4611 Chapter 13 PDF Lecture Slides

Chapter 13: Glucose Metabolism

Overview

  • Reading spans pages 366-393.

Learning Objectives

  • Glycolysis:

    • Purpose: Convert glucose to pyruvate, producing energy.

    • Location: Occurs in the cytosol of the cell.

    • Regulation: Involves various enzymes and intermediates.

    • Intermediates, cofactors, and enzymes to recall include:

      • Intermediates: Glucose, Glucose-6-phosphate (G6P), Fructose-6-phosphate (F6P), Fructose-1,6-bisphosphate (FBP), Dihydroxyacetone phosphate (DHAP), Glyceraldehyde-3-phosphate (GAP), 3-Phosphoglycerate (3PG), Phosphoenolpyruvate (PEP), Pyruvate.

      • Cofactors: ATP, ADP, NADH.

      • Enzymes: Hexokinase, phosphoglucose isomerase, phosphofructokinase, aldolase, triose phosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase, phosphoglycerate mutase, enolase, pyruvate kinase.

    • Advantage of hexokinase phosphorylation: Prevents glucose from leaving the cell, maintaining a concentration gradient that favors import.

    • Mechanistic studies used evidence from alanine screening, transition state analogs, and radiolabels.

    • Fate of pyruvate:

      • Under aerobic conditions: Converted to acetyl-CoA.

      • Under anaerobic conditions in humans: Reduced to lactate.

      • Under anaerobic conditions in yeast: Converted to ethanol.

Glycolysis Overview

  • Glycolysis has two stages:

    1. Stage 1: Glucose ➜ G-3-P (requires 2 ATP).

    2. Stage 2: G-3-P ➜ Pyruvate (produces 4 ATP).

  • Net Yield: 2 ATP + 2 NADH.

  • Location: Cytosol.

  • Energy recovery depends on conditions:

    • Anaerobic: 2 NADH → heat.

    • Aerobic: 2 NADH → ~5 ATP.

Detailed Steps of Glycolysis

Step 1: Hexokinase

  • Mechanism makes the reaction irreversible.

  • Phosphorylation prevents glucose from leaving the cell.

  • Reduces intracellular glucose concentration.

Step 2: Phosphoglucose Isomerase (PGI)

  • Reaction is reversible; direction depends on concentrations.

  • Converts G6P to F6P.

Enzymatic Mechanisms

Phosphofructokinase-1 (PFK-1)

  • Phosphorylates F6P to FBP using ATP.

  • Reaction similar to hexokinase step.

Aldolase

  • Catalyzes the cleavage of FBP to GAP and DHAP.

  • Uses a Schiff base mechanism supported by alanine screening.

Triose Phosphate Isomerase (TIM)

  • Evidence for the enediolate intermediate in reaction mechanisms using transition state analogs.

Additional Steps

Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH)

  • Mechanism involves direct hydride transfer.

  • Evidence from enzyme inactivation and substrate binding.

Phosphoglycerate Kinase (PGK)

  • Involves a reaction similar to the reverse of the hexokinase reaction for ATP generation.

Enolase

  • Lyase that converts 2-phosphoglycerate to phosphoenolpyruvate (PEP) via dehydration.

Pyruvate Kinase

  • Generates ATP by converting PEP to pyruvate; regulated by energy state of the cell.

Fate of Pyruvate

  • Under anaerobic conditions in animals: Converted into lactate via lactate dehydrogenase, regenerating NAD+.

  • Under anaerobic conditions in yeast: Pyruvate decarboxylated to acetaldehyde, then reduced to ethanol.

  • Aerobic conditions: Converted to Acetyl-CoA or Oxaloacetate for further processes.

Gluconeogenesis

  • Mainly occurs in the liver; energetically expensive (4 ATP + 2 GTP required).

  • Involves bypass of irreversible glycolytic steps.

Glycogen Metabolism

Synthesis

  1. Phosphoglucomutase converts G6P to G1P.

  2. G1P is activated to UDP-glucose by UTP.

  3. Glycogen synthase links glucose units.

  4. Branch points created by glycogen-branching enzyme moving glucose residues.

Breakdown

  • Glycogen phosphorylase functions via phosphorolysis to produce G1P.

  • Debranching enzyme removes branches at α(1→6) links when 4 glucose residues are left.

Pentose Phosphate Pathway

  • Produces ribose for nucleotides and NADPH.

  • Pathways depend on the cell's need:

    • Oxidative: Irreversible, produces NADPH and ribulose-5-phosphate.

    • Carbon rearrangement: Reversible reactions producing ribose-5-phosphate or intermediates based on metabolic needs.

Cancer Metabolism

  • Cancer cells exhibit the Warburg effect: higher rates of glycolysis.

  • Targeting glycolytic pathways can provide treatment avenues for cancers.

Glycolytic Mechanisms in Cancer Diagnostics

  • Understanding glycolytic pathways aids in diagnostic imaging for cancers, exemplified through 6-phospho-FdG detection.