MBB 321 Glycolysis Part 3: Phosphoryl Compounds

MBB 321 Glycolysis Part 3: Phosphoryl Compounds

Sections Overview

• Section 14.1 (pp. 511-521): Introduction to glycolysis and roles of phosphoryl compounds.

• Section 14.2 (pp. 521-523): Examination of kinase reactions in glycolysis.

• Section 14.3 (pp. 525-530): Discussion of ATP production and phosphoryl group dynamics.

Importance of Kinase Reactions

• Some kinase reactions can proceed in reverse to build ATP instead of hydrolyzing it.

• Reversed Kinase Reaction:

  • General Reaction: ATP synthesis from X-P + ADP.

  • Example from glycolysis:

  • Phosphoglycerate Kinase (Reaction 7)

  • Pyruvate Kinase (Reaction 10).

• For compounds X in these reactions, the forward direction involving ATP hydrolysis is endergonic, indicated by ext{ΔG}'° >> 0.

• The efficacy of the reaction relies upon the structure of X compared to X-P.

Definition of Kinase Reaction

• A kinase reaction is defined as:

  • $X + ext{ATP} \rightarrow X-P + ext{ADP}$.

• The forward direction is typically exergonic (where ext{ΔG}'° < 0) and is crucial during the phosphate tagging steps in the investment phase of glycolysis.

Specific Kinase Reactions in Glycolysis

• Investment Phase:

  • Rxn 1 - Hexokinase:

  • Uses 1 ATP to phosphorylate glucose at the investment phase.

  • Rxn 3 - Phosphofructokinase-1:

  • Uses 1 ATP.

• Payoff Phase:

  • Rxn 7 - Phosphoglycerate Kinase:

  • Transfers phosphate from 1,3-bisphosphoglycerate (1,3-BPG) to ADP, forming ATP.

  • Rxn 10 - Pyruvate Kinase:

  • Transfers phosphate from phosphoenolpyruvate (PEP) to ADP, generating ATP.

Phosphate Tagging and Energy Input/Output in Glycolysis

• Net Reaction for ATP Investment with Lactate Fermentation:

  • $ext{Glucose} + 2 ext{Pi} + 2 ext{ADP} \rightarrow 2 ext{Lactate} + 2 ext{ATP}$.

• ATP Investment & Payoff

  • Four kinase enzymes are involved in ATP investment, payoff, and recovery.

  • ATP tags facilitate the conversion of glucose into high-energy intermediates.

Energy Yield from Hydrolysis

• Types of Hydrolysis:

  • Anhydride Hydrolysis:

  • High-energy yield indicated by ext{ΔG}'° << 0 (approximately -7.6 kcal/mol).

  • Ester Hydrolysis:

  • Moderate energy yield indicated by ext{ΔG}'° < 0 (approximately -3.5 kcal/mol).

• Hydrolysis for acids involves exchanging one hydroxyl for another with $ext{ΔG}'° = 0$.

Structural Factors Influencing Phosphoryl Hydrolysis

• Structural Influence on Hydrolysis:

  • Phosphate groups in X stabilizes through resonance, whereas glucose does not exhibit resonance changes.

  • Hydrolysis yields different energy based on compound structure:

  • ATP has charge repulsion which may contribute to energy release; conversely, glucose 6-phosphate does not.

Hydrolysis Calculations

• Hydrolysis Energy Values:

  • $ext{ΔG}'° ext{ for Glc6P} ext{ hydrolysis} = -3.3 ext{ kcal/mol}$.

  • $ext{ΔG}'° ext{ for ATP hydrolysis} = -7.3 ext{ kcal/mol}$.

Role of Hexokinase and Phosphofructokinase-1 in Glycolysis

• Hexokinase Reaction:

  • Converts ATP to glucose-6-phosphate, transforming a high-energy bond into a lower-energy bond via the formation of a phosphoester bond.

  • The resulting energy enables further ATP production through subsequent oxidation steps.

  • Energetics:

  • $ext{ΔG}'° ext{ before Rxn 1} = +3.3 ext{ kcal/mol}, ext{ after = -7.3 kcal/mol}$.

ATP Payoff Phase in Glycolysis

• Phosphoglycerate Kinase (Reaction 7):

  • Transfers a phosphoryl from 1,3-bisphosphoglycerate to ADP, yielding ATP, with a net ext{ΔG}'° < 0 (approximately -11.8 kcal/mol), making the reaction highly favorable.

• Pyruvate Kinase (Reaction 10):

  • Transfers phosphoryl group from PEP to ADP, yielding ATP and pyruvate, with $ext{ΔG}'° ≈ -14.8 ext{ kcal/mol}$.

Understanding High-Energy vs Low-Energy Phosphoryl Derivatives

• Energy Storage Dynamics:

  • Phosphate tags engage in transformations that impact energy availibility between ATP and other phosphoryl compounds, pushing ATP synthesis and creating a gradient in ATP production during glycolysis.

Mechanisms Regenerating NAD⁺ During Anaerobic Metabolism

• GAPDH Reaction (Reaction 6):

  • Produces NADH from NAD⁺; essential for glycolytic operation.

• Fermentation:

  • NADH is re-oxidized to produce NAD⁺, facilitating glycolysis to continue.

  • In muscle cells, this involves reduction of pyruvate to lactate by lactate dehydrogenase, thereby regenerating NAD⁺.

Reaction Dynamics Highlighting Tautomerization and Hydrolysis

• PEP Tautomerization:

  • PEP exhibits unusual phosphate ester energy, making it capable of facilitating ATP production in glycolysis.

  • Enol-pyruvate transformation leads to a lower energy state after phosphorylation, stabilizing pyruvate.

• Final ATP Payoff Calculations:

  • Overall ATP synthesis reactions yield energy through considerations of reaction directionality and the structural energy of participating compounds.

Summary Insights: Glycolysis and Energy Content

• The energy available from phosphoryl compounds must be compared to the products derived from their phosphorylation reactions, adhering to thermodynamic principles.

• Glycolysis systematically constructs high-energy phosphoryl stores that subsequently allow for ATP synthesis through forward and reverse kinase reactions, revealing an intricate balance of energy dynamics across metabolic pathways.

Learning Objectives

• Understand the directionality of kinase reactions and which factors influence their forward/reverse pathway in cellular environments.

• Recognize various classes of phosphoryl compounds and distinguish their energy profiles via structural and thermodynamic analysis.

• Contrast the energy investments vs. payoffs during glycolysis, emphasizing how low-energy phosphoesters and high-energy intermediates participate in respective phases.

• Explain the regeneration of NAD⁺ in cells during anaerobic conditions and detail the mechanisms involved in GAPDH steps.

• Rationalize the separation of aldehyde oxidation and phosphorylation within glycolysis and outline the biochemical rationale motivating this coupling.