Glycolysis

ATP and Reaction Equilibrium

  • Reaction: ATP + Fructose → ADP + Fructose-6-phosphate

  • Given Free Energy Values:

    • AG (ATP to ADP + Pi) = -30.5 kJ/mol

    • AG (Fructose to Fructose-6-phosphate + Pi) = +15.9 kJ/mol

  • Calculation of Equilibrium Constant (Keq):

    1. Sum the free energy values:

      • AG = -30.5 kJ/mol + 15.9 kJ/mol = -14.6 kJ/mol

    2. Use equation:

      • AG° = RT ln(Keq)

      • -14,600 J/mol = (-8.315 J/mol K)(298 K)(2.303) log(Keq)

      • Rearranging gives:

      • log(Keq) = 2.6

      • Keq ≈ 398.1

Metabolism of Glucose

  • Glucose Catabolism:

    • Converts glucose into pyruvate via glycolysis.

    • Glycolysis occurs under both aerobic and anaerobic conditions.

  • Glucose Anabolism:

    • Converts pyruvate back into glucose via gluconeogenesis.

Preparatory and Payoff Phases of Glycolysis

Preparatory Phase

  • Key Steps:

    • Phosphorylation of glucose to Glucose-6-phosphate via hexokinase.

    • Formation of Fructose-6-phosphate from Glucose-6-phosphate via phosphohexose isomerase.

    • Conversion to Fructose-1,6-bisphosphate via phosphofructokinase-1 (PFK).

Payoff Phase

  • Key Steps:

    • Conversion of Glyceraldehyde 3-phosphate to pyruvate; coupled formation of ATP and NADH.

    • 2 ATP produced per glucose in substrate-level phosphorylation.

Functional Group Comparison

  • Most Reduced Functional Group: a) Aldehyde b) Alcohol c) Carboxylic Acid

Common Cofactors in Oxidation

  • Commonly Produced Cofactor in Oxidation of Carbon-Carbon Double Bonds: a) NAD+ b) NADH c) FAD d) FADH2

Reaction Overview – Glucose to Glucose-6-Phosphate

  • Reaction: Glucose + ATP → Glucose-6-phosphate + ADP

  • Enzyme: Hexokinase

  • ΔG’ = -16.7 kJ/mol

Reaction Overview – Glucose-6-Phosphate to Fructose-6-Phosphate

  • Reaction: Glucose-6-phosphate to Fructose-6-phosphate

  • Enzyme: Phosphohexose Isomerase

  • ΔG° = 1.7 kJ/mol

Regulation in Glycolysis

  • Reaction: Fructose-6-phosphate to Fructose-1,6-bisphosphate

  • Importance: Major regulatory step in glycolysis. PFK activity is ATP-dependent.

Endergonic Reactions in Glycolysis

  • Reaction: Breakdown of Fructose-1,6-bisphosphate into Dihydroxyacetone phosphate and Glyceraldehyde 3-phosphate.

  • Condition for Occurrence: Requires consideration of energy input or coupling reactions.

Isomerization of Dihydroxyacetone Phosphate

  • Reaction: Conversion from Dihydroxyacetone phosphate to Glyceraldehyde 3-phosphate.

  • Remains in equilibrium with constant depletion moving right.

Net Yield of Glycolysis

  • Net Reactions:

    • Glucose + ATP ➔ Glucose-6-P + ADP

    • Glucose-6-P ➔ Fructose-6-P

    • Fructose-6-P + ATP ➔ Fructose-1,6-bisP + ADP

    • Overall: Yields 2 Glyceraldehyde 3-P from glucose + 2 ATP (net loss of energy so far).

Oxidation of Glyceraldehyde 3-Phosphate

  • Reaction: Glyceraldehyde 3-phosphate to 1,3-Bisphosphoglycerate

  • Enzyme: Glyceraldehyde 3-phosphate dehydrogenase

  • ΔG' = 6.3 kJ/mol

Phosphoryl Transfer Reaction

  • Reaction: 1,3-Bisphosphoglycerate + ADP → 3-Phosphoglycerate + ATP

  • Enzyme: Phosphoglycerate kinase

  • ΔG'° = -18.5 kJ/mol (substrate-level phosphorylation).

Conversion of 3-Phosphoglycerate

  • Reaction: 3-Phosphoglycerate to 2-Phosphoglycerate

  • ΔG° = +4.4 kJ/mol; Actual ΔG = ~0 due to rapid conversion.

Dehydration Reaction in Glycolysis

  • Reaction: 2-Phosphoglycerate to Phosphoenolpyruvate

  • ΔG° = 7.5 kJ/mol

Final Conversion to Pyruvate

  • Reaction: Phosphoenolpyruvate + ADP → Pyruvate + ATP

  • ΔG' = -31.4 kJ/mol

Summary of Yield at Glycolysis End

  • At the end of glycolysis:

    • Gained 2 ATP

    • Gained 2 NADH

    • 2 molecules of Pyruvate

Energetics of Glycolysis

  • Note on Energetics: ΔG can vary greatly from ΔGo’, influencing spontaneity in cellular reactions.

Fate of Pyruvate

  • In Aerobic Conditions: Utilizes O2 as the final electron acceptor.

  • In Anaerobic Conditions: Converts pyruvate into lactate; linked to fermentation processes.

Gluconeogenesis Overview

  • Process: Synthesis of glucose from pyruvate/lactate, primarily occurring in the liver.

NADH Production in Glycolysis

  • Enzyme Producing NADH:

    • a) Glyceraldehyde-3-phosphate dehydrogenase

    • b) Hexokinase

    • c) Enolase

    • d) Pyruvate kinase

Common Enzyme Cofactors

  • Commonly Required Cofactor for Phosphodiester Bonds:

    • a) Coenzyme A

    • b) TPP

    • c) Mg++

    • d) Flavin Mononucleotide

Gluconeogenesis vs Glycolysis

  • Reversal of Glycolysis: Many gluconeogenesis reactions mirror glycolysis. Irreversible steps require bypass mechanisms.

Bypass #1 for PEP Production

  • Process: Convert Pyruvate to PEP involves pyruvate carboxylase and PEP carboxykinase.

  • Requires: 1 ATP and 1 GTP.

Bypass Mechanisms in Gluconeogenesis

  • Bypass #2: Hydrolysis of Fructose-1,6-Bisphosphate to Fructose-6-Phosphate

    • Enzyme: Fructose bisphosphate bisphosphatase

  • Bypass #3: Hydrolysis of Glucose-6-Phosphate to Glucose

    • Enzyme: Glucose-6-phosphatase

Regulation of Glycolysis & Gluconeogenesis

  • Key Regulatory Enzyme: Phosphofructokinase (PFK-1)/Fructose-1,6-bisphosphatase

  • High ATP signals energy sufficiency; favors gluconeogenesis.

  • High AMP signals energy need; favors glycolysis.

Reconfirming Regulation Points

  • Major Regulatory Step: PFK-1 response to ATP and AMP levels, influencing glycolysis and gluconeogenesis pathways.

Fructose 2,6-bisphosphate Role in Metabolism

  • Metabolic Mediator:

    • High glucagon activates kinase, inactivating phosphofructokinase 2.

    • High insulin activates phosphofructokinase 2, increasing glycolysis.

Summary of Fructose 2,6-Bisphosphate Function

  • Effect of F-2,6-BP:

    • Activates PFK1, promoting glycolysis when glucose levels are high.