KEY FATES OF PYRUVATE

1. Acetyl CoA: substrate for citric acid cycle and fatty acid synthesis
2. Oxaloacetate: intermediate in CAC and substrate for gluconeogenesis
3. Lactate: produced by eukaryotes in absence of oxygen
4. Ethanol: produced by yeast and some bacteria (including intestinal flora) in absence of oxygen.

AEROBIC CONDITIONS

1. Cellular respiration: Pyruvate converts to acetyl CoA

• Fed conditions (glucose abundant)
• Occurs in mitochondrial matrix
• Pyruvate dehydrogenase complex
• Irreversible reaction: produces 1 CO2 and 1NADH
• Acetyl CoA enters the citric acid cycle and oxidative phosphorylation
• Final product is ATP

2. Gluconeogenesis: Pyruvate converts to oxaloacetate

• Fasting conditions (glucose in demand)
• Occurs in liver (minor process in kidneys): mitochondrial matrix
• Pyruvate carboxylase
• Irreversible reaction
• Oxaloacetate is substrate for gluconeogenesis and CAC intermediate

ANAEROBIC CONDITIONS

3. Lactic acid fermentation (humans)

• Occurs in exercising muscle and red blood cells: cytosol
• Glycolysis: 1 glucose = 2 pyruvates + 2 ATP + 2 NADH
• Lactate dehydrogenase: 2 pyruvate + 2NADH = 2 lactate + 2 NAD+
• Reversible reaction
• Lactate can enter bloodstream and travel to liver: lactate dehydrogenase catalyzes reverse reaction (lactate to pyruvate)

Clinical correlation: intense exercise can produce lactic acidosis; lactate accumulates in muscle cells and causes intracellular drop in pH

4. Ethanol production (yeast and select bacteria)

• Can occur in inteestinal flora
• Glycolysis: 1 glucose = 2 pyruvates + 2 ATP + 2 NADH
• 2 step rxn: pyruvate to acetaldehyde to ethanol
• Ethanol formation consumes 2 NADH in second step and produces 2 NAD+ for reuse
• Irreversible reaction
• Fermentation in yeast used to make beer and wine