Gluconeogenesis

Page 1: Lecture Information

  • Course: BIOC 2300

  • Lecture Title: Gluconeogenesis: Reverse of Glycolysis?

  • Date: November 04 – 17, 2024

  • Instructor: Petra C. Kienesberger, PhD

  • Position: Associated Professor, Department of Biochemistry & Molecular Biology

  • Contact: pkienesb@dal.ca

Page 2: Case Study on Metformin Effects

  • **Mechanism of Action:

    • Lowers blood glucose through multiple pathways:

      • Suppresses hepatic gluconeogenesis

      • Increases peripheral insulin-mediated glucose uptake

      • Decreases fatty acid oxidation

      • Increases intestinal glucose consumption

  • Pharmacokinetics:

    • Maximal plasma concentration: Approximately 2 hours post-ingestion

    • Half-life: Ranges from 2.5 to 4.9 hours

    • Elimination: About 90% excreted in urine within 12 hours

  • Side Effects:

    • Common gastrointestinal issues: diarrhea, nausea, vomiting, abdominal bloating, anorexia

    • Occur in 20%-30% of patients; typically transient

  • Lactic Acidosis Risk:

    • Rare but serious side effect of metformin; incidence: ~6 cases per 100,000 patients

    • Associated with >30% mortality rate in critical care patients

Page 3: Concept Map of Gluconeogenesis

  • Definition: Process for forming glucose from non-hexose precursors:

    • Examples: glycerol, lactate, pyruvate, propionate, glucogenic amino acids

  • Occurrence: Ubiquitous in:

    • Plants

    • Animals

    • Fungi

    • Bacteria

    • Microorganisms

Page 4: Learning Objectives

  • **Explain the necessity for gluconeogenesis (GNG)

  • Address the limitations of reversing glycolysis

  • Compare and contrast glycolysis and GNG

  • Describe specific reactions in GNG

  • Understand the reciprocal regulation of glycolysis and GNG

  • Convert specific substrates (lactate, alanine, glycerol) to glucose using GNG

  • Link GNG to type 2 diabetes

Page 5: Problem of Glucose Dependence

  • Daily Glucose Requirement: 160 g for humans

  • Main Consumers of Glucose:

    • Brain: 120 g (major consumer)

    • Red blood cells

    • Muscles

  • Glycogen Storage Capacity:

    • About 190 g (sustains for ~1 day)

  • Concern: What happens when glucose reserves deplete?

Page 6: Solution - Gluconeogenesis (GNG)

  • Definition: Synthesis of glucose from non-hexose precursors:

    • Notably from pyruvate, lactate, glycerol, amino acids, citric acid cycle intermediates

  • Major Precursor Sources:

    • Lactate

    • Amino acids

    • Glycerol

  • Location of GNG: Primarily in the liver, also occurs in the kidneys

  • Importance: Critical during fasting/starvation - provides glucose for the brain and RBCs

Page 7: Glycolysis vs Gluconeogenesis

  • Key Point: Glycolysis features irreversible, exergonic steps

  • Question Raised: Why can't glycolysis be simply reversed to convert pyruvate back to glucose?

Page 8: Counteracting Irreversible Glycolysis Reactions

  • Irreversible Steps: Steps 1, 3, and 10 in glycolysis have large negative ΔG

  • Regulatory Steps: Require different enzymes for reversing glycolytic reactions in GNG; not exact reversals

Page 9: GNG Pathway Overview

  • Net Reaction of GNG: 2 Pyruvate + 2 NADH + 6 ATP → Glucose + 2 NAD+ + 6 ADP + 6 Pi

  • Key Point: GNG is not an exact reversal of glycolysis; different enzymes and products involved

Page 10: Regulation of Glycolysis and GNG

  • Futile Cycle Prevention: Glycolysis and GNG cannot occur simultaneously

  • Regulation: Pathways are governed by the energetic needs of the cell

Page 11: Unique Aspects of GNG

  • Non-Reversal Mechanism: Highly exergonic glycolysis steps are bypassed in GNG

  • Common Reversible Reactions: Shared between both pathways

  • Unique Enzymes in GNG:

    • Glucose phosphatase

    • Fructose bisphosphatase

    • Phosphoenolpyruvate carboxykinase (PEPCK)

    • Pyruvate carboxylase

Page 12: ATP Consumption in GNG

  • Process Description: Hydrolysis of phosphates is utilized rather than regeneration of ATP

  • Implications: ATP hydrolysis to ADP occurs in the context of GNG

Page 13: Final Steps of Glycolysis

  • Conversion Process: Pyruvate → Oxaloacetate → Phosphoenolpyruvate

  • Key Enzymes:

    • Pyruvate carboxylase: Converts pyruvate to oxaloacetate (requires ATP, biotin cofactor, mitochondrial enzyme)

    • PEPCK: Converts oxaloacetate to PEP (requires GTP, cytosolic enzyme)

Page 14: Substrates for GNG

  • Key Substrates Include:

    • Glycerol

    • Glucose-6P

    • Pyruvate

    • Lactate

    • Glucogenic amino acids

    • TCA cycle intermediates

  • Special Note: Acetyl-CoA cannot be converted to pyruvate, leading to ketone body production

Page 15: Cori Cycle

  • Mechanism Introduction: Lactate from anaerobic glycolysis enters bloodstream, reaches liver

  • Liver Function: Converts lactate to pyruvate then to glucose, which is released back into circulation

Page 16: Amino Acids as Gluconeogenic Sources

  • Key Insights:

    • Tissue protein hydrolysis provides amino acids during fasting

    • Metabolism generates α-keto acids, precursors for glucose or TCA cycle

    • Some amino acids are ketogenic and do not contribute to glucose synthesis

Page 17: Glycerol Conversion to Glucose

  • Process Overview: Glycerol from triacylglycerols is converted to G3P then to dihydroxyacetone phosphate—important in both glycolysis and GNG

Page 18: GNG Regulation by Metformin

  • Goals in Regulation:

    • Prevent concurrent glycolysis/GNG activity

    • Generate ATP when necessary

    • Avoid accumulation of pathway intermediates

  • Enzyme Activation: Pyruvate carboxylase activated by acetyl CoA, signaling fasting state requirements

Page 19: Further Regulation of GNG

  • Fructose 1,6-bisphosphatase Regulation: Inhibited by AMP/ATP ratio and fructose 2,6-bisphosphate, signaling the energy state and hormonal control

Page 20: Reciprocal Regulation Summary

  • Overlap in Regulation: Fructose-2,6-bisphosphate acts as an allosteric activator for PFK-1 and inhibitor for FBPase, managing flux through both pathways

Page 21: Feedback and Feedforward Regulation of Pathways

  • Control Mechanisms:

    • End product feedback inhibits upstream enzyme

    • Substrate activation promotes downstream enzyme activity

    • Ensures efficient pathway operation and avoids excessive product production

Page 22: Overview of Type 2 Diabetes

  • Primary Characteristics:

    • Accounts for ~90-95% of diabetes cases

    • Presents with hyperglycemia

    • Associated with insulin resistance in muscle, liver, fat

    • Major risk factors: Overweight, obesity, genetic predisposition

Page 23: GNG and Type 2 Diabetes Connections

  • Research References:

    • John Le Lay and Klaus H. Kaestner; Physiol Rev 2010; 90:1-22

    • Galbo T & Shulman GI; Aging (Albany NY), 5(8):582-3

Page 24: Homework Assignments

  • Compare glycolysis and gluconeogenesis on the following aspects:

    • Start Point

    • Energy Yield or Consumption

    • Endpoint

    • Location

    • Purpose

    • Regulatory Steps

    • Allosteric Regulation

Page 25: Self-Reflection Questions

  • Key Questions to Assess Understanding:

    • What is the purpose of GNG?

    • Where does GNG occur?

    • Describe the process starting from pyruvate.

    • How are lactate, alanine, and glycerol converted to glucose?

    • What is the role of lactate shuttling in muscle performance?

    • How is GNG regulated?

    • Why can't glycolysis be simply reversed for GNG?

    • Identify similarities and differences between glycolysis and GNG.