GLUCONEOGENESIS

DEFINITION

• Lippincott Illustrated Reviews: Biochemistry, 8e, p. 353

PHYSIOLOGICAL SIGNIFICANCE / WHY IS IT NEEDED?

• Brain and nervous tissue
• Red blood cells (RBCs) - lack mitochondria, rely entirely on glycolysis
• Kidney medulla, lens and cornea of the eye, testes
• Exercising skeletal muscle

1. Maintain blood glucose during prolonged fasting/starvation
2. Recycle metabolic waste products (lactate, glycerol, amino acids)
3. Respond to increased glucose demand (e.g., prolonged exercise)

SITES OF GLUCONEOGENESIS

SUBSTRATES (GLUCONEOGENIC PRECURSORS)

1. Lactate

• The most important substrate during exercise and anaerobic conditions
• Produced by RBCs, exercising skeletal muscle, skin
• Converted to pyruvate by lactate dehydrogenase (LDH) in the liver
• Forms the basis of the Cori cycle (see below)

2. Glycerol

• Released from hydrolysis of triacylglycerols (TAGs) in adipose tissue during fasting
• Converted to glycerol-3-phosphate by glycerol kinase
• Then oxidized to dihydroxyacetone phosphate (DHAP) by glycerol-3-phosphate dehydrogenase
• DHAP is a direct intermediate of both glycolysis and gluconeogenesis

3. Glucogenic Amino Acids

• All amino acids except leucine and lysine are glucogenic
• They are transaminated or deaminated to yield pyruvate, OAA, alpha-ketoglutarate, succinyl CoA, or fumarate - all of which can feed into the TCA cycle to form OAA and ultimately PEP
• Most important: Alanine → pyruvate (via alanine aminotransferase); forms the Glucose-Alanine Cycle

Note: Leucine and lysine are purely ketogenic (give rise only to acetyl CoA, which cannot make net glucose due to the irreversibility of pyruvate dehydrogenase). Fatty acids are also not gluconeogenic substrates for the same reason.

THE PATHWAY OF GLUCONEOGENESIS

THE 4 UNIQUE REACTIONS OF GLUCONEOGENESIS

Bypass 1: Pyruvate → Phosphoenolpyruvate (PEP)

• Location: Mitochondrial matrix
• Pyruvate + CO₂ + ATP → Oxaloacetate (OAA) + ADP + Pi
• Requires coenzyme Biotin (covalently bound to lysine residue of the enzyme)
• Allosteric activator: Acetyl CoA (signals that TCA cycle needs replenishment)
• This reaction has an anaplerotic function (replenishes TCA cycle intermediates)

• Location: Cytoplasm (and partly mitochondrial)
• OAA + GTP → PEP + CO₂ + GDP
• Requires GTP as energy source
• PEPCK is the key rate-limiting enzyme of gluconeogenesis
• Induced by glucagon and cortisol; inhibited by insulin

OAA cannot cross the inner mitochondrial membrane directly. It is first reduced to malate (by mitochondrial malate dehydrogenase), exits to cytoplasm, and is re-oxidized back to OAA (by cytoplasmic malate dehydrogenase). This transport also provides NADH to the cytoplasm for gluconeogenesis.

Bypass 2: Fructose-1,6-bisphosphate → Fructose-6-phosphate

• Location: Cytoplasm
• Fructose-1,6-bisphosphate + H₂O → Fructose-6-phosphate + Pi (simple hydrolysis, no ATP generated)
• Inhibited by: AMP, Fructose-2,6-bisphosphate
• Activated by: Citrate

Bypass 3: Glucose-6-phosphate → Glucose

• Location: Smooth ER (endoplasmic reticulum) membrane of liver and kidney cells
• Glucose-6-phosphate + H₂O → Glucose + Pi (hydrolysis)
• This enzyme is absent in muscle and brain - which is why these tissues CANNOT release free glucose
• Allows the liver to release free glucose into the bloodstream

ENERGY COST OF GLUCONEOGENESIS

• 6 ATP equivalents (4 ATP + 2 GTP)
• 2 NADH (used in the glyceraldehyde-3-phosphate dehydrogenase reverse step)

2 Pyruvate + 4ATP + 2GTP + 2NADH + 6H₂O → Glucose + 4ADP + 2GDP + 6Pi + 2NAD⁺

REGULATION OF GLUCONEOGENESIS

1. Hormonal Regulation

2. Allosteric Regulation

3. Substrate Availability

4. Enzyme Induction (Long-term)

• PEPCK is transcriptionally induced by glucagon (via cAMP-CREB pathway) and cortisol
• Insulin suppresses PEPCK gene expression
• This slow mechanism adapts the liver's gluconeogenic capacity over hours/days

IMPORTANT CYCLES LINKED TO GLUCONEOGENESIS

1. The Cori Cycle (Glucose-Lactate Cycle)

• Muscle → anaerobic glycolysis → Lactate → released into blood
• Liver → lactate oxidized to pyruvate → gluconeogenesis → Glucose → released into blood → back to muscle
• Significance: recycles lactate, prevents lactic acidosis, sustains muscle activity

2. The Glucose-Alanine Cycle

• Muscle → amino acids → transamination → Alanine → released into blood
• Liver → alanine → pyruvate (by alanine aminotransferase) → gluconeogenesis → Glucose
• Also transfers amino groups from muscle to liver for urea synthesis
• Important during prolonged fasting and starvation

COMPARISON: GLUCONEOGENESIS vs. GLYCOLYSIS

CLINICAL RELEVANCE

1. Hypoglycemia: Deficiency of gluconeogenic enzymes (PC, PEPCK, FBPase-1, Glucose-6-phosphatase) causes hypoglycemia, especially during fasting.
2. Type 2 Diabetes Mellitus: Uncontrolled gluconeogenesis in the liver contributes significantly to fasting hyperglycemia. Metformin acts partly by inhibiting hepatic gluconeogenesis (reduces PEPCK and G6Pase expression).
3. Alcohol and Gluconeogenesis: Alcohol metabolism generates excess NADH, which shifts OAA to malate, depletes OAA, and blocks gluconeogenesis → causes alcoholic hypoglycemia.
4. Gluconeogenesis in DKA (Diabetic Ketoacidosis): In insulin deficiency, glucagon rises, driving uncontrolled gluconeogenesis → severe hyperglycemia.
5. Von Gierke disease (GSD Type I): Deficiency of glucose-6-phosphatase → cannot release free glucose from liver → severe fasting hypoglycemia, hepatomegaly.

SUMMARY