Key to Exercise on Pyruvate Dehydrogenase and the Citric Acid Cycle |
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Pyruvate Dehydrogenase (PDH)
1.
Table 1: Enzymes and Coenzymes of the Pyruvate Dehydrogenase Complex |
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Enzyme |
Coenzyme |
Function for PDH |
E1 – P |
TPP | Removes the carboxyl group from pyruvate |
E2–P |
lipoate | Transfers the hydroxyethyl group to CoA-SH |
E2–C |
CoA-SH | An acyl carrier that accepts and carries acetate |
E3–P |
FAD | Oxidizes dihydrolipoate, regenerating lipoate |
E3–C |
NAD+ | Oxidizes FADH2, regnerating FAD |
2. a. NADH, AcCoA, ATP (the top);
b. ATP/AMP
c. NADH/NAD+
d. NADH/NAD+ and AcCoA/CoA-SH
3. Contrast covalent regulation of PDH with covalent regulation of PFK-2/FBPase-2 by filling in the table below.
Table 2: Covalent Regulation of PDH and PFK-2/FBPase-2 |
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Regulated enzyme |
PDH |
PFK-2/FBPase-2 |
What kinase modifies it? |
protein kinase (PDH subunit) |
PKA |
What is the effect of modification? |
PDH is less active. |
PFK-2 is inactive and FBPase-2 is active. |
What pathway is less active as a result? |
citric acid cycle |
glycolysis |
What activates the kinase? |
high [ATP] |
cAMP |
4. Energy is stored instead of accumulating more NADH and ATP.
5. 2 ATP (glycolysis) + 4 NADH (2 from glycolysis + 2 from PDH)
6. At the end of glycolysis, the number of NADH that must be recycled is equal to the number of pyruvates. After PDH, the number of NADH that must be recycled is greater than the number of AcCoA available.
7. Products = CO2, AcCoA, and NADH.
Citric Acid Cycle
1. The reactions catalyzed by citrate synthase (first) and aconitase.
CoA is the only coenzyme required.
2. The reactions catalyzed by isocitrate dehydrogenase (first) and α-ketoglutarate dehydrogenase. Both require NAD+.
3. Both enzymes are complexes that actually involve three enzymes, and both require the coenzymes TPP, lipoate, CoA-SH, FAD, and NAD+.
a-b. 
c. Change the CH3 to either R or to CH2–COO− .
4. The reaction catalyzed by succinyl-CoA synthetase.
a. succinyl-CoA + Pi + GDP ↔ succinate + CoA-SH + GTP
b. 1. succinyl-CoA + Pi ↔ succinyl-phosphate + CoA-SH
2. succinyl-phosphate + enzyme-His ↔ succinate + enzyme-His-phosphate
3. enzyme-His-phosphate + GDP ↔ enzyme-His + GTP
5. The reactions catalyzed by succinate dehydrogenase, fumarase, and malate dehydrogenase. Succinate dehydrogenase requires FAD, and malate dehydrogenase requires NAD+. 
6. a. malate + NAD+ ↔ oxaloacetate + NADH + H+ ; b. − 0.154 V; c. + 29.7 kJ/mole.
7.
The three missing molecules are α-ketoglutarate, fumarate, and oxaloacetate: 
8. Glycolysis produces 2 ATP + 2 NADH; PDH produces 2 NADH (1 per pyruvate);
citric acid cycle produces 6 NADH, 2 GTP (= 2 ATP), and 2 FADH2 .
Total per glucose = 10 NADH + 2 FADH2 + 4 ATP.