Gluconeogenesis Notes

Definition and Physiological Role

• Definition: It is the synthesis of new glucose from non-carbohydrate precursors.
• Importance: The main function is to supply blood glucose to vital organs (brain and RBCs) in case of carbohydrate deficiency (fasting, starvation and low carbohydrate diet) for more than $18\text{ hours}$.
• Site: Mainly in the cytosol and mitochondria of liver cells and to a lesser extent in kidneys.

Gluconeogenic vs Glycolytic Enzymes

• Key enzymes that drive gluconeogenesis (the four bypass enzymes):
  1. Glucose-6-phosphatase (G6Pase)
  2. Fructose-1,6-bisphosphatase (F1,6BPase)
  3. Pyruvate carboxylase (the only mitochondrial enzyme)
  4. Phosphoenolpyruvate carboxykinase (PEPCK)
• Glycolytic key enzymes (major control points in glycolysis):
  1. Glucokinase (GK) [also hexokinase in other tissues]
  2. Phosphofructokinase-1 (PFK-1)
  3. Pyruvate kinase (PK)
  4. Phosphoenolpyruvate carboxykinase (PEPCK) – noted here as part of the reciprocal step in gluconeogenesis (the reversal involves a split step via PEPCK and pyruvate carboxylase).
• Concept: Gluconeogenesis is the reversal of glycolysis, but the three irreversible glycolytic reactions are bypassed by the four gluconeogenic enzymes listed above.

Pathway overview: Reversal of glycolysis

• The three irreversible steps in glycolysis are bypassed by the gluconeogenic enzymes as follows:
  • Hexokinase/Glucokinase step (glucose to glucose-6-phosphate) is reversed by Glucose-6-phosphatase (G6Pase).
  • Phosphofructokinase-1 step (fructose-6-phosphate to fructose-1,6-bisphosphate) is reversed by Fructose-1,6-bisphosphatase (F1,6BPase).
  • Pyruvate kinase step (phosphoenolpyruvate to pyruvate) is bypassed by the two-step sequence: Pyruvate carboxylase (mitochondrial) to form oxaloacetate, followed by Phosphoenolpyruvate carboxykinase (PEPCK) to form phosphoenolpyruvate (PEP).
• Key sequence (conceptual):
  • Pyruvate → (pyruvate carboxylase, requires CO₂ and ATP) → oxaloacetate (OAA) → (PEP carboxykinase, uses GTP) → phosphoenolpyruvate (PEP) → … → glucose.

Detailed steps along the gluconeogenic path

• From glucose-6-phosphate to glucose:
  • Glucose-6-phosphate ⇄ glucose via Glucose-6-phosphatase (G6Pase) to release free glucose into blood.
  • Reaction: $\text{glucose-6-phosphate} \rightarrow \text{glucose} + \text{P}_i$
• From fructose-1,6-bisphosphate to fructose-6-phosphate:
  • Fructose-1,6-bisphosphatase removes a phosphate group.
  • Reaction: $\text{fructose-1,6-bisphosphate} \rightarrow \text{fructose-6-phosphate} + \text{P}_i$
• From pyruvate to phosphoenolpyruvate (two-step bypass):
  • Pyruvate + CO₂ + ATP → oxaloacetate + ADP + Pᵢ (pyruvate carboxylase, mitochondrial)
  • Oxaloacetate + GTP → phosphoenolpyruvate + GDP + CO₂ (PEPCK)
  • Reactions: $\text{pyruvate} + \text{CO}<em>2 + \text{ATP} \rightarrow \text{oxaloacetate} + \text{ADP} + \text{P}</em>i$
  • $\text{oxaloacetate} + \text{GTP} \rightarrow \text{phosphoenolpyruvate} + \text{GDP} + \text{CO}_2$
• Other glycolytic intermediates are interconverted in the cytosol to allow formation of glucose-6-phosphate and then glucose:
  • Glyceraldehyde-3-phosphate (G3P) and Dihydroxyacetone phosphate (DHAP) interconversion via triose phosphate isomerase.
  • Through steps catalyzed by glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate mutase, phosphoglycerate kinase, and enolase, PEP is formed and channeled toward G6P and then glucose.
• Overall end product:
  • Glucose is released into blood by the action of G6Pase in liver and kidney to a lesser extent.

Gluconeogenic precursors

• Glucogenic amino acids (protein-derived glucose):
  • Proteins are a major source of blood glucose especially after 18 hours due to depletion of liver glycogen by glycogenolysis.
• Lactate (from exercised muscle and anaerobic glycolysis in RBCs) via the Cori cycle:
  • Lactate released by RBCs and skeletal muscles is transported to the liver, converted to pyruvate, then to glucose.
  • Cori cycle concept: lactate
• Glycerol: Produced from lipolysis of fat; enters gluconeogenesis via DHAP/G3P branch.
• Odd-chain fatty acids: Propagation to succinyl-CoA which feeds into the TCA cycle and is converted to oxaloacetate to support gluconeogenesis; ultimately contributes to glucose formation via reversal of glycolysis.

The Cori cycle: Importance and energy context

• Importance:
  • Helps maintain blood glucose during fasting and strenuous activity.
  • Prevents loss of lactate in urine; preserves energy in red cells and contracting muscles.
• In liver, gluconeogenesis consumes energy that is provided mainly from oxidation of fatty acids.
• Consequence: Liver provides glucose to RBCs and muscles, supporting ATP production in those tissues.

Regulation of glycolysis (key control points that also influence gluconeogenesis via reciprocal control)

• Key glycolytic enzymes that are tightly regulated: GK/HK, PFK-1, PK.
• Energy-state regulation:
  • High AMP (low ATP) stimulates glycolysis.
  • High ATP (low energy demand) inhibits glycolysis.
• Hormonal regulation:
  • Insulin (fed state, hypoglycemia context) stimulates glycolysis.
  • Anti-insulin hormones (glucagon and adrenaline) (fasting, stress) inhibit glycolysis.

Regulation of gluconeogenesis

• Gluconeogenic regulatory enzymes reverse the glycolytic key enzymes; pathways are reciprocally controlled:
  • Insulin (secreted after a carbohydrate meal): decreases activity of gluconeogenic key enzymes and increases glycolytic enzymes; results in reduced gluconeogenesis and lower blood glucose.
  • Anti-insulin hormones (glucagon, epinephrine, glucocorticoids, growth hormone): increase activity of gluconeogenic key enzymes; results in increased gluconeogenesis and higher blood glucose.
• Physiological context:
  • Gluconeogenesis is upregulated during fasting, stress, or severe muscular exercise to maintain glucose availability.

Control of gluconeogenesis by response elements and transcriptional regulation

• A model for how response elements affect metabolism (example):
  • Cortisol and glucagon stimulate gluconeogenesis through enhancer mechanisms.
  • Pathway outline:
  • Cortisol and glucagon increase intracellular cAMP.
  • cAMP activates Protein Kinase A (PKA).
  • PKA activates CREB (cAMP response element-binding protein).
  • CREB binds to CRE (cAMP response element) and GRE (glucocorticoid response element) elements upstream of the PEPCK gene.
  • Activation leads to transcription of PEPCK mRNA and synthesis of PEPCK enzyme, increasing gluconeogenesis.
• Diagrammatic concept (textual):
  • Cortisol — CRE/ GRE elements — PEPCK gene transcription ↑ → PEPCK enzyme ↑ → gluconeogenesis ↑
• Summary: Cortisol and glucagon act via CRE and GRE enhancer elements to upregulate PEPCK transcription, increasing gluconeogenesis.

PEPCK: The rate-limiting step and transcriptional control

• Gluconeogenesis is a hepatic pathway with the major function of maintaining adequate glucose in the blood for brain and RBCs during fasting, and during stress.
• Glucagon, via membrane receptors, elevates cAMP and activates CRE-mediated transcription of gluconeogenic genes.
• Cortisol, via intracellular glucocorticoid receptor (a zinc-finger DNA-binding protein), acts through a glucocorticoid response element (GRE).
• Key transcriptional control point:
  • The PEPCK gene contains a cAMP response element (CRE) and a glucocorticoid response element (GRE) upstream from its transcription start site.
  • These elements integrate hormonal signals to regulate the rate-limiting enzyme of gluconeogenesis.

Summary of the pathway's significance and clinical relevance

• Gluconeogenesis is essential for maintaining blood glucose during fasting and stress, ensuring energy supply to brain and RBCs.
• The liver is the primary site; kidneys contribute to gluconeogenesis under certain conditions.
• Hormonal regulation (insulin vs anti-insulin hormones) orchestrates the balance between glycolysis and gluconeogenesis to meet organismal energy needs.
• Transcriptional regulation via CRE and GRE ensures that metabolic state (fed vs fasting, stress) is encoded at the level of gene expression for PEPCK, a key control point in gluconeogenesis.

Key equations and reactions (LaTeX-formatted)

• Lactate to pyruvate (Cori cycle precursor step):
  $\text{lactate} + \text{NAD}^+ \rightleftharpoons \text{pyruvate} + \text{NADH} + \text{H}^+$
• Glucose-6-phosphate to glucose (G6Pase):
  $\text{glucose-6-phosphate} \rightarrow \text{glucose} + \text{P}_i$
• Fructose-1,6-bisphosphatase (F1,6BPase):
  $\text{fructose-1,6-bisphosphate} \rightarrow \text{fructose-6-phosphate} + \text{P}_i$
• Pyruvate carboxylase (mitochondrial):
  $\text{pyruvate} + \text{CO}<em>2 + \text{ATP} \rightarrow \text{oxaloacetate} + \text{ADP} + \text{P}</em>i$
• Phosphoenolpyruvate carboxykinase (PEPCK):
  $\text{oxaloacetate} + \text{GTP} \rightarrow \text{phosphoenolpyruvate} + \text{GDP} + \text{CO}_2$
• Glycerol to DHAP (lipolysis-derived substrate for gluconeogenesis):
  $\text{glycerol} + \text{ATP} \rightarrow \text{glycerol-3-phosphate} + \text{ADP}$
  $\text{glycerol-3-phosphate} + \text{NAD}^+ \rightarrow \text{dihydroxyacetone phosphate} + \text{NADH} + \text{H}^+$
• Odd-chain fatty acids via succinyl-CoA pathway (propionyl-CoA to oxaloacetate):
  $\text{propionyl-CoA} \rightarrow \text{D-methylmalonyl-CoA} \rightarrow \text{succinyl-CoA} \rightarrow \text{oxaloacetate} \rightarrow \text{PEP}$
• CRE and GRE enhancer concept (transcriptional control):
  $\text{CRE} \quad \text{(cAMP response element)} \ \text{GRE} \quad \text{(glucocorticoid response element)}$
• Overall regulatory pathway (simplified):
  $\text{Cortisol} \rightarrow \uparrow [\text{cAMP}] \rightarrow \text{PKA} \rightarrow \text{CREB} \rightarrow \text{PEPCK mRNA} \rightarrow \text{PEPCK enzyme} \rightarrow \text{gluconeogenesis}$

Connections to broader concepts and practical implications

• Energy balance: Gluconeogenesis consumes energy; liver generates this energy primarily via fatty-acid oxidation to support glucose production for brain and RBCs.
• Metabolic flexibility: The liver can switch between glycolysis and gluconeogenesis in response to hormonal signals to maintain euglycemia.
• Clinical relevance: Dysregulation of gluconeogenesis can contribute to hyperglycemia in diabetes or hypoglycemia during fasting; understanding transcriptional control (PEPCK) helps explain metabolic adaptations during stress and fasting.