Name: _______KEY________
Date: _________ Blk: _____
Enzyme Review Worksheet
Part A: Define the following terms in your own word. Be clear and concise!
metabolism
all the chemical reactions that take place in living systems to maintain homeostasis
substrate
the substances that enter a specific reaction
enzyme
proteins that serve as catalysts (they speed up reactions)
active site
the place on an enzyme where substrate(s) bind
coenzyme
non-protein organic molecules (e.g. NAD+) that help enzymes to catalyze reactions or carry electrons,
hydrogen, or functional groups stripped from substrates. Often complete the active site.
metabolic pathway
a stepwise sequence of reactions in cells, with specific enzymes catalyzing each step.
activation energy
the minimum amount of energy that colliding reactants must have in order for a chemical reaction to
occur.
Part B: Fill the the blanks.
1.
A metabolic pathway begins with a particular reactant, terminates with an end product, and has many
minute steps in between. A reactant is a substance that participates in a reaction. A product is a substance
that is formed by the reaction. The reactants in an enzymatic reaction are called the substrates for that
enzyme.
2.
An enzyme is a protein molecule that functions as an organic catalyst to speed up a chemical reaction.
3.
The energy that must be added to cause molecules to react with one another is called the energy of
activation. Enzymes lower the amount of energy for activation to occur.
4.
When an enzyme forms a complex with its substrate, the small part of the enzyme that
complexes with the substrate is called the active site. In the induced fit model, the active site undergoes a
slight change in shape in order to accommodate the substrate. Only a small amount of enzyme is actually
needed in a cell because enzymes are not used up.
5.
Enzymes are very specific in their action and are named for their substrates.
6.
As the temperature rises, why is there an increase in enzyme activity?
more effective collisions between enzyme and substrate as the temperature rises
7.
When an enzyme's shape changes due to high temperature or extreme pH, the enzyme is said to be
denatures. Enzymes that phosphorylate certain enzymes are called kinases, whereas phosphatases remove
phosphate groups.
8.
In competitive inhibition, another molecule is so close in shape to the enzyme's substrate that it can
compete with the true substrate for the enzyme's active site. In non-competitive inhibition, a molecule binds
to an enzyme, but not at the active site. This other site is called the allosteric site and can cause a shift in
the three-dimensional structure.
9.
In feedback inhibition, a product produced in high amounts by an enzymatic reaction can inhibit the
enzyme's activity. The end product of an enzymatic pathway binds at an allosteric site on the first enzyme
of the pathway.
10.
Coenzymes are organic molecules that bind to enzymes and serve as carriers for chemical groups or
electrons. Vitamins are small organic molecules that are required in trace amounts in our diet for
synthesis of coenzymes.
11.
Consider this metabolic pathway:
A E1BE2CE3D
If E1-E3 represents different enzymes, letters A, B, and C are considered reactants or substrates in the
reactions, whereas letters B, C, and D would be considered products. As a result of the action of E2, B is
now called a substrate for E2, and C is the product.
Part B: Short Answers
1.
The equation ADP + Pi  ATP is energy (requiring or releasing) requiring.
2.
In the pathway below, the letters stand for substrates and the numbers stand for enzymes. Each and every reaction in a cell
requires a specific enzyme.
1
2
3
4
A
B
C
D
E




3.
If an enzymatic reaction is heated gently, it will speed up.
4.
Enzymes lower the amount of activation energy necessary for a reaction to take place by putting its substrates on a precise
“collision course.”
5.
In the equation S + E  SE  P + E, what do the letters stand for?
S:
substrate
P:
product
SE:
substrate-enzyme complex
E:
enzyme
6.
Name two environmental factors that can change the shape of an enzyme.
i.
temperature
ii.
pH
7.
Name two factors that can speed up enzymatic reactions
i.
increase temperature
ii.
increase concentration of substrate or enzyme
8.
Enzymes have helpers called coenzymes. A common example of the latter is NAD. What is the function of NAD in cells?
Carries H atoms in oxidation reduction reactions..
9.
Give the overall equation for aerobic cellular respiration. Indicate energy on the correct side.
C6H12O6
+
6 O2
-------->
6CO2
+ 6 H2O
+
38 ATP
Food (Glucose)
energy
10.
11.
+
oxygen
carbon dioxide
+
water
+
Label the parts on this diagram.
1
enzyme
2
coenzyme
3
substrate
4
product
Label all missing parts on the graphs to the right. Highlight the energy of activation on both graphs.
without enzyme
Ea
Energy
Level
Energy
Level
with enzyme
Ea
Substrate
Reactants
Product
Progress of Reaction
Product
Progress of Reaction
12a.
Which graph below best represents a graph of the enzyme activity vs pH? Explain what is occurring at each portion of the
graph. D
12b.
Which graph below best represents a graph of the enzyme activity vs temperature? Explain what is occurring at each portion
of the graph.
D
A
B
C
D
Part C: Answer on separate sheets of paper, in your own words.
1.
What advantages can you see in having complex metabolic pathways within body cells to produce various substances, such as
amino acids and ATP?
more control over reactions (can be halted/modified/sped up/slowed down at any step).
more sophisticated reactions possible, so more complex molecules can be made
intermediate products can be used in other pathways
cyclic pathways/feedback mechanisms possible
2.
What gland produces the hormone thyroxin? What is the function of thyroxin in metabolism?
Thyroid gland. Thyroxin increases cellular metabolism (increases oxygen uptake, protein synthesis etc.)
3.
Explain, using a good example, how a metabolic pathway can be self-regulating (that is, how it can shut itself on and off)
The amino acid aspartate becomes the amino acid threonine by a sequence of 5 enzymatic reactions. When threonine, the end product of this pathway,
is present in excess, it binds to an allosteric site on enzyme 1, and then the active site is no longer able to bind aspartate.
4.
How does the “Lock and Key” theory of enzyme action differ from the “Induced Fit” theory? Use diagrams to help your
explanation.
In Induced Fit model, once the substrate binds the enzyme, the enzyme changes shape to more tightly bind the substrate. In Lock & Key model,
enzyme and substrate fit each other perfectly before they bind. See notes for diagrams
5.
Why do you think each enzyme has its own preferred pH at which it operates?
Changes in pH cause conformation changes (denaturing) in proteins (because they disrupt bonds holding the enzyme in its precise shape). Changes in
enzyme shape at active site will impair or destroy its substrate-binding ability.
6.
What is the effect of lowering the temperature on enzyme activity. How about raising the temperature? Draw a graph to show
these relationships.
Lowering temperature lowers the rate of activity (but does not usually denature the enzyme). Raising the temperature moderately raises the rate of
reaction. Raising the temperature a large amount will denature the enzyme.
7.
Describe three factors that can lead to the denaturing of enzymes. How would denaturing an enzyme affect its activity?
pH
high temperature
heavy metals
specific chemicals e.g. HCN
8.
What happens to the rate of product formation if you continue to add to an enzyme-catalyzed reaction the following: a)
substrate b) enzyme c) an inhibitor d) Lead, mercury, or cadmium e) H + ions f) OH- ions
a) Increase until enzymes saturated
b) increase as long as substrate present
c) decrease
d) decrease and can cause denaturing
e) decrease and can cause denaturing
f) decrease and can cause denaturing
9.
Explain, using diagrams, how competitive inhibitors differ from non-competitive inhibitors in the way they act on enzymes.
S1
e
E
e
The inhibitor binds to the
enzyme, blocking active
site from real substrate.
No product can be
formed. If the Inhibitor
doesn’t leave, the enzyme
is “dead”!
Inhibitor
S2
e
e
EI Complex
e
S1
e
E
e
competitive inhibitors
S1
e
E
e
Inhibitor
e
S2
e
S2
e
When the inhibitor binds
allosterically, it causes the
active site to change shape,
so the substrates can no
longer bind.
non-competitive inhibitors
Both slow the rate of reaction.
10.
Discuss, using examples, the effects of reversible and non-reversible inhibitors on enzyme activity.
Reversible inhibitors will slow down enzyme action. The more inhibitor that is added, the more the activity slows. e.g. threonine. Non-reversible
inhibitors will slow down enzyme action. Each inhibitor will destroy an enzyme. If enough I added, enzyme activity will eventually cease. e.g.
HCN, Pb++, Hg++, penicillin
11.
Explain the role of vitamins in metabolic reactions. List at least 2 examples.
Many vitamins are integral (i.e. structural) parts of coenzymes and therefore are necessary for enzyme function. Vitamins are relatively small organic
molecules that our bodies can’t synthesize, and so must be ingested in trace amounts in our diets. For example
Vitamin
Coenzyme
Niacin
NADB2 (riboflavin)
FAD
B1 (Pantothenic acid)
Coenzyme A (CoA)
B12 (cobalamin)
B12 coenzymes
12.
Explain why a genetic defect that affects only one enzyme in a metabolic pathway can have serious consequences.
A genetic effect in an enzyme in a metabolic pathway means that the enzyme may no longer function. If it no longer functions, this means that the
pathway will stop at that point, and all the other steps “downstream” will also be affected or stop. This could cause disastrous effects on homeostasis.
For example, if enzyme 2 in the pathway below is non-functional due to a genetic defect, C, D, and E will not be produced, and any pathways
requiring C, D, and E will also be affected.
1
2
3
4
A
B
C
D
E

13.



An experiment was carried out to investigate the action of the enzyme catalase from homogenized (broken down) liver cells.
Catalase breaks down hydrogen peroxide (found in liver cells) into oxygen and water (2H2O2  2H2O + O2). Five test tubes
were set up as shown on the table below. The following steps were performed:
i. equal portions of liver homogenate were placed in the five test tubes.
ii. the test tubes were placed in the water baths at the temperature indicated in the table for 5 minutes.
iii. 10mL of hydrogen peroxide was added to teach test tube.
iv. the amount of oxygen released was recorded
TUBE
1
2
3
4
5
Temperature (º C)
18
28
38
48
80
mL of O2 produced in 1 min.
4.1
8.8
18
1.2
0
a.
The more H2O2 that is broken down, the better (worse/better) the enzyme catalase must be working.
b.
The amount of H2O2 that is broken down is directly proportional to the amount of oxygen gas that is produced, so by
measuring how much oxygen gas is produced, we can have a reliable measure of how well the enzyme catalase is
working.
c.
Explain why there was a difference in the amount of oxygen produced between test tubes 3 and 4.
38C is the optimum temperature for catalase to work. At 48C, the enzyme catalase may have been slightly denatured.
d.
Account for the low level of oxygen in test tube 1.
The temperature is too low for effective collisions to occur.
e.
Why was there no oxygen produced in test tube 5?
The 80C, the enzyme is catalase has been drastically denatured.
f.
Rank the test tube where the enzymes worked the best to the least.
Test tube #3, 2, 1, 4 and 5