2._Glycolysis

Learning Objectives

  • Discuss the hormonal regulation of carbohydrate metabolism.

  • Compare and contrast the actions of insulin and glucagon.

  • Describe the role of different glucose transporters.

    • Compare GLUT2 and GLUT4.

  • Describe regulatory enzymes of glycolysis.

    • Compare hexokinase and glucokinase.

    • Explain the role of Phosphofructokinase-1 (PFK-1) in glycolysis.

    • Discuss Pyruvate Kinase.

  • Describe differences in regulation of glycolysis in liver and muscle.

  • Discuss citrate's inhibition of glycolysis in muscle vs. liver.

  • Discuss epinephrine's activation of glycolysis in muscle vs. liver.

  • Compare aerobic and anaerobic glycolysis.

  • List energy gains and expenditures of aerobic versus anaerobic glycolysis.

  • Discuss biochemical basis of important pathological deficiencies in glycolysis:

    • PFK-1 deficiency.

    • GLUT-1 deficiency.

    • Glucokinase deficiency.

    • Pyruvate kinase deficiency.

Regulation of Metabolism-Hormonal Regulation

  • Major hormones involved:

    • Insulin

    • Glucagon

    • Stress hormones (Epinephrine, Cortisol)

  • Epinephrine: Short-term stress response, increases fuel supply for energy.

  • Cortisol: Long-term stress response.

Insulin and Glucagon Roles

  • Insulin: Released during fed state, activates enzymes via dephosphorylation, promoting fuel storage and complex molecule synthesis.

  • Glucagon: Released during fasting, maintains blood glucose levels by increasing glucose release from the liver.

Understanding Glycolysis

  • Glycolysis is the primary pathway for dietary carbohydrate metabolism and the fastest pathway to produce ATP.

  • Stages of catabolism include:

    • Capturing chemical energy as ATP from fuel molecules.

    • Producing building blocks for complex molecules.

Glucose Transport into Cells

  • Glucose uses specific glucose transport proteins for entry into cells, characterized by Km (affinity).

Glucose Transporters Overview

  • GLUT1:

    • Tissue: All mammalian tissues

    • Km: 1 mM

    • Insulin responsive: No

    • Role: Baseline glucose uptake.

  • GLUT2:

    • Tissue: Liver and intestinal cells; pancreatic beta cells.

    • Km: 15-20 mM

    • Insulin responsive: No

    • Role: Regulation of insulin in pancreas; glucose removal from blood in liver.

  • GLUT3:

    • Tissue: All mammalian tissues.

    • Km: 1 mM

    • Insulin responsive: No

    • Role: Baseline glucose uptake in neurons.

  • GLUT4:

    • Tissue: Muscle and fat cells.

    • Km: 5 mM

    • Insulin responsive: Yes

    • Role: Increases with endurance training.

  • GLUT5:

    • Tissue: Small intestine.

    • Km: Not specified

    • Insulin responsive: No

    • Role: Primarily a fructose transporter.

Glycolysis Overview

  • Pathway: Converts 1 glucose to 2 pyruvate.

    • Pyruvate fate:

      • Anaerobic: Lactate or ethanol.

      • Aerobic: Oxidized to Acetyl CoA for further oxidation in citric acid cycle.

  • Energy Yield: More ATP is generated through the citric acid cycle.

Net Reaction of Glycolysis

  • 2 ATP produced along with 2 NADH from:

    • Glucose + 2 ADP + 2 NAD+ + 2 Pi → 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

Stages of Glycolysis

  • Hexose Stage: Two ATP utilized per glucose.

  • Triose Stage: Four ATP generated per glucose.

  • Overall Net ATP: 2 ATP produced.

Key Enzymatic Steps in Glycolysis

  1. Hexokinase: Converts glucose to G6P, requires Mg2+.

    • Broad substrate specificity.

  2. Glucose-6-phosphate Isomerase: Converts G6P to F6P, stereospecific binding.

  3. Phosphofructokinase-1 (PFK-1): Catalyzes irreversible step, key regulatory point in glycolysis.

    • Allosteric regulation by ATP and AMP.

  4. Aldolase: Cleaves F1,6BP into GAP and DHAP; near-equilibrium.

  5. Triose Phosphate Isomerase (TPI): Rapid conversion of DHAP to GAP.

  6. Glyceraldehyde 3-Phosphate Dehydrogenase (GAPDH): Converts GAP to 1,3BPG with reduction of NAD+ to NADH.

  7. Phosphoglycerate Kinase (PGK): Transfers phosphate from 1,3BPG to ADP yielding ATP.

  8. Phosphoglycerate Mutase: Transfers phosphate within the substrate without requiring ATP.

  9. Enolase: Converts 2PG to PEP by dehydration.

  10. Pyruvate Kinase (PK): Catalyzes the last ATP-generating step, metabolically irreversible.

Fate of Pyruvate

  1. Aerobic Conditions: Oxidized to Acetyl CoA for citric acid cycle.

  2. Anaerobic Conditions: Converted to ethanol (microorganisms) or lactate (muscles, RBC).

Regulation of Glycolysis

  • Glycolysis is activated when ATP is needed, while its activity decreases when ATP levels are sufficient.

  • Key regulatory enzymes include:

    • Hexokinase/Glucokinase

    • Phosphofructokinase-1 (PFK-1)

    • Pyruvate kinase (PK)

Summary of Enzyme Regulation

  1. Hexokinase: Feedback inhibition by G6P.

  2. PFK-1: Requires allosteric activators (AMP) and is inhibited by ATP and citrate.

  3. Pyruvate Kinase: Regulated by allosteric effectors and hormonal signals (Insulin and Glucagon).

Other Sugars in Glycolysis

  • Fructose: Converted to Glyceraldehyde 3-Phosphate.

  • Galactose: Converted to Glucose 6-Phosphate.

  • Mannose: Converted to Fructose 6-Phosphate.

Questions for Review

  • Name the glucose carrier proteins and their specifics regarding tissue location and insulin responsiveness.

  • What are the pathways for pyruvate conversion under different conditions?

  • Define the hormonal regulators of glycolysis and their impact on key enzymes.