Selected Solutions to End of Chapter 16 Problems
Class 1
1. Here is the answer, but be sure you know these chemical structures of substrates and products,
but not necessarily the enzyme cofactors.
Citrate Synthetase: a. Acetyl-CoA + Oxaloacetate + H2O  Citrate + CoA
b. CoA.
c. condensation.
Aconitase: a. Citrate  Isocitrate
b. none.
c. isomerization
Isocitrate Dehydrogenase: a. Isocitrate + NAD+  α-ketoglutarate + CO2 + NADH
b. NAD+
c. oxidative decarboxylation.
α-Ketoglutarate Dehydrogenase: a. α-ketoglutarate + NAD+ + CoA  succinyl-CoA + CO2 + NADH
b. NAD+, CoA, thiamin pyrophosphate. c. oxidative decarboxylation
Succinyl-CoA synthetase: a. succinyl-CoA + Pi + GDP  succinate + CoA + GTP
b. CoA.
c. substrate level phosphorylation
Succinate Dehydrogenase: a. Succinate + FAD  Fumarate + FADH2 b. FAD. c. oxidation.
Fumarase: a. Fumarate + H2O  malate
b. None.
c. hydration.
Malate Dehydrogenase: a. Malate + NAD+  Oxaloacetate + NADH b. NAD+ c. oxidation
Net: Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O  2 CO2 + CoA + 3NADH + FADH2 + GTP
2. Now lets get this major part of Central Metabolism all Summed Up:
Glycolysis:
Glucose + 2 ADP + 2 Pi + 2 NAD+  2 pyruvate + 2 ATP + 2 NADH
Pyruvate Dehydrogenase:
2 Pyruvate + 2 NAD+ + 2 CoA  2 acetyl-CoA + 2 CO2 + 2 NADH
Citric Acid Cycle:
2 Acetyl-CoA + 6 NAD+ + 2 FAD + 2 Pi  2 CoA + 4 CO2 + 6 NADH + 2 FADH2 + 2 GTP
OVERALL: (converting GTPs to ATPs)
Glucose + 4 ADP + 4 Pi + 10 NAD+ + 2 FAD  6 CO2 + 4 ATP + 10 NADH + 2 FADH2
3. Oxidation or Reduction?
a. oxidation (alcohol to carbonyl).
b. oxidation (carbonyl to carboxyl)
c. reduction (CO2 to carboxyl)
d. reduction (carboxyl to carbonyl)
e. oxidation (alcohol to carbonyl)
f. oxidation (methyl to carboxyl)
g. oxidation (ane to ene)
h. oxidation (carboxyl to CO2)
4. Compare metabolism of glucose (C6H12O6) and hexanoic acid (C6H14O2). This is a sugar
compared to a short chain fatty acid (we will see this β-oxidation pathway in Chapter 17).
Answer: Consider which one is the most reduced, it has more electrons to give though oxidation
reactions (energy producing). It should be obvious that it is hexanoic acid. Later we will see that the
amount of energy produced from fatty acids (β-oxidation and respiratory electron transport) is far
more than through glycolysis + pyruvate dehydrogenase + citric acid cycle + respiratory electron
transport)…all because of the redox state of fatty acids (mostly methyls) vs sugars (mostly alcohols).
5. Use of NADH and NAD+ in some of the reactions in Problem 3 or others. (Note when writing
NADH you realize it is really NADH + H+), this way it makes it shorter to just write NADH.)
a. oxidized: ethanol + NAD+  acetaldehyde + NADH
b. reduced: 1,3 bisphosphoglycerate + NADH  glyceraldehyde-3-phosphate + NAD+ + Pi
c. unchanged: pyruvate  acetyl-CoA + CO2 (NAD+ not used in this reaction, all electron on
products).
d. oxidized: puryvate + NAD+  acetate + CO2 + NADH
e. reduced: oxaloacetate + NADH  malate + NAD+
f. unchanged: acetoacetate + H+  acetone + CO2
6. Describe the co-factor reactions in pyruvate dehydrogenase (enzymes 1, 2 and 3):
TPP – thiamine pyrophposhpate ring adds to and stabilizes the carbanion.
Lipoic acid – oxidizes pyruvate to acetate and activates CoA as a thioester.
CoA-SH - becomes the thioester.
FAD – oxidizes reduced lipoic acid.
NAD+ - oxidizes FADH2 to FAD becoming NADH +H+.
7. Thiamine deficiency patients (part of the classroom work) have high levels of pyruvate in blood.
Why? It’s because some of the pyruvate can not be converted to acetyl-CoA and is exported from
cells to blood. Note that this also makes α-ketoglutarate dehydrogenase much less active because it
has the same mechanism as pyruvate dehydrogenase.
8. Isocitrate dehydrogenase is an oxidative decarboxylase using either NAD+ or NADP+ as the
electron acceptor. But its mechanism does not use TPP, lipoate, FAD, CoA, etc. like pyruvate DH or
α-ketoglutarate DH.
9. This problem integrates the other function of the Citric Acid Cycle (CAC): CAC intermediates are
used as the starting molecules for several amino acid biosynthesis and pyrole (heme) biosynthesis.
This drains CAC intermediates away which could slow down CAC (CAC can only move if there is
sufficient oxaloacetate OAA). So when OAA or malate were added to pigeon muscle preps, their
oxygen uptake increased because the CAC got moving faster (having α-ketoglutarate, succinyl-CoA,
and OAA syphoned off for biosynthesis, reduces the amount of OAA to be the acceptor for acetylCoA). We will cover here the anaplerotic reactions that replace the CAC intermediates so the OAA
concentration is increased and stabilized.
Because CAC is in the mitrochondrion, most all of the NADH and FADH2 produced by the CAC is
immediately used by the electron transport system (cytochromes) to generate the proton motive force
using the energy of the electrons in NADH and FADH2 as they electron move through to eventually
reduce oxygen to water. Thus mitochondrial oxygen uptake is a measure of electron transport
system supplied by CAC and a bit by Glycolysis.
15. Malonate was one, if not the first, enzyme inhibitor whose kinetics were studied. Compare
malonate to succinate:
_OOC-CH -COO2
to
-OOC-CH -CH -COO2
2
What type of inhibition would you suspect?
a. competitive. b. uncompetitive. or c. mixed?
18. Another follow the carbon labeling experiment through Glycolysis and CAC. Where is 1-14Cglucose in:
a. Fructose-1,6-bisphosphate?
Carbon 1
b. Glyceraldehyde-3-phosphate? Carbon 3
c. Phosphoenolpyruvate? Carbon 3
d. Acetyl-CoA? Carbon 2 (the methyl carbon)
e. Citrate?
Carbon 2
f. α-ketoglutyrate? Carbon 4
g. Oxaloacetate? Carbon 2 and 3 (half and half distribution).
19. We have touched on this already: thiamine deficiency…the disease Beri Beri, and will do more in
class. Thiamine is obviously needed for the synthesis of thiamine pyrophosphate used in the two
dehydrogenases for pyruvate and α-ketoglutarate.
30. Coupling of CAC and electron transport (respiration). When oxygen is low or if electron transport
is inhibited, the CAC slows or stops. Why? This is easy to think about, without electron transport
taking electrons from NADH and FADH2 the concentration of these cofactors increases with the
decrease of NAD+ and FAD (without which the CAC cannot function). High [NADH] inhibits reactions
leading into and in the first part of the CAC….flow into the next question.
31. Additionally, the reduced forms are inhibitors of the control points in CAC: pyruvate
dehydrogenase, citrate synthase, and α-ketoglutartate dehydrogenase.
32. Lets get to the actual ΔG of citrate synthase in the mitochondria where [acetyl-CoA] = 1µM,
[OAA] = 1 µM, [citrate] is 220 µM, and [CoA] = 65 µM. The ΔG o’ = -32.2 kJ/mole which indicates it
should go forward fairly well.
Remember ΔG =
ΔGo’
+ RT lnQ
Q=
[citrate][CoA]
[OAA][acetyl−CoA]
Q = 1.48 x104
=
(220 x10−6 )(65 x 10−6 )
(1 x 10−6 )(1 x10 −6 )
so lnQ = 9.60
ΔG = -32.2 kJ/mole + 2.48 kJ/mole (9.6) = -32.2 kJ/mole + 23.8 kJ/mole
ΔG = -8.4 kJ/mole therefore still exothermic (=exergonic).