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Gluconeogenesis Overview

Definition

• Gluconeogenesis: The synthesis of glucose from noncarbohydrate precursors, aimed at maintaining blood glucose levels during fasting.
• Primary Organs: Liver and kidneys.
• Triggers: Fasting, prolonged exercise, a high-protein diet, and stress.

Specific Pathways in Gluconeogenesis

1. Pyruvate to Phosphoenolpyruvate (PEP)
2. Fructose-1,6-bisphosphate (F-1,6-BP) to Fructose-6-phosphate (F-6-P)
3. Glucose-6-phosphate (G-6-P) to Glucose

Precursors for Gluconeogenesis

1. Lactate (from Anaerobic Glycolysis)

• Conversion:

  $\text{Lactate} \rightarrow \text{Pyruvate}$

• Enzyme involved: Lactate Dehydrogenase.

• Reduction:

  $\text{NAD}^+ + \text{Lactate} \rightleftharpoons \text{NADH} + \text{Pyruvate}$

2. Amino Acids (from Muscle Proteins)

• Key Example: Alanine

• Conversion to Pyruvate via Alanine Aminotransferase:

  $\text{Alanine} + \alpha\text{-ketoglutarate} \rightarrow \text{Pyruvate} + \text{Glutamate}$

3. Glycerol (from Adipose Tissue)

• Conversion involves formation of Glycerol-3-phosphate:

  $\text{Glycerol} + \text{ATP} \rightarrow \text{Glycerol 3-phosphate} + \text{ADP}$

• Subsequent Conversion:

  $\text{Glycerol 3-phosphate} \rightarrow \text{Dihydroxyacetone phosphate (DHAP)}$

Conversion of Pyruvate

Key Steps:

1. Pyruvate to Oxaloacetate

   • Enzyme: Pyruvate Carboxylase
   • Input: Requires Biotin and ATP.

2. Oxaloacetate to Phosphoenolpyruvate (PEP)

   • Enzyme: Phosphoenolpyruvate Carboxykinase (PEPCK).

• This pathway is influenced by glucagon via cAMP signaling.

Glucagon Regulation:

• Leads to the induction of PEPCK and inhibition of pyruvate kinase (PK), converting PK to its inactive form.

Conversion of Phosphoenolpyruvate to Glucose

1. Fructose-1,6-bisphosphate (F-1,6-BP) to Fructose-6-phosphate (F-6-P)

• Enzyme: Fructose-1,6-bisphosphatase (located in the cytosol).

1. Glucose-6-phosphate (G-6-P) to Glucose

• Enzyme: Glucose-6-phosphatase (located in the endoplasmic reticulum (ER)).

Regulation of Gluconeogenesis

• Key Concept: Gluconeogenesis and glycolysis are reciprocally regulated.
• Main Points:

1. Pyruvate to PEP:

   • Induced by hormones: glucagon, epinephrine, cortisol.

2. F-1,6-BP to F-6-P:

   • Inhibited by Fructose-2,6-bisphosphate.

3. G-6-P to Glucose:

   • Induced during fasting periods.

Glycolysis vs. Gluconeogenesis

Glycolysis:

• Objective: Generation of ATP under anaerobic or aerobic conditions.
• Involves various regulatory enzymes such as Glucokinase, Phosphofructokinase-1, and Pyruvate Kinase.
• Produces intermediates like 3-Phosphoglycerate, Glyceraldehyde 3-phosphate, etc.

Gluconeogenesis:

• Activation during fasting, prolonged exercise, and a high-protein diet.
• Main precursors: lactate, glycerol, amino acids.
• Key reactions:
  1. Pyruvate to PEP
  2. F-1,6-BP to F-6-P
  3. G-6-P to Glucose
• Overall regulation and balancing with glycolytic processes are crucial for energy homeostasis.