L11- Pyruvate Metabolism

Question 1

LO1: Describe four enzymatic reactions that utilize pyruvate and indicate the subcellular localization of each

Answer 1

1. Oxaloacetate synthesis by pyruvate carboxylase in the mitochondria
2. Lactate by lactate dehydrogenase in the cytosol
3. Alanine by alanine aminotransferase in the cytosol
4. Acetyl co-A by pyruvate dehydrogenase complex in the mitochondria

Question 2

LO2: Predict the most likely pathway of pyruvate utilization under fed vs. fasted conditions in muscle vs. liver

Answer 2

FED

• utilization to make acetyl co-A by PDH complex is inhibited (by phosphorylation via PDH kinase)
• utilization to make oxaloacetate by pyruvate carboxylation is inhibited
• utilization to make lactate by LDH is inhibited
• utilization to make alanine by alanine aminotransferase inhibited

FASTED

• utilization to make acetyl co-A by PDH complex is activated (by dephosphorylation via PDH phosphatase)
• utilization to make oxaloacetate by pyruvate carboxylation is activated
• utilized to make lactate by LDH is activated
• utilization to make alanine by alanine aminotransferase is activated

Question 3

LO3: In what major tissues does anaerobic glycolysis occur, and how does the energy state of the cell determine the switch for aerobic to anaerobic glycolysis?

Answer 3

ANAEROBIC GLYCOLYSIS

• skeletal muscle (when stressed/exercising)
• red blood cells (no mitochondria, so no oxidative respiration)

-switch occurs in other tissues when oxygen levels are low or energy state of cell is low (low glucose, low glycogen)

Question 4

LO4: What reaction/enzyme links glycolysis to the TCA cycle? (reactants and products)

Enzymes in complex?
Coenzymes?
Cofactor?

Answer 4

-generation of acetyl-coA, CO2, and NADH from pyruvate, CoASH, and NAD+ by PDH complex

E1, E2, E3=PDH COMPLEX
E1: pyruvate dehydrogenase
E2: transacetylase
E3: dihydrolipoyl dehydrogenase

COENZYMES
Thiamine pyrophosphate
Lipoic acid
CoASH
NAD+
FAD

COFACTOR
Thiamin

Question 5

LO4: List the subcellular localization of the enzymes that link glycolysis to the TCA cycle

Answer 5

Mitochondrial matrix

Question 6

LO5: How are pyruvate dehydrogenase and pyruvate carbozylase regulated/how is this related to the energy state of the cell?

Answer 6

PDH INHIBITION: when energy state of cell is high

• negative feedback inhibition by products (acetyl-coA and NADH)
• PDH kinase inhibits PDH by phosphorylating it
  - PDH kinase activated allosterically when energy state is high (increase in ATP, acetyl-coA, NADH)
  - PDH kinase inhibited by accumulation of pyruvate

PDH ACTIVATION: when energy state of cell is low

• PDH phosphatase activates PDH by dephosphorylating it
  - PDH phosphatase activated by insulin, Ca+2 (especially upon contraction)

PC ACTIVATION: by acetyl-coA to induce gluconeogenesis

Question 7

LO6: Why would a thiamin (LO10) or biotin deficiency result in lactic acidosis?

Answer 7

THIAMIN DEFICIENCY

• thiamin is an essential co-factor of PDH complex; in a deficiency pyruvate won’t be converted into acetyl-coA, CO2 and NADH and will be diverted to lactate formation instead
  - beri-beri (when diet consists of primarily polished rice)
  - Wernicke-Korsakoff (excess ethanol consumption inhibits absorption of thiamine)

BIOTIN DEFICIENCY
-biotin is an essential co-factor of pyruvate carboxylase (carries CO2); in a deficiency pyruvate won’t be converted to oxaloacetate and will be diverted to lactate formation instead

Question 8

LO7: Explain why blood alanine levels would be elevated in a person with pyruvate carboxylase deficiency

Answer 8

• pyruvate carboxylase converts pyruvate to oxaloacetate
• if deficient, pyruvate will be converted to acetyl-coA, lactate, and alanine instead
  - equilibrium will shift to make more product due to excess pyruvate levels

Question 9

LO8: Compare/contrast acquired (LO10) vs. genetic deficiencies in PDH complex and their treatments

Answer 9

ACQUIRED CAUSES
Beri-beri: increase thiamin in diet
Wernicke Korsakoff: increase thiamin in diet and stop drinking
Arsenic poisoning: decontaminate/treat side effects, can use chelators

GENETIC CAUSES
-mutations in genes for any of the PDH enzymes: avoid large glucose loads

Question 10

LO9: Describe why arsenic is toxic/negatively affects the PDH complex

Answer 10

Arsenic’s toxic form arsenite can form a non-functional complex with lipoic acid, making any enzyme that requires lipoic acid non-functional

• E2 subunit of PDH complex
• alpha-ketoglutarate DH and branched chain keto acid-DH complexes