Chapter+15+Slides

Chapter 15: Glucose Catabolism

Overview of Glycolysis

  • Glycolysis:

    • Breakdown of glucose to pyruvate.

    • Free energy released used to synthesize ATP from ADP and Pi.

    • 10-reaction sequence divided into two stages:

      • Energy Investment: ATP is consumed.

      • Energy Recovery: ATP is produced.

Stage I : Energy Consumption/Investment

  • ATP Utilization:

    • 2 ATPs used in Stage I for phosphorylating glucose molecules.

    • Prepares glucose for breakdown.

  • Electron Loss:

    • Electrons lost through glucose oxidation.

    • Resulting electrons reduce NAD+ to NADH.

Stage II: Energy Generation/Energy Recovery

  • ATP Recovery:

    • 4 ATPs recovered in Stage II.

    • Results in a net gain of 2 ATPs.

  • Net Process:

    • Conversion of glucose's oxidation state from 0 to pyruvate's oxidation state of +2.

    • Synthesis of 2 NADH necessary.

Key Enzymes and Mechanisms in Glycolysis

  • Hexokinase:

    • Enzyme catalyzing the phosphorylation of glucose.

    • Transfers gamma phosphate from ATP to carbon six of glucose.

    • Magnesium needed to stabilize triphosphates such as ATP.

Hydrolysis Energetics

  • ATP hydrolysis is more energetically favorable than glucose phosphorylation.

  • The enzyme's active site prevents water from associating with the ATP, enhancing the reaction.

Conformational Changes of Hexokinase

  • Glucose induces large conformational changes in hexokinase.

  • This movement facilitates the attachment of ATP to glucose without the interference of water molecules.

Aldose and Enzyme Reactions

  • Step 1 of Glycolysis: Substrate binding initiates enzyme reactions.

  • Aldol Cleavage: Forming DHAP and GAP in reaction 4, these products are interconvertible.

  • Phosphofructokinase (PFK):

    • Central regulator of glycolysis.

    • Catalyzes nucleophilic attack by ATP on F6P, producing FBP.

    • Plays a critical role due to its negative ΔG, indicating high regulation.

Fermentation Processes

  • NADH Reoxidation:

    • Must occur for glycolysis to continue.

    • In muscles: Pyruvate reduced to lactate.

    • In yeast: Pyruvate decarboxylated to CO2 and ethanol (requires TPP).

Aerobic vs. Anaerobic Conditions

  • Aerobic: Pyruvate completely oxidized to CO2 in the TCA cycle yielding 32 ATP.

  • Anaerobic (Muscle): Converted to lactate to regenerate NAD+ and sustain high ATP demand.

Enzymatic Mechanisms in Anaerobic Glycolysis

  • Lactate Dehydrogenase (LDH):

    • Catalyzes the transfer of a hydride from NADH to pyruvate, facilitating lactate formation.

    • Reaction can also reverse, generating pyruvate for gluconeogenesis.

  • Ethanol Production in Yeast:

    • Involves decarboxylation of pyruvate to acetaldehyde, followed by NADH reduction to ethanol.

Regulation of Glycolysis

  • Key Regulatory Points: Reactions by hexokinase, PFK, and pyruvate kinase are candidates for flux control.

    • PFK: Allosterically inhibited by ATP and activated by AMP/ADP.

  • Substrate Cycling:

    • Allows rapid responses to changing metabolic needs, involving two different enzymes to control forward and reverse reactions.

Metabolism of Hexoses Other Than Glucose

  • Fructose:

    • Bypasses PFK regulation in the liver; fast metabolism can lead to lipid synthesis.

  • Galactose: Phosphorylated and converted into intermediates for glycolysis.

  • Mannose: Similar pathways converting it through phosphorylation to glycolytic intermediates.

Pentose Phosphate Pathway

  • Alternatives to glycolysis serving for NADPH production and nucleotide synthesis.

  • G6P Dehydrogenase: Key regulatory reaction, controlling NADPH production rates.

  • Transketolase and Transaldolase: Convert pentose phosphate products back to glycolytic intermediates based on cellular needs.

Conclusion

  • In conclusion, glycolysis is a critical metabolic pathway for glucose breakdown and ATP production. Understanding regulation, enzyme mechanisms, and alternative pathways provides a comprehensive view of cellular energy metabolism.