Chapter 17: (PPT) Pyruvate Dehydrogenase and the Citric Acid Cycle

Introduction to the Citric Acid Cycle

• The citric acid cycle (also known as the tricarboxylic acid or TCA cycle, or Krebs cycle) is a series of oxidation-reduction reactions.

• It results in the oxidation of an acetyl group to two molecules of CO2, serving as the final pathway for the oxidation of fuel molecules.

• Critical for generating high-energy electrons that power ATP synthesis and provide key biosynthetic precursors.

Acetyl CoA

• Most fuel molecules enter the cycle as acetyl CoA (acetyl coenzyme A).

• Structure: Contains a sulfur atom bonded to an acetyl group, which is critical for its role in metabolism.

Pyruvate Dehydrogenase Complex

• The pyruvate dehydrogenase complex is a large enzyme complex that:

  • Converts pyruvate into acetyl CoA under aerobic conditions.

  • Takes place in the mitochondrial matrix, linking glycolysis to the citric acid cycle.

Overview of the Citric Acid Cycle

• The citric acid cycle extracts electrons from acetyl CoA to reduce NAD+ and FAD, forming NADH and FADH2.

Electron-Transport Chain (ETC)

• The ETC consists of membrane proteins that facilitate the flow of electrons from NADH and FADH2, leading to the generation of a proton gradient across the inner mitochondrial membrane.

• This gradient powers ATP synthase, converting ADP and inorganic phosphate into ATP.

Steps of the Pyruvate to Acetyl CoA Conversion

1. Coupled Steps: The decarboxylation of pyruvate drives formation of NADH and acetyl CoA.

2. Cofactors Used:

   • Catalytic cofactors: thiamine pyrophosphate (TPP), lipoic acid, FAD.

   • Stoichiometric cofactors: CoA, NAD+.

The Citric Acid Cycle Reactions

1. Formation of Citrate: Catalyzed by citrate synthase which adds acetyl CoA to oxaloacetate.

   • Produces citryl CoA, facilitating the aldol addition and hydrolysis reaction.

2. Isomerization of Citrate:

   • Catalyzed by aconitase, converting citrate to isocitrate through dehydration and hydration steps.

3. Oxidation of Isocitrate:

   • Isocitrate dehydrogenase converts isocitrate to α-ketoglutarate, producing NADH and releasing CO2.

4. Decarboxylation of α-Ketoglutarate:

   • Catalyzed by the α-ketoglutarate dehydrogenase complex, producing succinyl CoA and NADH.

5. Formation of Succinate:

   • Succinyl CoA synthetase cleaves succinyl CoA to succinate, generating ATP or GTP in the process.

6. Oxidation of Succinate:

   • Succinate dehydrogenase catalyzes conversion to fumarate, generating FADH2.

7. Hydration of Fumarate:

   • Catalyzed by fumarase, resulting in the formation of malate.

8. Oxidation of Malate:

   • Malate dehydrogenase catalyzes the final step, converting malate back to oxaloacetate while generating more NADH.

Regulation of the Citric Acid Cycle

• Entry into the cycle and its metabolic pathway is tightly regulated through allosteric mechanisms and reversible phosphorylation, particularly at key enzymes:

  • Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase regulate cycling rates.

• High concentrations of products (acetyl CoA, NADH, ATP) signal to reduce flux through the cycle.

Biosynthetic Role of the Citric Acid Cycle

• The cycle integrates a variety of biosynthetic pathways, serving as a source of precursors for metabolic products like amino acids and fatty acids.

• Anaplerotic reactions, such as the carboxylation of pyruvate to oxaloacetate, are crucial for replenishing cycle intermediates when they are diverted for biosyntheses.

Conclusion

• The citric acid cycle plays a critical role in cellular respiration and energy metabolism, serving as a major pathway connecting various metabolic processes and providing key intermediates for biosynthesis.

REVIEW GUIDE:

• Understand the acetyl coA’s entrance into the citric acid cycle, pyruvate decarboxylase

• Structure of the mitochondria- where do things take place? Outer membrane, inner membrane, intermembrane space, matrix

• How does glycolysis connect to the citric acid cycle? (Slide 10)

• How does citric acid connect to the electron transport chain? (Slide 8)

• Know overall conversion of pyruvate to acetyl coA; don’t need to know the intermediate steps (slide 11)

• Know the steps of the citric acid cycle- substrates, enzymes, where is NADH and FADH2 created? Slide 28 shows the complete cycle. Where is ATP or GTP made? You can skip mechanism of action slides 14, 20, 21

• What is the net scorecard for the citric acid cycle? (Slide 26)

• What controls balance of the citric acid cycle? (Slides 29-31, but you can skip 32), know regulation under biological conditions (slide 33)

• Skip slides 35 and 36 (we will see more about these in other chapters)

• How is the citric acid cycle replenished if various components are consumed for other pathways? (slide 37)

REVIEW GUIDE ANSWERS + CONTENT FOR ANKI DECK

Acetyl CoA’s Entrance into the Citric Acid Cycle

• Acetyl CoA enters the citric acid cycle through its combination with oxaloacetate to form citrate. This reaction is catalyzed by citrate synthase.

• The pyruvate dehydrogenase complex converts pyruvate into acetyl CoA in the mitochondrial matrix, linking glycolysis to the citric acid cycle.\n

Structure of the Mitochondria

• The mitochondria consist of several parts:

  • Outer Membrane: Contains porins that allow the passage of small molecules.

  • Inner Membrane: Highly folded into cristae; contains proteins involved in the electron transport chain and ATP synthesis.

  • Intermembrane Space: The space between the inner and outer membranes; involved in the electron transport chain.

  • Matrix: The innermost compartment; where the citric acid cycle and pyruvate decarboxylation occur.

Connection of Glycolysis to the Citric Acid Cycle

• Glycolysis, which occurs in the cytoplasm, converts glucose to pyruvate. Under aerobic conditions, pyruvate enters the mitochondria and is converted to acetyl CoA by the pyruvate dehydrogenase complex. This conversion links glycolysis to the citric acid cycle.

Connection of Citric Acid to the Electron Transport Chain

• The products of the citric acid cycle (NADH and FADH2) serve as high-energy electron carriers. These carry electrons to the electron transport chain located in the inner mitochondrial membrane, facilitating ATP production through oxidative phosphorylation.

Conversion of Pyruvate to Acetyl CoA

• Pyruvate is converted to acetyl CoA in a two-step process: decarboxylation (which releases CO2) and oxidation (which produces NADH). The net result is the formation of one molecule of acetyl CoA for each pyruvate.

Steps of the Citric Acid Cycle

1. Formation of Citrate: Acetyl CoA + oxaloacetate = citrate (catalyzed by citrate synthase).

2. Isomerization of Citrate: Citrate -> isocitrate (catalyzed by aconitase).

3. Oxidation of Isocitrate: Isocitrate -> α-ketoglutarate (catalyzed by isocitrate dehydrogenase); NADH is produced, CO2 is released.

4. Decarboxylation of α-Ketoglutarate: α-Ketoglutarate -> succinyl CoA (catalyzed by α-ketoglutarate dehydrogenase); NADH produced, CO2 released.

5. Formation of Succinate: Succinyl CoA -> succinate (catalyzed by succinyl CoA synthetase); ATP or GTP is produced.

6. Oxidation of Succinate: Succinate -> fumarate (catalyzed by succinate dehydrogenase); FADH2 is produced.

7. Hydration of Fumarate: Fumarate -> malate (catalyzed by fumarase).

8. Oxidation of Malate: Malate -> oxaloacetate (catalyzed by malate dehydrogenase); NADH is produced.

• NADH and FADH2 Creation: Occurs during step 3 (isocitrate to α-ketoglutarate), step 4 (α-ketoglutarate to succinyl CoA), step 6 (succinate to fumarate), and step 8 (malate to oxaloacetate).

Net Scorecard for the Citric Acid Cycle

• The overall yield from one acetyl CoA entering the cycle is:

  • 3 NADH

  • 1 FADH2

  • 1 ATP (or GTP)

  • 2 CO2 emitted.

Regulation of the Citric Acid Cycle

• The cycle is regulated by the availability of substrates and product concentrations, especially for key enzymes such as isocitrate dehydrogenase and α-ketoglutarate dehydrogenase.

• High levels of ATP, NADH, or acetyl CoA signal a reduction in the cycle's activity, while low levels indicate a need for a more active cycle.

Replenishment of the Citric Acid Cycle

• The cycle can be replenished through anaplerotic reactions, such as the carboxylation of pyruvate to oxaloacetate, ensuring that intermediates remain available despite being consumed for biosynthesis.