Chapter 7 pt. 2 (March 5, 2025)

Overview of Glycolysis and Subsequent Pathways

• Glycolysis converts one glucose molecule (6 carbons) into two pyruvate molecules (3 carbons each) through 10 enzymatic reactions.

Glycolysis Steps

Energy Investment Phase

• First Five Reactions:

  • Consume 2 ATP to phosphorylate glucose, making it more reactive.

  • Aids in capturing the sugar within the cell and enhancing its chemical reactivity.

  • Phosphorylation facilitates weak interactions with enzymes, lowering the activation energy barrier.

Energy Payoff Phase

• Second Five Reactions:

  • Produce a total of 4 ATP via substrate-level phosphorylation, resulting in a net gain of 2 ATP (4 produced - 2 consumed).

  • Also produce 2 NADH, which carry high-energy electrons to be later used in the electron transport chain for ATP synthesis.

Substrate-Level Phosphorylation

• Involves an enzyme's active site converting ADP and a phosphoryl donor molecule into ATP.

• The phosphoryl donor must have higher free energy than ATP to facilitate the transfer of the phosphate group.

• Key donor molecules mentioned: bisphosphoglycerate and phospho-glutamate.

Pyruvate Dehydrogenase Complex

• Converts pyruvate into acetyl CoA before entering the Krebs cycle:

  • Composed of three enzymes that facilitate:

    • Decarboxylation of pyruvate (release of CO2).

    • Link the remaining two carbons to CoA to form acetyl CoA.

    • Reduction of NAD+ to NADH, harvesting high-energy electrons.

Krebs Cycle (Citric Acid Cycle)

• Overview:

  • Comprises eight reactions, four of which are redox reactions.

  • Each turn generates 3 NADH, 1 FADH2, and 1 ATP (or GTP).

• Each acetyl CoA that enters the Krebs cycle aids in the generation of activated carriers, which are critical for ATP production in the electron transport chain.

Electron Transport Chain

• Located in inner mitochondrial membrane and is responsible for generating a proton motive force:

  • Proton Pumps: Complexes I, III, and IV pump protons into intermembrane space as electrons pass through them.

  • End of the chain features molecular oxygen, which acts as the final electron acceptor, forming water and driving the entire process.

ATP Synthase Mechanism

• Uses the proton gradient established by the electron transport chain to synthesize ATP:

  • Protons flow back into the mitochondrial matrix through ATP synthase, producing ATP from ADP and inorganic phosphate.

  • The process operates via mechanical rotation driven by proton flow.

Yield from Complete Oxidation of Glucose

• Theoretical maximum yield is 32 ATP per glucose molecule under aerobic conditions.

• Total includes contributions from glycolysis, Krebs cycle, and electron transport chain.

Anaerobic Conditions and Fermentation

• Without oxygen, organisms must regenerate NAD+ through fermentation processes:

  • In muscle cells, pyruvate is converted to lactate, allowing for minimal ATP production and lactate accumulation leading to muscle burn.

  • In yeast, pyruvate is decarboxylated to form acetaldehyde, which is then reduced to ethanol, allowing glycolysis to continue.

Key Concepts to Remember

• Importance of phosphorylation in glycolysis.

• Roles of NADH and ATP in energy conservation and generation.

• Sequential steps of oxidation and reduction during Krebs cycle and electron transport chain.

• Mechanism of ATP synthesis from proton motive force.