26 Gluconeogenesis

Gluconeogenesis Overview and Rationale

Gluconeogenesis Overview

• Gluconeogenesis is a metabolic process that generates glucose from non-carbohydrate substrates.

• It is crucial for maintaining blood glucose levels during fasting and rigorous exercise.

Key Enzymes Involved in Gluconeogenesis

1. Pyruvate Carboxylase: An enzyme that converts pyruvate to oxaloacetate (OAA) in the mitochondria.

2. Phosphoenolpyruvate Carboxykinase (PEPCK): Converts oxaloacetate to phosphoenolpyruvate (PEP).

3. Fructose-1,6-bisphosphatase (FBPase-1): Catalyzes the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate.

4. Glucose-6-Phosphatase: Converts glucose-6-phosphate to glucose, facilitating its export from the cell.

Rationale for Gluconeogenesis

• The process requires energy to generate glucose, making it fundamentally an anabolic pathway.

• The net reaction can be summarized as:
  $ext{2 pyruvate} + 4 ext{ ATP} + 2 ext{ GTP} + 2 ext{ NADH} + 2 ext{ H}^+ + 6 ext{ H}<em>2 ext{O} ightarrow ext{glucose} + 4 ext{ ADP} + 2 ext{ GDP} + 6 ext{ P}</em>i + 2 ext{ NAD}^+$

• Energy sources for gluconeogenesis include fatty acid oxidation, lactate, amino acids (e.g., alanine), and glycerol.

Where and When Does Gluconeogenesis Occur?

• When:

  • During fasting periods.

  • The brain requires approximately 120 g of glucose per day.

  • After the depletion of glycogen stores, such as during exercise.

• Where:

  • Primarily in the liver, but also to a lesser extent in the kidneys, intestines, and in astrocytes of the brain.

• Analogy: The liver acts like the Federal Reserve of Energy, both storing energy (as glycogen) and producing glucose through gluconeogenesis.

The Cori Cycle

• The Cori cycle describes how lactate produced during anaerobic glycolysis in muscles is transported to the liver where it is converted back into glucose through gluconeogenesis.

• The question arises: How can glycolysis and gluconeogenesis be thermodynamically favorable simultaneously?

  • Reactions with significant free energy drops provide regulatory points.

  • The drive for glucose synthesis must overcome these energy drops, typically through the input of ATP and GTP.

Reciprocal Irreversible Reactions in Glycolysis and Gluconeogenesis

• Reciprocal reactions between glycolysis and gluconeogenesis allow for both pathways to exist without counteracting each other.

• Glycolysis uses glucose oxidation to produce energy (net gain of 2 ATP and 2 NADH) while gluconeogenesis requires energy input (4 ATP, 2 GTP, and 2 NADH).

• Gluconeogenesis takes place across three cellular compartments: the mitochondria, cytosol, and endoplasmic reticulum.

Bypasses in Gluconeogenesis

Bypass 1: Conversion of Pyruvate to PEP

• This step is challenging due to energy investment requirements.

• Includes:

  • Pyruvate Carboxylase: Converts pyruvate to OAA, requires ATP.

  • PEPCK: Converts OAA to PEP using GTP, involves decarboxylation.

• The combined reaction is:
  $ext{Pyruvate} + ext{ATP} + ext{GTP} <br>ightarrow ext{PEP} + ext{ADP} + ext{GDP} + ext{Pi} + ext{H}^+ \ \ ext{ΔG}^ ext{°'} = -2.6 ext{ kJ/mol} \ \ ext{ΔG} ext{ approximately } -25 ext{ kJ/mol}$

Key Enzymatic Reactions in Gluconeogenesis

Fructose-1,6-Bisphosphatase (FBPase-1)

• Catalyzes the reverse of the phosphofructokinase-1 (PFK-1) reaction in glycolysis.

• Standard free energy changes of the reactions involved:

  • For FBPase-1: $ext{ΔG}^ ext{°'} = -16.3 ext{ kJ/mol}$

  • For PFK-1: $ext{ΔG}^ ext{°'} = -15.9 ext{ kJ/mol}$

• Both reactions are thermodynamically favorable under standard conditions, but regulatory mechanisms will dictate directionality in a cellular context.

Glucose-6-Phosphatase

• Responsible for the final step of gluconeogenesis, converting glucose-6-phosphate to free glucose.

• Located in the endoplasmic reticulum (ER) membrane, allowing newly synthesized glucose to evade glycolytic enzymes in the cytosol, facilitating more efficient glucose release.

Structure and Function

• Glucose-6-phosphatase is an integral membrane protein with:

  • 9 transmembrane helices in the catalytic subunit.

  • Transporters for glucose-6-phosphate and inorganic phosphate (Pi).

• Why the ER?

  • Protects glucose from immediate glycolytic degradation, aiding in export.

  • Allows for use in glycosylating new proteins in the ER lumen.

Mechanisms of Glucose Export

• GLUT2 Transport Protein: Facilitates bidirectional transport of glucose, allowing gluconeogenic glucose to exit cells.

• Hexokinase IV (Glucokinase): Has low affinity for glucose, is only engaged in glycolysis if glucose levels are high, preventing unnecessary glucose use when it’s scarce.

Regulation of Gluconeogenesis

• Regulates flux between glycolysis and gluconeogenesis through kinetic control of their reciprocal irreversible reactions.

• Key regulatory enzymes include:

  • PFK-1 and FBPase-1 regulated by fructose-2,6-bisphosphate.

  • PFK-2/FBPase-2 is a bifunctional enzyme that modulates levels of fructose-2,6-bisphosphate, which serves as a glycolytic activator.

Dance of Hormonal Regulation

• Fed state: Insulin enhances glycolysis and inhibits gluconeogenesis.

• Fasted state: Glucagon promotes gluconeogenesis by stimulating FBPase-2 activity, decreasing fructose-2,6-bisphosphate levels which alleviates inhibition of FBPase-1.

Summary of Hormonal Effects

• Insulin stimulates pathways leading to glucose storage and reduces gluconeogenic activity, while glucagon induces glucose release through gluconeogenesis and glycogenolysis.

Conclusions and Review

• Review of glycolytic steps, particularly those catalyzed by key enzymes:

  • Pyruvate kinase, FBPase-1, PFK-1, and their interactions with fructose-2,6-bisphosphate and other metabolic signals.

• Important Concepts to Remember:

  • Mechanisms of gluconeogenesis in the liver, the overall reaction, key locations and enzymes required, regulation by hormonal signaling, and how glucose is generated and exported during varying metabolic states.