Download (18 pts) Pyruvate can be converted to a variety of othe

MCB 11
Spring 2005
May 3, 2005
M. D. Alper
Third Midterm Examination-Key
(18 pts) Pyruvate can be converted to a variety of other molecules.
a. Name three molecules into which pyruvate can be converted either directly (one step)
or using two steps.
Alanine, Acetyl CoA, Ethanol, Carbon Dioxide, Lactate (Lactic Acid)
b. The environment in which an organism lives, or the type of organism itself, can have a
significant effect on its metabolism. Take two of the molecules you named in part a
and describe how the cell’s “choice” of which one of them is made depends either on
the environment in which the cell lives or on which organism it is.
Ethanol vs Lactate—is it a yeast cell or a bacterial cell or a muscle cell
Ethanol or Lactate vs Acetyl CoA—is it growing anaerobically (the former) or aerobically
Acetyl CoA vs Alanine—does it need the energy from the Krebs Cycle or does it need to make
amino acids or protein/
c. Your roommate thinks that any given molecule can be converted to only one other
molecule. How would you explain the fact that a molecule such as pyruvate could be
converted to any one of several different products. (We are not interested here in the
question of what controls which of the several possible products is made, just how it is
possible to make those different products from the same starting molecule.)
The molecule could undergo any of a variety of different chemical reactions. Each would lead
to a different product. Starting with pyruvate, for example, a carbon-carbon bond break
would lead to ethanol and carbon dioxide. Reduction would lead to lactate.
2. (14 pts) Electron transport has some fundamental differences from glycolysis.
a. In electron transport, a cytochrome interacts with oxygen, which is converted to water.
Does the cell then have to make more cytochrome to have that process repeated?
Explain.
No. The cytochrome is not destroyed or used up. In this reaction it has simply transferred its
electrons (with protons accompanying from the solution) to oxygen. It can then immediately
accept electrons from the carrier before it in the chain and repeat the process.
b. In glycolysis, fructose-6-phosphate interacts with ATP, which is converted to ADP.
Does the cell have to make more fructose-6-phosphate to have that process repeated?
Explain.
Yes it does. The fructose-6-phosphate is converted to what we will call here for clarity, fructose
di-phosphate. This is converted to the next intermediate in the pathway. Thus there is no more
fructose-6-phosphate and more must be made (from glucose).
3. (14 pts) ATP is produced in oxidative phosphorylation and in glycolysis. What is the
immediate energy source for the production of ATP in each of these processes. Explain.
In oxidative phosphorylation, the immediate source of energy is the proton gradient, with a
higher concentration of protons on the outside of the membrane than the inside. The process
of protons moving across the membrane, which will lead to the equilibrium state of equal
concentrations on both sides has a negative free energy change, that it, is releases energy that
can be used—here to make ATP.
In glycolysis, ATP is produced when one of the intermediates in the pathway is converted to
another one at a much lower energy. Thus, again energy is released and it can be captured in
the production of ATP.
4. (8 pts) A student who has been generally sickly for her entire life, is given a full metabolic
check-up and it is determined that reactions that depend on ingestion of normal levels of
niacin do not seem to be going at the normal rate. She then goes out and spends a great deal
of money buying the purest, finest tablets of niacin she can find. She takes twice the
recommended daily allowance but still feels tired and without energy. Further medical
examination of her cells shows that the levels of niacin and the various enzymes involved in
these reactions are within normal range. What do you think might be wrong with her?
Niacin itself is not active in the oxidation reactions in which it participates. It must be
converted to its coenzyme form, NAD. Clearly here, the chemical reaction that converts niacin
to NAD is not functioning well. (After we complete our discussion of enzymes—although this
answer is acceptable now) we could also propose that the enzyme involved in the use of NAD in
oxidation reactions has somehow been altered so that it does not recognize NAD and cannot use
it.)
5. (10 pts) Acetyl CoA that has been made from radioactive carbon atoms is added to a cell that
has a very active Krebs cycle. A bit later, all the molecules in the cell are examined, and those
that have radioactive carbon are isolated and identified. Other than the Krebs cycle
intermediates themselves, what sorts of molecules do you think are now radioactive. Explain
your answer.
Virtually all molecules of the cell will be labeled. Clearly all the intermediates of the Krebs
Cycle will be labeled because they were produced through chemical reactions in the steps
from Acetyl CoA. For the same reason carbon dioxide will also be labeled. Also, from the
various intermediates in the Krebs Cycle, we make amino acids, we make the components of
genes, we make fatty acids and lipids, we make the heme etc etc.
6.
(36 pts) In glycolysis, glucose-6-phosphate is converted to fructose-6-phosphate. Only the
appropriate enzyme is required.
a. What type of reaction is this? Explain.
Isomerization. Each molecule has the same number of each type of atom. Only the
arrangement is altered.
b. Alternatively, a different reaction can occur in which glucose-6-phosphate is converted
to the following molecule.
What type of reaction is this? Why do you think so?
This is an oxidation reaction. The aldehyde (carbon 1) has been oxidized to an acid.
c. What molecule other than glucose-6-phosphate and the enzyme is required for this
reaction? Explain
NAD is required as it is for all oxidation reactions.
d. Later on in this pathway, the following reaction takes place in which the two
molecules on the right of the arrow are converted to the two molecules to the left of the
arrow:
+
Despite the apparent complexity, only one fundamental type of reaction is occurring
here, although it is occurring twice. Name that type of reaction and tell why you think
it is occurring here.
This is a carbon-carbon bond breaking or making reaction. In fact both are occurring. The top
two carbons of the “ribulose” are broken off (giving the 3 carbon “glyceraldehydes”) and they
are then reattached to the 5 carbon “xylulose” to make the 7 carbon “sedoheptulose”.
e. This type of reaction occurs multiple times in glycolysis and the Krebs cycle. Identify
a single example by naming one of the products of the reaction and briefly explain
your reasoning.
All reactions that break a carbon-carbon bond to produce carbon dioxide. Also, the reaction
that splits the six carbon fructose to the two three carbon molecules in glycolysis.
f. What class of molecules are these four a member of? What two features that we see in
examining these four that relate to the diversity of the members of this class?
Carbohydrates. They differ from one another in length, here from 3 carbons up to 7 carbons.
They are also modified, here with a phosphate group attached.