GLUCONEOGENESIS

• Synthesis of glucose from non-carbohydrate precursors
• Occurs mostly in the liver and minor process in kidney
• Kidney produces 10% total glucose during overnight fast

ENZYMES UNIQUE TO GLUCONEOGENESIS

1. Pyruvate carboxylase (mitochondrial matrix)

• Converts pyruvate to oxaloacetate
• Requires 1 ATP, biotin and 1 CO2 (ABC Reaction)

2. Phosphoenol carboxykinase (cytosolic and mitochondrial isozymes)

• Converts oxaloacetate to phosphoenolpyruvate (PEP)
• Consumes 1 GTP and releases 1 CO2

Cytosolic PEPCK

• Used in the malate shuttle: shuttles oxaloacetate from mitochondrion to cytosol via malate
• Pathway dominates when pyruvate is the gluconeogenic substrate
• Mitochondrion: oxaloacetate –> malate (consumes 1 NADH)
• Cytosol: malate –> oxaloacetate (releases 1 NADH)
• Released NADH used in G3P synthesis
• Pyruvate substrate uses cytosolic PEPCK: consumes 2 NADH

Mitochondrial PEPCK

• Pathway dominates when lactate is substrate
• Cytosol: lactate –> pyruvate (releases 1 NADH)
• Released NADH used in G3P synthesis
• Mitochondrial PEPCK: oxaloacetate –> PEP (can cross mitochondrial membranes)
• Lactate substrate uses mitochondrial PEPCK: does NOT consume NADH

PEP converted to glyceraldehyde 3-phosphate in 4 reversible reactions: 1 ATP and 1 NADH consumed (double energy inputs, substrates and products)

Glyceraldehyde 3-phosphate reversibly combines w/ DHAP to form fructose 1,6-bisphosphate

3. Fructose 1,6-bisphosphatase (cytosol)

• Produces fructose 6-phosphate
• Consumes 1 H20 and releases 1 Pi

Fructose 6-phosphate reversibly converts to glucose 6-phosphate

4. Glucose 6-phosphatase (ER membrane-bound)

• Enzyme complex with 4 proteins
  i. Transport protein: G6P from cytosol to ER lumen
  ii. Phosphatase: removes Pi from G6P to form glucose (consumes 1 H2O)
  iii. Transport protein: transports glucose to cytosol
  iv. Transport protein: transports Pi to cytosol

FINAL BOOKKEEPING:
2 Pyruvate + 4ATP + 2GTP + 2NADH + 6H20 –> 1 Glucose + 4ADP + 2GDP + 2NAD+ + 6Pi + 6H+
2 Lactate + 4ATP + 2GTP + 6H2O –> 1 Glucose + 4ADP + 2GDP + 6Pi + 6H+

Lactate substrate lacks net NADH requirement

CLINICAL CORRELATION

Avidin

• Protein in egg whites
• Binds biotin very tightly (biotin required by pyruvate carboxylase)
• Consuming large amounts of raw eggs over an extended period of time can produce biotin deficiency
• Symptoms: CNS problems (lethargy)