Week 2

Metabolic Energy from Glucose and Pyruvate Production

  • Anaerobic Glycolysis:

    • Approximately 90% of metabolic energy from glucose remains in two moles of pyruvate produced through this process.

    • Under anaerobic conditions, pyruvate can’t be oxidized to CO₂ and H₂O, leading to its conversion to lactate.

    • This conversion regenerate NAD⁺, crucial for continuous glycolysis and ATP production in the cytoplasm.

    • This process is quantitatively vital for the reoxidation of NADH in vertebrate cell cytoplasm.

    • Enzyme Involved: Lactate dehydrogenase (LDH, #22).

  • Alternative NAD⁺ Regeneration Methods:

    • Conversion of pyruvate to alanine via an amino transferase reaction.

    • Aerobic conditions utilize glycerol 3-phosphate and malate shuttles for NAD⁺ regeneration.

  • Clinical Utility of LDH:

    • Damaged tissues release intracellular enzymes into the bloodstream; measuring these activities can indicate affected tissues (refer to Chapters 5 and 6).

    • LDH Isozymes:

    • Exist in multiple forms useful for differentiating between conditions like myocardial infarctions and liver diseases (e.g., infective hepatitis).

    • Two genes code for LDH: one for the M (muscle) form and another for the H (heart) form.

    • LDH contains four subunits, leading to five possible isozymes: M₄, M₃H, M₂H₂, MH₃, and H₄.

    • Predominance in tissues:

      • Skeletal muscle: Mostly M₄

      • Heart: Mostly H₄

    • Inhibition: H₄ isozyme is strongly inhibited by pyruvate.

Overview of Pyruvate Metabolism

  • Unique Position of Pyruvate:

    • Acts as a central node in carbohydrate, protein, and lipid metabolism.

  • Exercise Interaction:

    • Heart muscle absorbs lactate from circulation during physical activity.

  • Amino Acid Functionality:

    • Alanine serves primarily in protein incorporation and involvement in transamination.

  • Sources of NADPH:

    • Cytoplasmic malate and malic enzyme contribute to NADPH production during lipogenesis, as well as generating pyruvate.

Enzymatic Regulation and Function

  • Inhibition of Pyruvate Dehydrogenase (PDH):

    • Inhibited by:

    • Acetyl-CoA

    • Increases in mitochondrial ATP/ADP and NADH/NAD⁺ ratios.

  • Activation of Pyruvate Carboxylase:

    • Acetyl-CoA acts as an allosteric activator.

  • Nutritional Deficiencies Affecting Enzyme Activity:

    • Deficits in thiamin or niacin reduce PDH activity.

    • Biotin or Zn²⁺ deficiencies diminish pyruvate carboxylase activity.

Pyruvate-Branching Reactions

  • Metabolism Intermediates:

    • Pyruvate can branch into various metabolic pathways, including:

    • Conversion to alanine via alanine aminotransferase (ALT).

    • Interaction with malic enzyme for alternative NADPH and pyruvate sources.

  • Alanine and Pyruvate Reconversion:

    • Reaction Details:

    • Formation of alanine from pyruvate involves the substitution of an α-amino group for carbonyl oxygen.

    • Reversible under normal conditions:

      • Usually favors alanine → pyruvate conversion in the liver (gluconeogenesis).

      • Favors pyruvate → alanine in muscles, especially during exercise.

    • Alanine transports nitrogen as NH₃ from peripheral tissues to the liver for reutilization or urea synthesis.

Pyruvate’s Role in TCA Cycle and Other Pathways

  • Transformation to Acetyl-CoA:

    • Pyruvate's conversion to acetyl-CoA is irreversible in animal cells.

    • Catalyzed by pyruvate dehydrogenase (PDH).

  • Fates of Acetyl-CoA:

    • 1) Oxidation to CO₂ and H₂O in TCA cycle.

    • 2) Incorporation into various compounds such as acetylcholine or ketone bodies.

  • Importance of Acetyl-CoA:

    • Potential for citrate formation where citrate can either:

    • Oxidize in TCA cycle.

    • Diffuse into the cytoplasm for lipid synthesis.

Regulation of Pyruvate Dehydrogenase (PDH)

  • Control Mechanisms:

    • PDH activity is regulated by concentrations of NADH/NAD⁺, ATP/ADP, and GTP/GDP.

    • Increased mitochondrial energy levels inhibit PDH, while a buildup of acetyl-CoA does as well.

    • Insulin elevation and increased pyruvate stimulate PDH activity.

  • Reversible Modifications on PDH:

    • PDH inhibition occurs through phosphorylation.

    • Activation through phosphatase activity facilitates dephosphorylation.

  • Impact of PDH Deficiency:

    • Serious consequences for tissues like muscle, kidney, and brain, which rely majorly on aerobic metabolism for ATP.

Additional Enzyme Information

  • Pyruvate Carboxylase Role:

    • Catalyzes the addition of CO₂ to form oxaloacetic acid (OAA) from pyruvate.

    • Requires ATP for energy, with a high activity present in liver and kidneys for gluconeogenesis.

  • Cofactor and Activation Details:

    • Essential cofactors include magnesium and manganese, while biotin transmits CO₂.

    • Noteworthy that acetyl-CoA serves as an allosteric activator of pyruvate carboxylase without participating directly in the reaction.

  • Anaplerotic Functions of Pyruvate Carboxylase:

    • Important for replenishing TCA cycle intermediates, particularly during intense muscle activity.

Major Objectives in Pyruvate Metabolism Study

  • Outline the three primary stages of glycolysis and discuss regulatory reactions at each.

  • Identify four NAD+ regeneration pathways essential for sustaining anaerobic glycolysis.

  • Describe the regulation mechanisms for LDH isozymes across different tissues.

  • Review ALT and AST catalyzed transamination reactions and their importance.

  • Explain how branched-chain amino acid (BCAA) oxidation nitrogen is transported to the liver.

  • Discuss malate dehydrogenase and malic enzyme roles in hepatic NADPH transfer results.

  • Illustrate reasons for lack of net conversion of hydrocarbons from acetyl-CoA to glucose.

  • Control mechanisms for both PDH and pyruvate carboxylase, including vitamin cofactors.

Assessment Questions

  1. Which enzyme requires biotin as a cofactor?

    • a. Malic enzyme

    • b. Pyruvate dehydrogenase

    • c. Alanine dehydrogenase

    • d. Pyruvate carboxylase

    • e. Lactate dehydrogenase

  2. Which enzyme has an anaplerotic action?

    • a. Pyruvate dehydrogenase

    • b. Alanine dehydrogenase

    • c. Lactate dehydrogenase

    • d. Pyruvate carboxylase

    • e. Malic enzyme

  3. LDH isozyme prevalent in liver tissue?

    • a. LDH1

    • b. LDH2

    • c. LDH3

    • d. LDH4

    • e. LDH5

  4. True statement about LDH in heart muscle?

    • a. Stimulated by pyruvate

    • b. Converts lactate to pyruvate during exercise

    • c. Similar LDH isozyme in liver tissue

    • d. Similar to skeletal muscle LDH

    • e. Converts alanine to pyruvate

  5. Cytoplasmic enzyme for NADPH generation?

    • a. Lactate dehydrogenase

    • b. Malic enzyme

    • c. Alanine aminotransferase

    • d. Pyruvate dehydrogenase

    • e. Pyruvate carboxylase

  6. Most vital reaction for NAD+ generation under anaerobic conditions?

    • a. Glucose → Glucose 6-P

    • b. 3-Phosphoglycerate → 2-Phosphoglycerate

    • c. Alanine → Pyruvate

    • d. Fructose 6-P → Fructose 1,6-bisphosphate

    • e. Pyruvate → Lactate

  7. Which enzyme is involved in oxidative decarboxylation of pyruvate?

    • a. Malic enzyme

    • b. Lactate dehydrogenase

    • c. Alanine aminotransferase

    • d. Pyruvate carboxylase

    • e. Pyruvate dehydrogenase

  8. Nitrogen transported from muscle to liver during exercise primarily in what form?

    • a. Urea

    • b. Uric acid

    • c. NH₄⁺

    • d. Lactate

    • e. Alanine

  9. Acetyl-CoA from fat oxidation activates:

    • a. Pyruvate carboxylase

    • b. Lactate dehydrogenase

    • c. Pyruvate dehydrogenase

    • d. Malic enzyme

    • e. Alanine aminotransferase

Answers:

  1. d

  2. d

  3. e

  4. b

  5. b

  6. e

  7. e

  8. e

  9. a