BCH 4053 Mini-Exam NAME _____________________
August 2, 1996
Answer either question 1 or 2
Points
(25) 1. Give the step-wise mechanism catalyzed by the
enzyme pyruvate dehydrogenase, showing the
partial structure of all the enzyme-bound
intermediates. Identify the protein components
catalyzing each step and the coenzymes involved
as either cosubstrates or prosthetic groups.
(25) 2. Glucagon is a peptide hormone produced in
response to low blood glucose. Binding of
glucagon to a receptor in the membrane of the
liver causes a number of reactions to occur in
the liver which result in covalent modification of several enzymes.
Diagram the series of intermediate steps between glucagon binding and
enzyme covalent modification, indicate which enzymes we have
discussed that are modified and how the modification changes their
activity, and explain the overall consequence of these modifications
on the metabolism of liver glycogen. How is the effect of glucagon
reversed when blood glucose levels increase? How is glycogen
breadown in liver stimulated in the absence of hormonal stimulation?
Answer either question 3 or 4
(25) 3. Starting with [1-14C]-glucose, trace the radioactivity through
glycolysis and into the oxaloacetate formed after the first turn of
the citric acid cycle. To do so, give the structure of each
intermediate in the pathway, and circle the carbon atom(s) of each
intermediate which are derived from [1-14C]-glucose.
(25) 4. To demonstrate how the pentose-phosphate pathway can convert pentoses
to hexoses, show a sequence of steps by which ribose-5-phosphate can
be converted to fructose-6-phosphate. Show the structures of the
intermediates and the names of the enzymes catalyzing each step.
(You may only use intermediates derived from ribose-5-phosphate, but
it is not necessary to convert the ribose-5-phosphate completely into
fructose-6-phosphate, i.e. you might have other byproducts from your
pathway).
Answer only 2 of questions 5-8
(25) 5. One of the proton dissociations of hemoglobin occurs near neutral pH.
This dissociation is affected by the binding of oxygen to Hb. Assume
that the dissociations for oxyhemoglobin (HbO2) and deoxyhemoglobin
(Hb) can be treated as a simple monoprotic acids as follows:
HHbO2 þ H+ + HbO2 , pK = 6.6; and HHb þ H+ + Hb , pK = 8.2;
(a) Assume that blood is 2.5 mM in hemoglobin. At the plasma pH of
7.4, and the hemoglobin in the lungs fully oxygenated, what are
the concentrations of the protonated (HHbO2) and unprotonated
(HbO2) forms?
(b) In tissues, the pressure of O2 drops, and the oxygen dissociates
from hemoglobin. The deoxyhemoglobin is a stronger base, and
must be titrated with protons for the pH to remain constant at
7.4. Assuming the oxygen were completely dissociated, how many
protons would be required per liter to produce the proper Hb/HHb
ratio for pH 7.4?
(25) 6. Given the following data for a peptidase hydrolysing the dipeptide
gly.gly:
S (mM) 1.5 2.0 3.0 4.0 8.0 16.0
v (æmol/min) 0.21 0.24 0.28 0.33 0.40 0.45
Plot these data in both the Lineweaver-Burke plot and the Eadie-
Hofstee plot, and determine Vm and Km from both graphs. Show your
work below, but obtain a piece of graph paper to use for the graphs.
Use care in selecting your axes. Be sure to put your name on the
paper, and staple it to the test.
(25) 7. Aldolase catalyses the following reaction of glycolysis:
fructose-1,6- ----> dihydroxyacetone + glyceraldehyde-3-
diphosphate phosphate phosphate
(a) Give the structures of the reactant and products of this
reaction.
(b) delta Go' for this reaction is +23.9 kJ mol-1. Calculate K',
the equilibrium constant.
(R = 8.314x10-3 kJ-mol-1-deg-1; T = 310 K; ln x = 2.3log x.)
(c) Calculate delta G' for the reaction when fructose-1,6-
diphosphate is 1.0x10-4 M, dihydroxyacetone phosphate is 4.0x10-
5 M, and glyceraldehyde-3-phosphate is 2.5 x 10-6 M.
(25) 8. Give the structure of 6 of the following:
(a) phenylalanine (b) N-acetylgalactosamine
(c) phosphatidyl choline (d) histidine
(e) GTP (f) biotin
(g) thymine (h) linoleic acid