DEPARTMENT OF BIOCHEMISTRY AND BIOTECHNOLOGY

SCHOOL OF PURE AND APPLIED SCIENCES

KENYATTA UNIVERSITY

BACHELOR OF MEDICINE AND BACHELOR OF SURGERY

HMB 102: FUNDAMENTALS OF ENZYMOLOGY

INSTRUCTIONS

Section A: Answer ALL the Questions.

1. Enzymes are potent catalysts. They;
A) Drive reactions to completion while other catalysts drive reactions to equilibrium.
B) Are consumed in the reactions they catalyze.
C) Are very specific and can prevent the conversion of products back to substrates.
D) Increase the equilibrium constants for the reactions they catalyze.
E) Lower the activation energy for the reactions they catalyze.

2. Which of the following statements about a plot of V o versus [S] for an enzyme that
follows Michaelis-Menten kinetics is false?
A) KM is the [S] at which Vo = Vmax/2.
B) the shape of the curve is a hyperbola.
C) the y-axis is the rate term with units of μmoles/min
D) as [S] increases, the initial velocity of reaction, V o also increases.
E) at very high [S], the velocity curve becomes a horizontal line that intersects the y-
axis at KM.

3. The equilibrium assumption, as applied to enzyme kinetics, implies:

A) KM = KS
B) The maximum velocity occurs when the enzyme is saturated.
C) The ES complex is formed and broken down at equivalent rates.
D) The KM is equivalent to the cellular substrate concentration.
E) The enzyme is regulated.

4. In a plot of 1/Vo against 1/[S] for an enzyme catalyzed reaction, the presence of a
competitive inhibitor will alter the:
A) Vmax.
B) Intercept on the 1/Vo axis.
C) Intercept on the 1/[S] axis.
D) Curvature of the plot.
E) pK of the plot.

5. In competitive inhibition, an inhibitor:

A) Binds at several different sites on an enzyme.
B) Binds reversibly at the active site.
C) Binds only to the ES complex.
D) Binds covalently to the enzyme.
E) Lowers the characteristic Vmax of the enzyme.

6. An allosteric interaction between a ligand and a protein is one in which:

A) Two different ligands can bind to the same binding site.
B) The binding of the ligand to the protein is covalent.
C) Multiple molecules of the same ligand can bind to the same binding site.
D) The binding of a molecule to a binding site affects the binding of an additional
molecule to the same site.
E) The binding of a molecule to a binding site affects the binding properties of
another site on the protein.

7. In hemoglobin, the transition from T to R state (low to high affinity) is triggered by:
A) Subunit association.
B) Subunit dissociation.
C) Fe2+ binding.
D) Heme binding.
E) Oxygen binding.

8. Which of the following is not correct concerning cooperative binding of a ligand to

a protein?
A) It is usually associated with proteins with multiple subunits.
B) It is usually a form of allosteric interaction.
C) It results in a non-linear Hill plot.
D) It results in a sigmoidal binding curve.
E) It rarely occurs in enzymes.

9. According to IUB nomenclature system, enzymes are grouped in how many main
classes?
A) 10
B) 6
C) 4
D) 2

10. How is trypsinogen converted to trypsin?

A) Two inactive trypsinogen dimers pair to form an active trypsin tetramer.
B) An increase in Ca2+ concentration promotes the conversion.
C) Proteolysis of trypsinogen forms trypsin
D) Trypsinogen dimers bind an allosteric modulator, cAMP, causing dissociation into
active trypsin monomers.
E) A protein kinase catalyzed phosphorylation converts trypsinogen to trypsin.

11. Which of the following enzymes in serum in specifically elevated in alcoholism?

A) Acid phosphatase
B) ᵧ-glutamyl transpeptidase
C) sGOT
D) sGPT

12. All the enzymes are increased in myocardial infarction except:

A) Lactate dehydrogenase
B) sGOT
C) CPK
D) Alkaline phosphatase

13. ‘Lock’ and ‘Key’ model of enzyme action proposed by Fisher implies that:
(A) The active site is flexible and adjusts to substrate
(B) The active site requires removal of PO 43- group
(C) The active site is complementary in shape to that of the substrate
(D) Substrates change conformation prior to active site interaction

14. Regulation of some enzymes by covalent modification involves addition or

removal of:
(A) Acetate
(B) Sulphate
(C) Phosphate
(D) Coenzyme

15. Kinetics of allosteric enzymes is explained by:

(A) Michaelis-Menten equation
(B) Lineweaver-Burk plot
(C) Hill plot
(D) All of these

16. The kinetic effect of purely competitive inhibitor of an enzyme:

(A) Increases KM without affecting Vmax
(B) Decreases KM without affecting Vmax
(C) Increases Vmax without affecting KM
(D) Decreases Vmax without affecting KM

17. Enzymes requiring NAD+ as co-substrate can be assayed by measuring change

in absorbance at:
(A) 210 nm
(B) 290 nm
(C) 340 nm
(D) 365 nm

18. Foetal haemoglobin contains:

(A) Two α- and two γ-chains
(B) Two β- and two γ-chains
(C) Both (A) and (B)
(D) None of these
19. Enzymes activity is controlled by:
(A) pH of the solution
(B) Temperature
(C) Concentration of the enzyme
(D) Concentration of the substrate
(E) All of these

20. If the substrate concentration is much below the km of the enzyme, the velocity
of the reaction is:
(A) Directly proportional to substrate concentration
(B) Not affected by enzyme concentration
(C) Nearly equal to Vmax
(D) Inversely proportional to substrate concentration

Section B: Fill in the empty gaps (5 marks each)

1. Enzymes have properties that are pH sensitive. Most proteins are active only
within a narrow pH range, 5-9. This is a result of the effects of pH on a combination
of factors:
(a) __________________,
(b) __________________,
(c) __________________, and
(d) __________________ .
2. A (i) __________ is a substance that competes directly with the normal substrate
for an enzymatic binding site. Such an inhibitor resembles the (ii) ________ since it
specifically binds to the (iii) ________ but differs from it by being unreactive. (iv)
_________ is a competitive inhibitor of dihydrofolate reductase. (v)_________, which
chemically resembles succinate, is a competitive inhibitor of succinate
dehydrogenase.
3. Vertebrates express two globins: (i) ________, which is present mainly in (ii)
_______, (iii) ________, and endocrine tissues, and (iv) _______, which occurs in
most tissues. Neuroglobin protects neurons (nerve cells) from damage under
conditions of hypoxia by preventing reperfusion injury in much the same way as does
(v) ________ in muscle.
4. Certain small molecules such as (i) _______, (ii) ______, (iii) ______ and
(iv)______ coordinate to the sixth liganding position of Fe 2+ in (v) _______ and Mb
with a greater affinity than oxygen. This together with their similar binding to the
hemes of cytochromes accounts for their highly toxic nature.
5. (i) __________, exhibits both homotropic and heterotropic effects. The binding of
O2 to Hb results in a (ii) __________ effect since it increases hemoglobin’s O 2
affinity. (iii) _______, (iv) ________, H +, and (v) ________ are negative heterotropic
effectors of O2 binding to Hb because they decrease its affinity for O 2 (negative) and
are chemically different from O2 (heterotropic).
6. E. coli ATCase exhibits positive (i) ________ cooperative binding of its substrates,
(ii) _______ and (iii) ____________. ATCase is heterotropically inhibited by (iv)
__________, a pyrimidine nucleotide, and is heterotropically activated by (v)
_________, a purine nucleotide.
Section C: Answer ALL the Questions (5 marks each)
1. For a Michaelis-Menten reaction, k 1 = 5 x 107 s-1, k-1 = 2 x 104 s-1 and k2 = 4 x 102 s-
1
. Calculate KS and KM for this reaction. Does substrate binding achieve equilibrium
or the steady state?

2. Explain the detoxification of NO in the blood and muscle.

3. Show the steps involved in the conversion of the Michaelis-menten kinetics

equation to . What are drawbacks of this equation in terms of
determining the kinetic parameters’ KM and Vmax?

4. Using two examples illustrate and explain the term ‘zymogen’.

5. Explain mathematically, how the initial reaction velocity of an enzyme (V o) can be

expressed in terms of the free substrate concentration [S] and free enzyme
concentration [E].

6. Write out a well labelled reaction scheme that describes a bisubstrate reaction
occurring via a random mechanism.

Section D: Answer ANY TWO Questions (10 marks each)

1. Explain the use of the dehydrogenase system in the assay of a blood metabolite.
2. Using relevant examples identify the clinical use of five suicide inhibitors.
3. Explain the use of asparaginase in therapy.