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Enzymes
What do you remember about enzymes?
Create a mind map
• Include
– What they are made of
– How they work
– What factors affect them
– Any good examples
All enzymes are proteins
Enzymes are similar in many ways:
• Globular proteins with a specific 3D shape
(tertiary structure)
A catalyst is a molecule (or
• They act as catalysts element) that speeds up a
chemical reaction but does not
• Specific
get used up in the reaction. At
• Active site
the end of the reaction the
(pocket or cleft area) catalyst remains unchanged.
• Activity affected by temperature and pH
Enzymes
sucrose + water
glucose + fructose
substrate(s) –
product(s) –
substance(s) used
up in an enzymecontrolled reaction
substance(s) formed
in an enzymecontrolled reaction
Sucrase – the
enzyme
Active site
• area on an enzyme to which
substrate binds (not bonds or
joins)
• Tiny part of enzyme
• Very few (often fewer than
10) amino acids form the
actual active site
Individual cell may contain
as many as 1000 different
Inside and out
• Intracellular
(enzymes catalyse reactions inside the cell)
• Extracellular enzymes:
released from cells that make them and their action
takes place outside cells
eg
Salivary amylase
Fungal hyphae secrete digestive
enzymes
Why do reactions need enzymes?
• Increase rate of reaction
• Enzymes increase the rate of reaction by 107
(10 million times faster!)
Reactions need a level of energy
before they can proceed =
activation energy
Lowering the amount of energy
required to get reaction started
makes the reaction more likely
to occur
Enzymes control metabolic processes?
E.g.
• Photosynthesis
• Respiration
Draw a similar diagram to the one above and
explain how the end product can control the rate
of the reaction
Active site shape is
determined by
tertiary (3D)
structure of enzyme
Due to the 3D
shape of the
active site
Specificity
The active site is
only
complementary to
a specific substrate
Each enzyme can
only catalyse one
reaction
Determined by
the primary
structure of the
protein which
forms the enzyme
Formed during
protein synthesis
and coded for by
DNA
Enzyme
action
Theories of Enzyme Action
• Lock and Key Hypothesis
• Induced Fit Hypothesis
Lock and key hypothesis
Lock = Enzyme
Key = Substrate
This theory relies on
the complementary
shapes of the
substrate and active
site creating enzymesubstrate complexes
Textbook p126-127
Induced Fit Hypothesis
substrate molecule changes shape to fit enzyme molecule
more closely as they bind, putting a strain on the substrate
bonds.
(Think of it as a hand in a glove)
• Charges on amino acids at active site
also hold substrate in place.
• Enzyme-substrate complexes are
formed as before.
• Products no longer fit the active site so
they leave.
Animation – hydrolysis
Examiners tip:
Don’t write:
“the enzymes shape is complementary to the substrate”
Do write:
“the shape of the enzymes active site is complementary to
the shape of the substrate molecule”
Enzyme action
1. Enzyme and substrate molecules have their own
kinetic energy and are moving – they collide randomly
Enzyme action
2 substrate molecule binds to active site of enzyme
forming an enzyme-substrate complex (lock and key
/ induced fit)
Enzyme action
3 Enzymes reduce activation energy by holding
substrate in a way which causes reaction to occur
more easily forming enzyme-product complex
Enzyme action
4 The products no longer fit active site and move out –
enzyme is free to catalyse same reaction with
another substrate molecule
Enzyme action
‘a single enzyme molecule typically acts on a thousand
substrate molecules per second’
Straw is plentiful and it could be used for the production of
many organic substances. The first step is the conversion of
cellulose to glucose. It has been suggested that an enzyme
could be used for this process. There is a difficulty - the lignin
that covers the cellulose would protect the cellulose from
attack by the cellulose-digesting enzyme.
•
Use your knowledge of the way in which enzymes
work to explain why cellulose-digesting enzymes do not
digest the lignin.
(2 marks)
•
•
Enzymes are specific;
Shape of lignin molecules
will not fit active site of
enzyme;
Enzyme action - Catalase
1
2
3
4
Catalase is a globular
protein made from
four polypeptide
chains (an example of
a protein with a
quaternary structure)
One molecule of catalase can break down
83 000 molecules of hydrogen peroxide
every second!
Catalase is found in living cells and breaks down toxic
hydrogen peroxide into harmless water and oxygen
2H2O2
catalase
2H2O + O2
Enzyme structure
and action
Investigating the time course for an
enzyme controlled reaction
Enzyme: catalase
Substrate: H2O2
(3 different sources – peas, potato, yeast )
Independent Variables
Dependent Variable
Controlled variables:
Time, Source of catalase
Volume of oxygen
apparatus
temperature
Volume of H2O2
Method:
•
•
•
•
•
•
Prepare measuring cylinder(full of water)
Add enzyme source to test-tube
Add 10cm3 hydrogen peroxide to test-tube (use syringe)
Quickly place bung into top of test-tube
Ensure delivery tube carries gas into cylinder
Measure volume of O2 produced every 30 seconds for 10 minutes
Follow the progress of an enzyme-catalysed
cylinder
1. Prepare a results table
reaction delivery tube measuring
– select an
appropriate size
clampstand
2. Set up the apparatus
(minus the hydrogen
peroxide) as shown
bung
water
boiling tube
3. Add the hydrogen
peroxide, return bung
and immediately start
a stop clock
4. Record volume of O2
collected in measuring
cylinder every 10s until
reaction stops
source of
catalase –
adjust size etc
as needed
10cm³ hydrogen
peroxide – add last
using a syringe
plastic tub
5. If reaction is too fast /
slow – adjust volume /
surface area of
catalase source/ size
of measuring cylinder/
time intervals
Results table:
Total Volume O2 produced (cm3)
Time (s)
30
60
90
120
150
180
210
240
270
300
330
360
Potato
Peas
Yeast
390
420
450
480
510
540
570
600
Graph:
Volume of O2 produced
(cm3)
Graph to show the effect of enzyme source on the time
course of an enzyme controlled reaction
Time (s)
Volume of O2 produced (cm3)
Calculating initial rate of reaction
Comparing the initial rate
of an enzyme controlled
reaction is useful as it
should be the fastest
point of a reaction.
b
a
Time (s)
Initial rate =
Volume O2
Time taken (s)
a
b
cm3 s-1
cm3 s-1
Tangents taken later on
the reaction curve allow
you to calculate the rate
at later stages.
You can also use these
calculations to plot a
graph of rate against time
Page 142-143
Calculating Mean Rates of a Reaction
Factors Affecting Enzyme Activity
•
•
•
•
•
Temperature
pH
Concentration of the substrate
Concentration of the enzyme
Presence of inhibitors