inhibition can be reversible

23. – Example:
– Digitalis – binds with ATPase = increase in the
contraction of heart muscle
Inhibition of enzyme function can be lethal, but in
many situations inhibition is essential!

25. • End product inhibition
– Using end product of a chain of reactions as an
enzyme inhibitor
– As the enzyme converts substrate to product, it is
slowed down because the end-product binds to
another part of the enzyme and prevents more
substrate binding
– As product levels fall, the enzyme is able to top
them up again
– non-competitive reversible inhibition