Krebs Cycle

Exercise Science 223: Aerobic Metabolism of Glucose

Introduction

• Presented by Dr. Ray Thompson

• Focus on aerobic metabolism of glucose

Glycolysis Recap

• Pyruvate molecule has two possible fates:

  • Anaerobic: Produces lactate to recycle NAD+

  • Aerobic: Transports pyruvate into mitochondria along with hydrogens and electrons from glycolysis

• Glycolysis Overview:

  • Splitting of glucose into two glyceraldehyde-3-phosphate (G3P) molecules

  • Production of pyruvate through substrate-level phosphorylation

Turning Pyruvate into Acetyl CoA

• Pyruvate Dehydrogenase: Converts pyruvate to acetyl CoA

  • Electrons from G3P are transferred into mitochondria

  • Produced NADH helps to recycle NAD in the cytosol

The Krebs Cycle

• Key Ingredients for Krebs Cycle:

  • Acetyl CoA: Starting material from pyruvate

  • Oxaloacetate: Four-carbon molecule needed to initiate the cycle

• First step of Krebs Cycle:

  • Acetyl CoA reacts with oxaloacetate to produce citrate, catalyzed by citrate synthase

  • Coenzyme A is cleaved off and recycled

Energy Extraction in the Krebs Cycle

• Initial Steps:

  • Citrate undergoes reactions to remove carbons and regenerate NADH

  • First carbon is removed as CO2, transitioning from a six-carbon to a five-carbon molecule.

• Second Step:

  • Second CO2 is released; five-carbon reduced to a four-carbon molecule.

  • No ATP produced yet, focus remains on producing NADH for electron transport.

Substrate-Level Phosphorylation

• One occurrence in the Krebs cycle:

  • ATP produced directly without electron transfer.

  • Precise moment of phosphorylation occurs while retaining the four-carbon structure.

Energy Carriers and Final Steps

• Additional hydrogen and electrons are stripped off, involved in the creation of:

  • FADH2: New electron carrier produced in one step.

  • Another reaction regenerates NADH with the formation of oxaloacetate.

• Krebs cycle is cyclic:

  • Returns to oxaloacetate to join with new acetyl CoA.

Importance of the Krebs Cycle

• Significant for energy harvesting:

  • Electrons (from NADH and FADH2) are utilized in the electron transport chain to generate ATP.

• Although the Krebs cycle does not directly involve oxygen, it is classified as an aerobic process since it depends on prior oxygen availability.

Summary Points for Exam

• Know starting and ending materials of the Krebs cycle.

• Understand the function of NADH and FADH2 as energy carriers.

• Recognize the relationship between Krebs cycle intermediates and ATP production.

• Differentiate between the linear pathway of glycolysis and the cyclic nature of the Krebs cycle.

• Understand the Krebs cycle’s role in aerobic metabolism and energy production.