chem 454-tca (and glycolysis) problems 2-15pts name key . 1. in liver, vmax for fructose bisphosphatase is 3-4 times higher

Chem 454-TCA (and glycolysis) Problems 2-15pts
NAME KEY .
1.
In Liver, Vmax for fructose bisphosphatase is 3-4 times higher
than Vmax for Phosphofructokinase, whereas in muscle it is only
about 10% of that of Phosphofructokinase. Explain this difference.
Greater Vmax allows greater flux through a pathway. Remember the liver
is the only organ that contributes a significant amount of glucose to
the blood. Fructose bisphosphatase is on the pathway to glucose
synthesis and release and so much more flux in that direction is
needed by the liver. PFK on the other hand is used for glucose
catabolism which is less active in the liver and MUCH more active in
the muscle. The muscle has very little need for the gluconeogenesis
pathway.
2.
In muscle, lactate dehydrogenase preferentially produces lactate
from pyruvate, wheras in the heart it preferentially synthesizes
pyruvate from lactate. Explain how this is possible (without
violating any thermodynamic laws!!).
Recall LDH reaction: Pyruvate + NADH  lactate + NAD+
So Keq = [lactate] [NAD+]/ [pyruvate] [NADH], thus [NAD+]/[NADH] ratio
is very important in determining the direction of this reaction. In
the muscle, high NADH from anaerobic metabolism pushes the equilibrium
to lactate whereas in heart which is strictly aerobic, higher NAD+
levels push the equilibrium toward pyruvate.
3.
The conversion of citrate to isocitrate occurs by a
dehydration/rehydration reaction with cis-aconitate as an
intermediate. At equlibrium, a mixture of citrate, aconitate and
isocitrate contains about 90, 4, and 6% of these acids
respectively.
a.
Why must citrate be converted to isocitrate before subsequent
oxidation? (hint: consider the result of oxidizing citrate).
You cannot further oxidize the 3o –OH group on citrate wheras you can
oxidize the 2o –OH on isocitrate.
b.
Using the above data what are the equilibrium constants and
standard free energy changes for each of the 2 steps (ctirate
aconitate isocitrate)? For the overall process ?
Keq = [Aconitate]/[citrate]= 4/90= 4.44 * 10-2 , DGo’= -RTlnKeq= +1.84
kcal/mol
Similarly , aconitate/isocitrate DGo’=- 0.24 kcal/mol
So overall citrateisocitrate = DGo’=+1.60 kcal/mol
c.
With the answer to (b) in mind, how can the citric acid cycle
proceed under cellular conditions?
Cellular conditions are not standard nor equilibrium conditions and
forward flux will proceed as [citrate] rises. Later favorable
reactions (isocitrate a-KG) will also drive the overall reactions by
removing product effectively.
4.
Oysters and some other mollusks live their entire adult lives
permanently cemented to an object on the sea floor (talk about a
dull life!). This means that sometimes there will be inadequate
oxygen for aerobic life and they will have to survive as
facultative anaerobes. When oysters are deprived of oxygen, they
accumulate and secrete succinate. Even though the TCA cannot
operate as a cycle in the absence of oxygen (why?), the reactions
can be exploited in a way that maintains redox balance. The
4-carbon TCA reactions can run in reverse from oxaloacetate to
succinate.. This produces NAD+ and FAD. Simultaneously the cycle
runs forward from citrate to succinate. producing two NADH.
Assuming the oysters have a steady supply of oxaloacetate (from
amino acids), how much energy could they derive from this process
(per “cycle”)?
One ATP “equivalent” is generated by succinyl CoA synthetase. The NADH
used cancels the NADH produced and the second NADH can reduce FAD via
the electron transport system (and “reverse” electron transport by
NADH oxidation by NADH-Q oxidoreductase? QH2FAD). This may produce
another ATP in the proton gradient formed. The end product would be
succinate.
5.
Some organisms can grow using ethanol as their sole carbon source
(and I believe I have spotted them on Water Street). Propose a
pathway for the utilization of this two carbon compound. The
pathway(s) should convert ethanol into one or more molecules that
can be used for energy generation and as biosynthetic precursors.
Ethanol can be converted to acetate (alcohol and aldehyde
dehydrogenase) and then to Acetyl CoA. Using the glyoxylate cycle, net
production of oxaloacetate and thus other TCA cycle intermediates is
possible which can be drawn off for synthesis of amino acids, sugars,
nucleic acids, fats, etc. Complete oxidation of Acetyl CoA can
generate energy aerobically by TCA and ETS.

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