10.2 TCA Cycle

Introduction to Metabolism

Focus on Tricarboxylic Acid Cycle (TCA Cycle)

Also known as: Krebs Cycle, Citric Acid Cycle

Learning Outcomes

  • Understand the central role of the TCA cycle in metabolism, serving as a crucial pathway for energy production and the intersection of carbohydrate, fat, and protein metabolism.

  • Name and draw structures of all intermediates in the cycle, associating each with its biochemical role.

  • Identify intermediates used in biosynthesis, highlighting their importance in anabolic pathways.

  • List enzymatic reactions in the TCA cycle and the enzymes involved, providing insight into the catalytic processes.

  • Draw a complete diagram of the TCA cycle, including all inputs and outputs.

  • Know control points, activators, and inhibitors of the TCA cycle, understanding regulatory mechanisms.

Summary of Previous Lectures

  • Glycolysis produces pyruvate as the endpoint, converting glucose into pyruvate under anaerobic or aerobic conditions.

  • The link reaction decarboxylates pyruvate to produce acetyl coenzyme A (acetyl CoA), which serves as the fuel for the TCA cycle.

  • Acetyl CoA enters the TCA cycle, combining with oxaloacetate to initiate the cycle.

Overview of the TCA Cycle

  • Acetyl CoA combines with oxaloacetate (4 carbons) to form citrate (6 carbons). This condensation reaction is a key entry point into the cycle.

  • Key processes include:

    • Loss of carbon as CO2 (two molecules of CO2 are released per cycle).

    • Reduction of NAD+ to NADH at three key steps, facilitating energy capture.

    • Reduction of FAD to FADH2 during the oxidation of succinate, another crucial energy carrier.

    • Production of GTP, which can be readily converted to ATP, highlighting energy production efficiency.

TCA Cycle Intermediates

  • Key intermediates and their functions:

    • Citrate: Acts as an allosteric inhibitor for phosphofructokinase in glycolysis.

    • Isocitrate: Involved in the dehydrogenation process that produces NADH.

    • Alpha-ketoglutarate: Key substrate for amino acid synthesis; also produces NADH and CO2.

    • Succinyl CoA: Key in heme synthesis and produces GTP.

    • Succinate: Oxidized to fumarate while producing FADH2.

    • Fumarate: Prepares for hydration to malate.

    • Malate: Reduced to oxaloacetate, producing NADH, thus continuing the cycle.

    • Oxaloacetate: Regenerated to combine with another acetyl CoA, sustaining the cycle.

  • Enzymes responsible for each step:

    • Citrate synthase

    • Isocitrate dehydrogenase

    • Alpha-ketoglutarate dehydrogenase complex

    • Succinyl CoA synthetase

    • Succinate dehydrogenase

    • Fumarase

    • Malate dehydrogenase

Key Features of the TCA Cycle

  • The cycle operates using a 2-carbon fragment (acetate) entering from acetyl CoA, emphasizing the important role of fatty acid and amino acid metabolism.

  • Key Points:

    • No oxygen is involved directly in the cycle, although the cycle is part of aerobic respiration.

    • Only one ATP (or GTP) is produced directly from one turn of the cycle, emphasizing the cycle's efficiency.

    • The reduction of NAD+ and FAD to NADH and FADH2 is vital for the electron transport chain, where further ATP is generated.

    • Sequential coupling of reactions drives the cycle and utilizes unfavorable reactions by coupling to favorable ones.

    • The large negative delta G (Gibbs free energy change) of the reactions drives the cycle forward, ensuring continuous operation under physiological conditions.

Step-by-Step Breakdown of the TCA Cycle

  1. Oxaloacetate to Citrate:

    • A condensation reaction with acetyl CoA, facilitated by citrate synthase, combines these two molecules, producing citrate and liberating coenzyme A.

  2. Citrate to Isocitrate:

    • Structural isomerization occurs via aconitase, with a dehydration followed by hydration process transforming citrate to isocitrate.

  3. Isocitrate to Alpha-Ketoglutarate:

    • A two-step reaction involving dehydrogenation and decarboxylation via isocitrate dehydrogenase, producing NADH and releasing CO2, forms alpha-ketoglutarate.

  4. Alpha-Ketoglutarate to Succinyl CoA:

    • Further dehydrogenation and decarboxylation via alpha-ketoglutarate dehydrogenase complex produces Succinyl CoA, releasing another CO2 and generating more NADH.

  5. Succinyl CoA to Succinate:

    • GTP is produced during this step through succinyl CoA synthetase, highlighting energy production mechanisms.

  6. Succinate to Fumarate:

    • The oxidation of succinate occurs via succinate dehydrogenase, with the reduction of FAD to FADH2, facilitating electron transport.

  7. Fumarate to Malate:

    • A simple hydration reaction catalyzed by fumarase converts fumarate to malate.

  8. Malate to Oxaloacetate:

    • Catalyzed by malate dehydrogenase, this step generates NADH while restoring oxaloacetate, allowing the cycle to restart with another acetyl CoA.

Conclusion

  • The TCA Cycle is crucial for energy production, as it provides high-energy electron carriers (NADH and FADH2) that drive ATP synthesis in the electron transport chain.

  • Intermediates provide the backbone for various biosynthetic pathways, linking catabolism and anabolism.

  • Operates under aerobic conditions, and its efficiency is crucial for meeting cellular energy demands.

  • Essential for cellular energy generation, integrating pathways of carbohydrate, lipid, and protein metabolism.

Acknowledgment

  • A heartfelt appreciation for students' continued participation in this learning journey, and anticipation of further deep dives into metabolic pathways in the next lecture.