Transition Step in Cellular Respiration

Overview

• Continuation from glycolysis: formation and fate of pyruvate.
• Focus on the transition step/preparation phase.

Pyruvate and Oxygen

• Pyruvate is formed as a 3-carbon molecule from glycolysis.
• Transport into mitochondria requires oxygen (aerobic conditions).
• Oxygen allows NADH from glycolysis to donate hydride ions to the electron transport chain.
  • If no oxygen is present, NADH converts pyruvate to lactic acid instead.

Pyruvate Transport and Conversion

• Pyruvate must be transported from the cytosol to the mitochondria.
• Enzyme involved: Pyruvate Dehydrogenase.
• Conversion involves:
  • Pyruvate (3-carbons) to Acetyl CoA (2-carbons).
  • Loss of a carbon as CO2 (decarboxylation).
  • Addition of Coenzyme A (CoA).
  • Conversion of NAD+ to NADH by transferring hydride ions.

Carbon and Molecule Balancing

• From 2 pyruvate molecules (end product of glycolysis):
  • Acetyl CoA: 2 molecules formed.
  • CO2: 2 molecules generated (1 from each pyruvate).
  • NADH: 2 molecules formed.
  • CoA: 2 CoA molecules added.

Pyruvate Dehydrogenase (PDH)

• Key enzyme catalyzing the reaction.
• Irreversible step: one-directional conversion from pyruvate to Acetyl CoA.
  • Cannot convert back to pyruvate.

Summary of the Transition State

• Pyruvate enters the mitochondria under aerobic conditions.
• Enzymatic action by Pyruvate Dehydrogenase results in:
  • Loss of CO2.
  • Formation of Acetyl CoA.
  • Generation of NADH.
• Prepares Acetyl CoA for entry into the Krebs cycle.

Next Steps

• Detailed mechanism and regulation of this process will be covered in the next video.