kreb cycle

Introduction to Cell Biology & Biochemistry

Glycolysis and Gluconeogenesis

Regulation of Glycolysis and Gluconeogenesis

  • Glycolysis and gluconeogenesis are strictly regulated to avoid "futile cycles."

  • Energy-Poor Products:

    • ADP and AMP promote glycolysis (catabolic pathway).

  • Energy-Rich Products:

    • ATP and citrate promote gluconeogenesis (anabolic pathway).

Fructose 2,6-Bisphosphate (F2,6BP)

  • F2,6BP is a key allosteric regulator of both:

    • PFK-1 (Phosphofructokinase-1): Activated by F2,6BP.

    • FBPase-1 (Fructose-1,6-bisphosphatase): Inhibited by F2,6BP.

  • Mechanism:

    • F2,6BP binds to both enzymes, inducing conformational changes, thereby activating PFK-1 and inhibiting FBPase-1.

Allosteric Regulation

Definition of Allosteric Regulation

  • Allosteric Regulation:

    • The regulation of an enzyme by the binding of an effector molecule at a location other than the enzyme's active site.

    • This binding causes a conformational change in the enzyme, affecting its activity.

Components of Allosteric Regulation

  • Modulator Types:

    • Allosteric activator: Increases enzyme activity.

    • Allosteric inhibitor: Decreases enzyme activity.

  • Visual Representation:

    • Standard enzymes vs. allosteric enzymes show different activity profiles based on modulator presence and substrate concentration.

Cellular Respiration

Overview of Cellular Respiration Stages

  1. Stage 1: Acetyl-CoA Production

    • Input: Glucose

    • Result: Production of 2 Pyruvate, 2 NADH, and 2 ATP.

    • Involves fatty acids and pyruvate dehydrogenase complex.

  2. Stage 2: Acetyl-CoA Oxidation

    • Entry into the Krebs Cycle (Citric Acid Cycle).

    • Converts Acetyl-CoA into Citrate and proceeds through multiple steps producing NADH and FADH₂.

  3. Stage 3: Electron Transfer and Oxidative Phosphorylation

    • Utilizes NADH and FADH₂ to drive ATP synthesis via the electron transport chain.

    • Key outputs: ATP production, H₂O generation, CO₂ release.

Krebs Cycle (Citric Acid Cycle)

General Information

  • The Krebs Cycle is a series of chemical reactions employed by all aerobic organisms to release stored energy through the oxidation of acetyl-CoA.

  • Takes place in the mitochondria of eukaryotic cells.

  • Key aspects include:

    • Production of CO₂ as a byproduct.

    • High ATP and NADH yields.

    • Essential in energy metabolism.

Steps of the Krebs Cycle

  1. Formation of Citrate:

    • Enzyme: Citrate synthase

    • Reaction: Acetyl-CoA + Oxaloacetate → Citrate.

    • Involves water release.

  2. Conversion of Citrate to Isocitrate:

    • Enzyme: Aconitase

    • Requires conversion through cis-Aconitate.

  3. Isocitrate to Alpha-Ketoglutarate:

    • Enzyme: Isocitrate dehydrogenase

    • Gains NADH, releases CO₂.

  4. Alpha-Ketoglutarate to Succinyl-CoA:

    • Enzyme: Alpha-ketoglutarate dehydrogenase complex

    • Produces NADH, releases CO₂.

  5. Succinyl-CoA to Succinate:

    • Enzyme: Succinyl-CoA synthetase

    • Produces GTP (or ATP) via substrate-level phosphorylation.

  6. Succinate to Fumarate:

    • Enzyme: Succinate dehydrogenase

    • Produces FADH₂.

  7. Fumarate to Malate:

    • Enzyme: Fumarase

    • Involves hydration.

  8. Malate to Oxaloacetate:

    • Enzyme: Malate dehydrogenase

    • Produces NADH.

    • Completes the cycle by regenerating oxaloacetate.

Key Outputs

  • From one cycle:

    • 3 NADH, 1 FADH₂, 1 GTP (ATP), and 2 CO₂ molecules produced.

  • Acetyl-CoA contributes carbon units to the cycle, entering with 2 carbons and exiting as CO₂.

Summary of the Citric Acid Cycle

  • The citric acid cycle contributes to energy production by:

    • Oxidizing the acetyl group from Acetyl-CoA.

    • Producing high-energy electron carriers (NADH and FADH₂).

  • Overall, it is a crucial metabolic pathway in aerobic respiration, connecting to oxidative phosphorylation for ATP production.