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Bio301 Biochemistry I
Midterm Examination II, December 4, 2006
Question 1 (10 points). Answer the following with true or false. If false explain why.
a) The initial rate of an enzyme-catalyzed reaction is independent of substrate
concentration.
b) At saturating levels of substrate, the rate of an enzyme catalyzed reaction is
proportional to the enzyme concentration.
c) The Michaelis constant KM equals the substrate concentration at which v=1/2Vmax.
d) The KM for a regulatory enzyme varies with enzyme concentration.
e) If enough substrate is added, the normal Vmax of an enzyme-catalyzed reaction can be
attained even in the presence of a noncompetitive inhibitor.
f) The sigmoidal shape of the v versus [S] curve for some regulatory enzymes indicates
that the affinity of the enzyme for substrate decreases as the substrate concentration is
increased.
Question 2 (5 points). If the standard free-energy change (Go) for a reaction is zero,
which of the following statements about the reaction are true?
a) The entropy (S) of the reaction is zero.
b) The enthalpy (H) of the reaction is zero.
c) The concentration of the reactants and products are all 1 M at equilibrium.
d) The equilibrium constant for the reaction is 1.0
e) The reaction is at equilibrium.
Question 3 (10 points). Calculate the Go’ for a reaction that correspond to K’eq of
1.5x10-4 at 37oC. (The value of the gas constant R is1.98x10-3 kcal/mol.oK)
Question 4 (20 points). The KM of a certain enzyme is 1.0x10-5M in a reaction that is
described by Michaelis-Menten kinetics. At a substrate concentration of 0.10 M, the
initial rate of the reaction is 37 moles/min for a certain concentration of enzyme.
However, you observe that at a lower substrate concentration of 0.010 M the initial
reaction rate remains 37 moles/min.
a) Using numerical calculations, show why this tenfold reduction in substrate
concentration does not alter the initial reaction rate.
b) Calculate v as a fraction of Vmax for [S]=0.20KM, 0.50KM, 1.0KM, 2.0KM, 4.0KM,
and 10KM.
c) From the results in (b), sketch the curve relating v/vmax to [S]/KM. What is the
best range of [S] to use in determining KM or investigating the dependence of v on
[S]?
Question 5 (20 points). Five reaction mixturels containing equal concentrations of an
enzyme are made up to the substrate concentrations indicated below. The experiment is
repeated with an enzyme inhibitor present at a concentration of 2.2x10-4M in each
reaction mixture. The enzyme concentration is 2x10-6 M.Initial rates at various substrate
concentration in the presence and absence of an inhibitor for an enzyme catalyzed
reaction is as follows:
[S] (moles/liter)
v (moles/min)
v (moles/min)
without inhibitor
with inhibitor
-4
1.0 x10
28
18
1.5x10-4
36
24
-4
2.0x10
43
30
5.0x10-4
63
51
-4
7.5x10
74
63
Make a Eadie-Hofstee plot (v versus v/[S]) from the data above.
a) Graphically determine KM for the substrate, KI for the inhibitor, and Vmax in the
absence and in the presence of inhibitor.
b) Determine the turnover number for the enzyme in the absence of the inhibitor.
c) Is the inhibitor competitive or noncompetitive?
Question 6 (10 points). Fill in the blanks:
a) Under normal conditions, the heme iron in myoglobin and hemoglobin is in the
____________ oxidation state.
b) The ability of myoglobin to bind oxygen depends on the presence of a bound
prosthetic group called _____________.
c) In hemoglobin, the iron of the heme is bonded to the four nitrogens of porphyrin and to
the proximal ______________ residue of the globin chain.
d) The binding of 2-3-bisphosphogycerate to hemoglobin ____________ (increases,
decreases) its affinity of oxygen binding.
e) Carbon dioxide reacts with the amino terminal groups of hemoglobin to form
carbamate groups, which carry a ______________ charge.
Question 7 (5 points). A mutant hemoglobin gene has been isolated and shown to
encode an abnormal a chanin by endonuclease digestion. The electrophoretic mobility of
the mutant hemoglobin and its affinity for O2 are essentially the same as for hemoglobin
A. With regard to these observations, which of the following statements are correct?
a) The mutation is most likely at the active site.
b) There has been a substitution of a basic amino acid residue for an acidic one.
c) There has been a substitution of an acidic amino acid residue for a neutral one.
d) There has been a substitution that involves a homologous residue in terms odf
charge.
e) The mutation is most likely on the surface of the hemoglobin.
Question 8 (20 points). The partial pressure of oxygen in the venous blood of a human at
rest at sea level is approximately 40 torrs. (Remember the Hill coefficient is 2.8 for
Hemoglobin, and the P50, the pressure at which half of the sites on Hemoglobin are filled
with oxygen, is 26 torr.
a) Calculate the fractional saturation of hemoglobin under these conditions.
b) What percentage of the oxygen originally bound to hemoglobiln in the alveoli
remains unreleased?