Module 3 Lecture 4 - Pyruvate Oxidation and the Fates of Glycolytic End Products

Overview of Glucose Metabolism and Glycolytic End Products

Goal of Glucose Metabolism: When cells receive glucose, it must be oxidized to produce energy in the form of Adenosine Triphosphate ( $ATP$ ).
The First Step: Glycolysis: * Glucose is a six-carbon molecule ( $6C$ ). * It is broken down into two three-carbon molecules called pyruvate ( $3C$ ). * The process results in the synthesis of $ATP$ and a high-energy electron-carrying substance called Nicotinamide Adenine Dinucleotide ( $NADH$ ). * Net Output of Glycolysis: * $2$ Pyruvate molecules. * $2$ $ATP$ molecules. * $2$ $NADH$ molecules.
Independence from Oxygen: Glycolysis does not depend on oxygen; glucose can be converted to pyruvate and synthesize energy without it.
The Fate of NADH and Pyruvate: * While $ATP$ is immediately usable, energy remains stored in $NADH$ and pyruvate. * If oxygen is present, these proceed to the next stage of energy production. * If oxygen is absent (during strenuous exercise or hypoxic conditions), $NADH$ cannot be passed to the next stage, leading to a potentially harmful accumulation of $NADH$ and pyruvate in the cells.

The Four Fates of Pyruvate

Fate One: Anaerobic Metabolism in Microorganisms: Pyruvate is metabolized under anaerobic conditions to form ethanol.
Fate Two: Anaerobic Metabolism in Humans and Animals: Pyruvate is converted into lactate when oxygen supply is insufficient to support aerobic oxidation.
Fate Three: Anaerobic Metabolism in Rumen and Soil Microbes: Pyruvate can form short-chain fatty acids (this fate is mentioned but not explored in depth in this lecture).
Fate Four: Aerobic Conditions: Pyruvate enters the mitochondria and proceeds through the Tricarboxylic Acid cycle ( $TCA$ ), also known as the Citric Acid cycle or Krebs cycle, followed by the electron transport chain ( $ETC$ ) to produce a high yield of $ATP$ .

Anaerobic Glycolysis: Fermentation Pathways

Definition of Fermentation: A general term for processes that extract energy as $ATP$ without consuming oxygen or changing the concentration of $NAD^{+}$ or $NADH$ .
Types of Fermentation: * Lactate Fermentation: Occurs in mammalian and human cells. * Ethanol Fermentation: Occurs in yeast, plants, and other microorganisms.

Lactate Fermentation (Humans and Animals)

Mechanism: Under insufficient oxygen supply, $NADH$ accumulates. To avoid cell death, the enzyme Lactate Dehydrogenase catalyzes the reduction of pyruvate into lactate.
NADH Recycling: During this reduction, $NADH$ donates an electron (and hydrogen) to become $NAD^{+}$ . This $NAD^{+}$ is recycled back into the glycolytic pathway to ensure it can continue.
Red Blood Cells (RBCs) Exception: $RBCs$ lack mitochondria. Consequently, they cannot oxidize pyruvate even under aerobic conditions and must produce lactate via glycolysis.
The Lactate-Glucose Cycle (Cori Cycle context): * Lactate formed in active skeletal muscle is not wasted; it is carried through the blood to the liver. * In the liver, lactate is converted back into glucose (gluconeogenesis) and returns to the bloodstream to act as a precursor for future glycolysis.
Clinical Implications: A deficiency in the enzyme Lactate Dehydrogenase constitutes a metabolic disorder. In such cases, the accumulation of pyruvate and $NADH$ during exercise can halt the glycolytic pathway entirely, preventing $ATP$ production.

Ethanol Fermentation (Yeast and Microorganisms)

Process Detail: Yeast and plants lack Lactate Dehydrogenase. Instead, they utilize a two-step process to handle pyruvate under anaerobic conditions. * Step One: Pyruvate ( $3C$ ) is converted into acetaldehyde ( $2C$ ) via the enzyme Pyruvate Decarboxylase, which removes a carbon atom as $CO_{2}$ . * Step Two: Acetaldehyde is converted into ethanol via the enzyme Alcohol Dehydrogenase. During this step, $NADH$ is converted to $NAD^{+}$ for recycling.
Waste vs. Recycling: Unlike lactate, ethanol is a waste product for the organism and is not recycled back into glucose.
Biological Utility: In microbiology, organisms may produce ethanol as a defense mechanism to kill competing bacteria.
Commercial Utility: This principle is the basis for brewing, wine production, and the fermentation of products like milk.

Comparison of Fermentation Pathways

Oxidative Decarboxylation: Transition to Aerobic Metabolism

Definition: The process by which pyruvate is oxidized to form Acetyl Coenzyme A ( $Acetyl\,CoA$ ) and $CO_{2}$ .
Irreversibility: This is an irreversible oxidation. Once pyruvate is converted to $Acetyl\,CoA$ , it must proceed toward the electron transport chain ( $ETC$ ).
The Reaction: A carboxyl group is removed from pyruvate ( $3C$ ) as $CO_{2}$ . The remaining two carbons become the acetyl group of $Acetyl\,CoA$ ( $2C$ ).
Location: While glycolysis occurs in the cytoplasm (cytosol), oxidative decarboxylation prepares the molecules for entry into the mitochondrial matrix.

The Pyruvate Dehydrogenase (PDH) Complex

The Enzyme Complex: This is a highly coordinated "enzyme complex" composed of three distinct enzymes that perform three different jobs consecutively: 1. $E_{1}$ : Pyruvate Dehydrogenase. 2. $E_{2}$ : Dihydrolipoyl Transacetylase. 3. $E_{3}$ : Dihydrolipoyl Dehydrogenase.
Coenzymes: The complex requires five coenzymes, many of which are derived from $B$ Vitamins: 1. Thiamine pyrophosphate ( $TPP$ ) (derived from Vitamin $B_{1}$ ). 2. Flavin adenine dinucleotide ( $FAD$ ) (derived from Vitamin $B_{2}$ ). 3. Coenzyme A ( $CoA$ ) (derived from Vitamin $B_{5}$ ). 4. Nicotinamide adenine dinucleotide ( $NAD$ ) (derived from Vitamin $B_{3}$ ). 5. Lipoate.
Efficiency: The complex structure allows substrates to move quickly between active sites, ensuring the three chemical reactions occur efficiently without intermediates drifting away.

Mitochondrial Membrane Dynamics and Shuttle Systems

The Permeability Barrier: Glycolysis occurs in the cytosol, but the $TCA$ cycle and $ETC$ occur in the mitochondria.
Impermeability: The inner mitochondrial membrane is highly impermeable to large, ionized molecules like $NADH$ and $NAD^{+}$ .
The Solution: Shuttle systems are required to carry the energy (electrons) sequestered in cytosolic $NADH$ into the mitochondria, even when the molecule itself cannot cross.

The Malate-Aspartate Shuttle System

Mechanism: 1. In the cytosol, Oxaloacetate accepts electrons from $NADH$ , converting it to Malate via Malate Dehydrogenase. This creates $NAD^{+}$ in the cytosol. 2. Malate can cross the inner mitochondrial membrane into the matrix. 3. Inside the matrix, Malate is re-oxidized back into Oxaloacetate, donating electrons to $NAD^{+}$ to recreate $NADH$ inside the mitochondria. 4. Because Oxaloacetate cannot cross back into the cytosol, it is converted into Aspartate by Aspartate Aminotransferase. 5. Aspartate crosses back into the cytosol, where it is converted back into Oxaloacetate to restart the cycle.
Reversibility: This system is readily reversible and depends on the ratio of $NADH$ to $NAD^{+}$ being higher in the cytosol than in the mitochondria.

The Glycerol 3-Phosphate Shuttle System

Mechanism: 1. Electrons from cytosolic $NADH$ are transferred to Dihydroxyacetone Phosphate ( $DHAP$ ), an intermediate from glycolysis. 2. This reduction forms Glycerol 3-Phosphate. 3. Glycerol 3-Phosphate transfers its electrons to $FAD$ within a membrane-bound protein, forming $FADH_{2}$ . 4. $FADH_{2}$ then transfers these electrons directly to Coenzyme Q ( $Ubiquinone$ ) in the electron transport chain.
Tissue Specificity: This shuttle is prominent in brain tissues and skeletal muscles.

Questions & Discussion

Question: "Three fates of pyruvate?"
Response: Anaerobic (Lactate in humans, Ethanol in yeast) and Aerobic (conversion to $Acetyl\,CoA$ for the Krebs cycle).
Question: "Can you explain the shuttle systems briefly?"
Response: $NADH$ is impermeable to the inner mitochondrial membrane. Shuttle systems like the Malate-Aspartate shuttle transfer electrons via intermediates (Malate) into the matrix where they are donated back to $NAD^{+}$ to form $NADH$ for the electron transport chain.
Question: "Is Alcohol Dehydrogenase found in humans?"
Response: No, it is false. It is found in yeast, plant cells, and microorganisms.
Question: "Where does the Krebs cycle take place?"
Response: In the mitochondrial matrix.
Question: "Why is the Pyruvate Dehydrogenase ( $PDH$ ) complex important?"
Response: It is a complex of three enzymes and five coenzymes (derived from $B$ Vitamins) that coordinate the oxidative decarboxylation of pyruvate to $Acetyl\,CoA$ efficiently.
Question: "What happens if someone lacks Lactate Dehydrogenase?"
Response: This is a metabolic disorder where pyruvate and $NADH$ accumulate during exercise, potentially stopping the glycolytic pathway and preventing the generation of $ATP$ .