Download File

ANSWERS 1 (53 Marks)
IB Standard and Higher level Biology Dulwich College Shanghai
Topic 3 (SL): Chemistry of Life
Topic 7 (HL): Nucleic Acids and Proteins
3.6
3.6.1
3.6.2
3.6.3
3.6.4
3.6.5
7.6
7.6.1
7.6.2
7.6.3
7.6.4
7.6.5
Enzymes (SL)
Define enzyme and active site
Explain enzyme-substrate specificity (the lock-and-key model can be used as a basis for
the explanation. Refer to the 3-D structure).
Explain the effects of temperature, pH and substrate concentration on enzyme activity.
Define denaturation
Explain the use of lactase in the production of lactose-free milk
Enzymes (HL)
State that metabolic pathways consist of chains and cycles of enzyme-catalysed
reactions.
Describe the induced-fit model. (This is an extension of the lock-and-key model. Its
importance in accounting for the ability of some enzymes to bind to several substrates
should be mentioned).
Explain that enzymes lower the activation energy of the chemical reactions that they
catalyse. (Only exothermic reactions should be considered).
Explain the difference between competitive and non-competitive inhibition, with
reference to one example of each. (Reversible inhibition, as compared to irreversible
inhibition, is not required).
Explain the control of metabolic pathways by end-product inhibition, including the role
of allosteric sites.
Paper 1
Multiple Choice (6 Marks)
1.
Consider the metabolic pathway shown below.
A
1
B
2
C
3
D
4
E
If there is end-product inhibition, which product (B to E) would inhibit which
enzyme (1 to 4)?
Product
Enzyme
A.
C
4
B.
B
3
C.
B
4
D.
E
1
2.
What effect do enzymes have on the activation energy of exergonic and endergonic
reactions?
Activation energy of Activation energy of
exergonic reactions endergonic reactions
A.
increases
increases
B.
decreases
decreases
C.
increases
decreases
D.
decreases
increases
3.
The graph below shows the effect of changing the substrate concentration on an
enzyme controlled reaction.
40 % substrate
20 % substrate
10 % substrate
Amount of product formed
5 % substrate
2.5 % substrate
0 % substrate
Time
What is the correct interpretation of these data?
A.
The rate of reaction increases continuously with increase in substrate
concentration.
B.
The rate of reaction decreases continuously with increase in substrate
concentration.
C.
The rate of reaction increases up to a point and then remains constant.
D.
The rate of reaction is not affected by any change in the substrate
concentration.
4.
In the enzyme controlled pathway shown below, which compound is most likely to
inhibit enzyme (w)?
Precursor
A.
B.
C.
D.
enzyme
enzyme
enzyme
enzyme
w
x
y
z

I

II

III

IV
I
II
III
IV
5.
Which of the following could cause denaturation of an enzyme?
A.
Substrate concentration
B.
A competitive inhibitor
C.
High temperature
D.
Low salt concentration
6.
The reaction below shows the energy changes in a chemical reaction.
What would happen to the changes in energy if this reaction was controlled by an
enzyme?
A.
I would increase.
B.
II would decrease.
C.
I and IV would decrease.
D.
II and III would decrease.
Paper 2
Section A
Data Analysis (7 Marks)
1.
Alcohol dehydrogenase is an enzyme that catalyses the reversible reaction of
ethanol and ethanal according to the equation below.
+
+
NAD + CH3CH2OH
CH3CHO + NADH + H
ethanol
ethanal
The initial rate of reaction can be measured according to the time taken for NADH
to be produced.
In an experiment, the initial rate at different concentrations of ethanol was
recorded (no inhibition). The experiment was then repeated with the addition of
–3
l mmol dm 2,2,2-trifluoroethanol, a competitive inhibitor of the enzyme. A third
–3
experiment using a greater concentration of the same inhibitor (3 mmol dm ) was
performed. The results for each experiment are shown in the graph below.
1.2
1.1
1.0
0.9
0.8
0.7
Initial rate of reaction 0.6
0.5
/ arbitrary units
0.4
0.3
0.2
0.1
0.0
no inhibition
1 mmol dm–3 inhibitor
3 mmol dm–3 inhibitor
0
10
20
30
40
50
60
70
Ethanol concentration / mmol dm
80
90
100
–3
[Source: R Taber, Biochemical Education, (1998) 26, pages 239-242]
(a)
(b)
Outline the effect of increasing the substrate concentration on the control
reaction (no inhibition).
directly proportional / greater concentration, greater rate of reaction;
at high concentrations the increase is smaller / plateau / levels-off (at
–3
approximately 70 mmol dm );
(i)
(2)
–
State the initial rate of reaction at an ethanol concentration of 50 mmol dm
3
–3
–3
in the presence of the inhibitor at 1 mmol dm and 3 mmol dm
(1)
1 mmol dm–3 : 0.70 (± 0.02)
3 mmol dm–3 : 0.55 (± 0.02)
1 max
Both needed for [1]. For 1 mmol dm–3 accept 0.7.
(ii)
(c)
State the effect of increasing the concentration of inhibitor on the initial
rate of reaction.
(1)
–3
lower reaction rate at inhibitor concentration of 3 mmol dm / the
greater the
inhibitor concentration the slower the rate of reaction;
trend / overall shape are the same / increases but then levels-off;
but lower at greater concentration of inhibitor;
Explain how a competitive inhibitor works.
substrate and inhibitor (structurally) similar;
inhibitor binds to active site;
prevents substrate from binding;
activity of enzyme prevented;
named example (eg malonate inhibits succinate dehydrogenase as
it is similar to succinate);
(3)
Paper 2
Section A
Short Structured (21 Marks)
1.
(a) Explain the secondary and tertiary levels of protein structure.
secondary structure: [2 max]
folding / pleating of polypeptides to form b - pleated sheets / coiling of
polypeptides to form a -helix;
held in place by hydrogen bonds;
make structure stable;
contributes to strength of fibrous proteins;
provide structural role in organisms;
eg a -helix is keratin / b-sheet is silk;
(4)
tertiary structure: [2 max]
3-D shape;
due to bonding between amino R-groups / residues;
hydrogen bonds / disulphide bridges / sulphur bonds / ionic bonds;
form globular proteins;
which are soluble;
eg lysozyme / enzymes;
2.
(b)
Outline the induced fit model for enzyme action.
change in shape of enzyme’s active site;
improves fit of enzyme and substrates;
brought about when the substrate molecules bind with the enzyme;
enzyme changes from inactive to active form;
permits some enzymes to bind with several substrates;
distorts / weakens bonds in substrates;
lowers activation energy:
Accept any of the above if clearly explained in a labelled diagram.
(2)
(a)
State the function of a named protein.
name and function;
eg hemoglobin carries oxygen
(1)
(b)
Outline the significance of the primary structure in a protein.
order of amino acids in polypeptides;
primary structure determines higher structure;
primary structure reflects genetic information / is coded for by the DNA;
(2)
(c)
Discuss the statement, “Enzyme inhibitors function by binding to the active site of
the enzyme”.
(3)
competitive inhibitors are structurally similar to the substrate;
competitive inhibitors bind / block active site;
non-competitive inhibitors do not bind to the active site / bind somewhere
else
on enzyme;
they function by changing the shape of the enzyme so the substrate cannot fit
in to the active site;
3.
(a)
The table below compares prokaryotic and eukaryotic cells. Place a tick (
wherever the organelle is present.
Organelle
Prokaryotic
)
(2)
Eukaryotic
Nucleus
Mitochondrion
Ribosomes
(b)
(i)
The graph below shows the energy changes in a reaction.
(1)
Key:
Energy
original reaction
with enzyme
Progress of reaction
On the above graph draw the result you would obtain in this same
reaction if an enzyme that catalyses this reaction were added.
(ii)
(c)
Explain how the enzyme produces this effect.
enzyme binds to substrate;
lowers activation energy;
by weakening bonds;
making substrate more likely to react;
Outline the process of glycolysis.
one hexose sugar / glucose is converted to two 3-carbon compounds /
pyruvate;
at start 2 ATP are used / phosphorylation of glucose;
net gain of 2 ATP / 4 ATP produced in total;
+
production of 2NADH + H / reduced NAD;
(3)
(3)
Section B
Extended Response (19 Marks)
1.
Outline enzyme-substrate specificity.
active site of enzyme binds to specific substrate;
shape of the active site and substrate fit / complement each other;
lock and key model;
chemical properties of substrate and enzyme attract / opposite charges;
enzyme / active site is not rigid and substrate can induce slight changes in shape;
allows substrates of similar structure to bind with same enzyme;
induced fit;
causes weakening of bonds in substrate to lower activation energy;
(5)
2.
Explain the effects of temperature, pH and substrate concentration on enzyme activity.
(8)
enzymes have an active site;
that fits the substrate precisely;
changes in the chemical environment of the enzyme can lead to a
shape / conformational change in the protein;
leading to a change in the shape of the active site;
may interfere with the binding of the substrate with the active site;
altering pH can alter intermolecular interactions within the protein;
or within the active site;
enzymes have an optimum pH;
increase in temperature can increase molecular motion leading to
disruption of intermolecular interactions;
increases chance of enzyme substrate collisions so enzyme activity increases;
optimal temperature;
temperature changes / pH changes can denature the protein;
the more substrate, the more product / more enzyme-substrate complex forms;
after a point, all active sites are bound to substrate / all active sites occupied;
additional substrate will not lead to a greater rate of product formation at
this point;
8 max
For full marks all three conditions must be included, otherwise award [6 max].
[Plus up to [2] for quality]
3.
Outline two examples of the commercial application of named enzymes in biotechnology.
(6)
the name of the enzyme and the substrate;
the name(s) of the product(s);
a statement as to why the application is useful commercially;
Award [3 max] for each example.
eg pectinase acts on soluble pectin;
produces smaller, more soluble carbohydrates;
used in fruit juice clarification / improving fruit juice yield;
eg DNA endonuclease acts on DNA;
produces DNA fragments;
used in genetic engineering;
eg protease acts on insoluble proteins;
produces amino acids;
washing powders – stain removal;