Glycolysis and Acetyl CoA Formation Study Notes

Glycolysis and the Citric Acid Cycle

Glycolysis

• Definition: The metabolic pathway that converts glucose to pyruvate.

• Location: Occurs in the cytoplasm.

Key Steps:

• Input: 1 molecule of Glucose.

• Output: 2 molecules of Pyruvate.

• Energy Yield: 2 ATP produced or GTP.

• Byproducts: 2 CO2 and high-transfer potential electrons are generated, which are used in the electron transport chain for further ATP production.

Citric Acid Cycle

• Definition: The metabolic pathway that utilizes Acetyl CoA to produce energy through the oxidation of acetyl groups.

• Location: Occurs in the mitochondrial matrix.

• Entry of Acetyl CoA: Acetyl CoA enters the citric acid cycle after glycolysis has occurred.

Key Elements of the Cycle:

• Acetyl CoA: Formed from pyruvate by the Pyruvate Dehydrogenase complex.

• Four-carbon acceptor: Combines with acetyl group from Acetyl CoA to form a six-carbon compound.

• End products: 2 CO2, ATP or GTP, and high-energy electrons are released.

Synthesis of Acetyl CoA by Pyruvate Dehydrogenase

• Process Overview: Conversion of Pyruvate to Acetyl CoA involves three main steps followed by a restoration step for the enzyme.

• Steps Involved:

  1. Decarboxylation - Removal of CO2.

  2. Oxidation - Transfer of electrons resulting in the formation of NADH.

  3. Transfer Acetyl to CoA - Formation of Acetyl CoA.

  4. Enzyme Reoxidation - Restores the enzyme to its original state for another cycle of activity.

Redox Reaction in Acetyl CoA Synthesis

• Definition: A redox reaction involves the transfer of electrons. Here, the reduction of NAD+ to NADH and oxidation of Pyruvate occurs, losing CO2.

Pyruvate Dehydrogenase Complex (PDH)

• Key Role: PDH catalyzes the conversion of Pyruvate to Acetyl CoA.

• Location: Found in the mitochondrial matrix.

• Function: Links glycolysis to the citric acid cycle through an irreversible enzymatic complex with three primary activities:

  1. Decarboxylation of the substrate (Pyruvate).

  2. Oxidation of the intermediate product.

  3. Transacetylase action transferring the product to CoA.

• Overall Reaction (when Pyruvate reacts with CoA and NAD+):
  $ext{Pyruvate} + ext{CoA} + ext{NAD}^+ <br>ightarrow ext{Acetyl CoA} + ext{CO}_2 + ext{NADH} + ext{H}^+$

Detailed Steps of Acetyl CoA Formation

1. Close-Up on the Decarboxylation Step

• Enzyme Involved: E1 (pyruvate dehydrogenase component).

• Coenzyme: Thiamin pyrophosphate (TPP) is utilized as a prosthetic group.

  • Vitamin B1 is an essential part of TPP which facilitates the reaction.

• Reaction: Pyruvate undergoes decarboxylation, resulting in the release of CO2 and the formation of hydroxyethyl-TPP.

2. Close-Up on the Oxidation Step

• Involvement of E1: Transfers hydroxyethyl-TPP to E2 (dihydrolipoyl transacetylase).

• Coenzyme: Lipoic acid, another prosthetic group.

• Detail: As hydroxyethyl is transferred, it gets oxidized creating an acetyl group and reducing lipoamide's disulfide bond.

3. Close-Up on the Transfer to CoA Step

• Enzyme: E2 transfers the acetyl group from acetyl-lipoamide to Coenzyme A, forming Acetyl CoA.

• Coenzyme: CoA, which is derived from Pantothenic acid (Vitamin B5).

• Reaction Product: Results in Acetyl CoA and dihydrolipoamide.

Fate of Acetyl CoA

• Reversibility: The formation of Acetyl CoA from Pyruvate is irreversible, indicating a key metabolic crossroads.

  • Implication: This is a main reason why animals cannot produce glucose from fatty acids because they cannot revert Acetyl CoA to glucose.

• Main Pathways for Acetyl CoA:

  • Citric Acid Cycle: Primarily utilized for energy production.

  • Lipid Synthesis: Can be diverted for synthesis and storage of fats from excess carbohydrates.

Summary of Acetyl CoA Pathways:

• Input: Acetyl CoA from lore glycolysis and Pyruvate dehydrogenase complex.

• Processes: Citric Acid Cycle & Fatty Acid Synthesis.

Conclusion

• The understanding of glycolysis and the citric acid cycle, including the conversion of Pyruvate to Acetyl CoA, underscores the intricate workings of metabolic pathways in cellular respiration.