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ENZYMES
Enduring Understanding
 All biological systems need catalysts to
alter speed of chemical reactions in the
system.
 Organic catalysts are proteins, therefore
they are affected by extremes of
temperature and pH changes.
Catalysts
 Chemicals that speed up chemical
reactions
 Not changed at end of reaction
Why do we need catalysts in biological
systems?
 Without them the chemical reactions
vital to life could not take place quickly
enough.
Eg. digestion, respiration
Biological Molecules
3.2 Enzymes
Enzymes and Activation Energy
THEME ONE:
PRINCIPLES OF BIOLOGY
3.2Enzymes
Enzymes
• Biological catalysts made up of tertiary proteins.
• Produced by living cells (organic)
• Speed up chemical reactions
• Not changed at end of chemical
reaction
• Specific in their reaction
Structure of enzymes
 Polypeptides (chains of
amino acids) are folded
around each other to form
a roughly spherical
structure.
 Results in 3-dimensional
shape.
 The shape is maintained
by hydrogen bonds.
 Folding gives rise to
active site (place at which
enzyme reaction takes
place)
ACTIVE SITE
 Pockets or clefts on enzyme surface
 Shape is complementary to substrate,
therefore specific
 This is where the enzyme substrate
reaction actually occurs.
 High temperature results in loss or
alteration of active site, ie. denatured
TYPES OF ENZYMES
 Intracellular
These are both made and have their action
inside cells. Eg. photosynthetic enzymes
inside chloroplasts
 Extracellular
These are made inside the cells but have their
action outside the cell.
Eg. digestive enzymes in the human gut,
enzymes released by fungi and bacteria
(decomposers)
Naming enzymes
 Based on what it reacts with (substrate)
 Add -ase to ending
Eg. enzyme lactase – reacts with
substrate lactose, a sugar
enzyme salivary amylase reacts with
substrate starch
Example
 Hydrolytic / digestive enzymes
- chemically breaks down larger
molecules in the presence of water.
Eg. amylase, lipase, proteases,
HOW ENZYMES WORK
enzyme
substrate
Click on the link below
http://www.bbc.co.uk/education/asguru/biology/02biolo
gicalmolecules/01proteins/11enzymes/index.shtml
STEPS IN ENZYME REACTION
 Substrate molecule fits into complementary
active site
 Enzyme-substrate complex formed. Reaction
takes place
 Substrate leaves active site. Enzyme is
ready for another reaction
NB: A small quantity of enzyme can bring about
a large amt of reaction.
For an enzyme to work, it must ....
 1. Come into physical contact with its
substrate
 2. Have an active site which fits the
substrate
Any factor that influence the rate at which an
enzyme works do so by affecting one or both
of the above conditions.
Effect of temperature on enzyme
catalysed reaction. (Pg 68, txtbook)
 At low temperature, the enzyme is less active. Why?




Recall kinetic theory + condition 1.
As temp. increases, enzyme activity increases.
At optimum temperature, enzyme is most active.
Beyond optimum temperature, the enzyme begins to
denature, ie. The weak bonds within the enzyme start
to break down, the enzyme begins to lose its 3-D
structure and active site. Therefore, rate of reaction
declines.
At very high temperature, reaction stops. Enzyme is
completely denatured, irreversible structural change
into a simple chain of protein.
Enzymes are proteins, so .....
 Sensitive to temperature
 Sensitive to pH changes.
Effect of Substrate Concentration
 At low substrate concentrations, the active
sites of enzyme molecules are not all
occupied. So reaction is fast. WHY??
 As substrate concentration increases, more
and more of the active sites are being
occupied for reaction. Further increase in
substrate concentration will not increase
speed of reaction. WHY??
Enzyme concentration
 When there are excess substrate
molecules, the speed of reaction is
directly proportional to enzyme
concentration.
A student who had seen a cook place slices of fresh
pineapple on top of meat before cooking it, decided to carry
out some experiments using fresh pineapple juice.
A milk agar was prepared by mixing fat-free milk and warm
water with agar powder. It was then poured into a sterilised
petri dish and allowed to cool. After it had set, five holes were
made in the milk agar. Each hole was filled with a different
substance, as shown in Fig. 1.1
The petri dish was then kept at 400C for 2 hours. The results obtained
are shown in
Fig. 1.2.
Explain the results obtained for each of the holes 1-5.
THE END