Quiz 8 - PPP & PDC

Purpose

• NADPH Generation: Provides reducing power for biosynthetic reactions such as fatty acids and cholesterol synthesis, as well as for detoxification processes.

• Ribose-5-Phosphate Production: Essential for nucleotide synthesis, serving as a building block for DNA and RNA.

• Sugar Interconversion: Facilitates the interconversion of sugars containing 3 to 7 carbon atoms, critical for generating glycolytic intermediates.

Phases

A. Oxidative Phase (Steps 1-3)

1. Step 1: Conversion of Glucose-6-phosphate to 6-phosphoglucono-d-lactone

   • Enzyme: Glucose-6-phosphate dehydrogenase

   • Product: NADPH (one molecule produced)

2. Step 2: Conversion of 6-Phosphoglucono-d-lactone to 6-phosphogluconate

   • Enzyme: Lactonase

   • Reaction: Hydrolysis of the lactone ring takes place to form 6-phosphogluconate.

3. Step 3: Conversion of 6-Phosphogluconate to Ribulose-5-phosphate and CO2

   • Enzyme: 6-phosphogluconate dehydrogenase

   • Product: NADPH and carbon dioxide (CO2) are produced in this reaction.

B. Non-Oxidative Phase (Steps 4-8)

4. Step 4: Conversion of Ribulose-5-phosphate to Ribose-5-phosphate

   • Enzyme: Ribulose-5-P isomerase

   • Significance: Interconverts ribulose and ribose forms.

5. Step 5: Conversion of Ribulose-5-phosphate to Xylulose-5-phosphate

   • Enzyme: Phosphopentose epimerase

   • Mechanism: Involves epimerization at the C-3 position of the pentose sugar.

6. Step 6: (Note: Step 6 can be omitted as not mentioned in the transcript)

7. Step 7: Combined reaction

   • Components: Sedoheptulose-7-phosphate (S7P) + Glyceraldehyde-3-phosphate (G3P) → Erythrose-4-phosphate + Fructose-6-phosphate

   • Enzyme: Transaldolase

   • Process: A C7 fragment donates a 3-carbon unit to a C3 fragment.

8. Step 8: Combined reaction

   • Components: Xylulose-5-phosphate + Ribose-5-phosphate → Glyceraldehyde-3-phosphate + Sedoheptulose-7-phosphate

   • Enzyme: Transketolase with cofactor thiamine pyrophosphate (TPP)

   • Process: Transfer of 2-carbon units.

End Products of the Carbon Rearrangements

• Glyceraldehyde-3-phosphate (G3P): A 3-carbon intermediate of glycolysis.

• Fructose-6-phosphate (F6P): A 6-carbon sugar, also an intermediate in glycolysis.

Net Reaction

• Transketolase + Transaldolase:

  • $2 ext{C5} <br>ightleftharpoons ext{C6} + ext{C4}$

Pyruvate Dehydrogenase Complex (PDC)

Purpose

• Conversion: Pyruvate is converted to acetyl-CoA, enabling the link between glycolysis and the citric acid cycle (Krebs cycle).

• NADH Production: Generates NADH, which is used in electron transport for ATP production.

Overall Reaction

• Reaction:
  $ext{Pyruvate} + ext{CoA} + ext{NAD}^+ <br>ightarrow ext{Acetyl-CoA} + ext{CO}_2 + ext{NADH} + ext{H}^+$

Enzyme Components of PDC

1. E1 - Pyruvate Dehydrogenase

   • Function: Decarboxylates pyruvate.

   • Cofactor: Thiamine pyrophosphate (TPP).

2. E2 - Dihydrolipoyl Transacetylase

   • Function: Transfers acetyl group to CoA, facilitating the production of acetyl-CoA.

   • Cofactor: Lipoamide, linked to lysine.

3. E3 - Dihydrolipoyl Dehydrogenase

   • Function: Regenerates oxidized lipoamide.

   • Cofactor: Flavin adenine dinucleotide (FAD).

Electron Flow in PDC

• Sequence: The flow of electrons proceeds through:

  • TPP → Lipoamide → FAD → NAD⁺

Carbon Fate of Pyruvate in PDC

• C-1 (Carboxyl): Converted to CO2.

• C-2 (Carbonyl): Becomes the carbonyl of acetyl-CoA.

• C-3 (Methyl): Corresponds to the methyl unit in acetyl-CoA.

Vitamin Cofactors Required for PDC

• Thiamine (B1) → Required for synthesizing TPP, essential for E1.

• Riboflavin (B2) → Necessary for FAD, a cofactor in E3.

• Niacin (B3) → Source of NAD⁺, crucial for overall reaction.

• Pantothenic Acid (B5) → Required for CoA formation.

• Lipoic Acid → Acts as lipoamide, cofactor for E2.

Practice Questions

1. Which PPP step is mechanistically identical to glycolysis step 5?

   • Answer: Step 5 of the PPP (Ribulose-5-phosphate → Xylulose-5-phosphate) is mechanistically similar to glycolysis step 5 (which involves isomerization of dihydroxyacetone phosphate to glyceraldehyde-3-phosphate).

2. What is the cofactor for transketolase?

   • Answer: Thiamine pyrophosphate (TPP).

3. Where does the carboxyl carbon of pyruvate end up in PDC?

   • Answer: It is released as carbon dioxide (CO2).

4. What enzyme in PDC contains a lipoyl group attached to lysine?

   • Answer: Dihydrolipoyl transacetylase (E2).

5. Net reaction of transketolase + transaldolase steps?

   • Answer: $2 ext{C5} <br>ightleftharpoons ext{C6} + ext{C4}$