Enzymes are globular proteins that catalyze metabolic reactions. They have an active site that binds to substrate molecules in a "lock and key" fashion. This lowers the activation energy needed for reactions to occur. Enzyme activity is affected by concentration, temperature, pH, and inhibitors. Inhibitors bind to the active site and prevent substrate binding, competitively or non-competitively. This inhibition allows control of reaction pathways and is important in processes like glycolysis.

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