Module 12

NSCI 4153: Biological ChemistryFocus: Citric Acid Cycle and Aerobic RespirationModule: 12, Fall 2024

Biochemical Redox Reactions

  • NAD+ accepts electrons and becomes reduced to NADH; NADH donates electrons and becomes oxidized.

  • Key roles:

    • NAD+ serves as an electron acceptor (oxidizing agent) in various metabolic pathways.

    • NADH acts as an electron donor (reducing agent) crucial in cellular respiration and specific biosynthetic reactions.

Energy Captured in Glycolysis

  • Only a small fraction of the energy from glucose is captured in glycolysis.

  • ∆G′° for glycolysis: –146 kJ/mol indicates a spontaneous process that leads to energy release.

  • Total oxidation (with O2): –2,840 kJ/mol, which reflects the energy potential of glucose when fully oxidized.

  • Energy yield: 2 ATP are generated through substrate-level phosphorylation in glycolysis and up to 36 ATP from the complete oxidation process in aerobic respiration.

Cellular Respiration Overview

  • Cells utilize O2 to metabolize glucose, resulting in the production of CO2 as a byproduct.

  • This process evolved approximately 2.5 billion years ago, contributing to the development of aerobic life forms.

  • Major stages of cellular respiration:

    1. Acetyl CoA production: Conversion of pyruvate from glycolysis into Acetyl CoA in the mitochondrial matrix.

    2. Acetyl CoA oxidation (TCA cycle): Further breakdown of Acetyl CoA to release stored energy.

    3. Electron transfer and oxidative phosphorylation: Involves electron transport chain (ETC) and chemiosmosis, leading to ATP synthesis.

Mitochondrion Structure

  • The citric acid cycle occurs in the mitochondrial matrix where enzymes necessary for the cycle are located.

  • Key enzyme: succinate dehydrogenase is uniquely bound to the inner mitochondrial membrane, facilitating both the TCA cycle and the electron transport chain.

  • The electron transport chain is embedded in the inner membrane, where it transfers electrons from NADH and FADH2 to oxygen.

Conversion of Pyruvate to Acetyl-CoA

  • Process: oxidative decarboxylation of pyruvate, which is an essential step linking glycolysis to the TCA cycle.

  • Catalyzed by: pyruvate dehydrogenase complex, involving multiple subunits and a crucial regulatory step.

  • Requires 5 coenzymes (e.g., TPP, lipoyllysine): Each plays a specific role in the decarboxylation process and the transfer of the acetyl group.

  • This conversion is irreversible under physiological conditions; Acetyl-CoA cannot be converted back to pyruvate, making it a central metabolite in energy metabolism.

Citric Acid Cycle (CAC)

  • Also known as Krebs cycle or TCA cycle, it is a series of enzymatic reactions that oxidizes acetyl groups from pyruvate.

  • Net reaction yields:

    • 2 CO2,

    • 3 NADH,

    • 1 FADH2,

    • 1 GTP per cycle, representing energy-rich molecules for subsequent ATP production.

Key Steps in the Citric Acid Cycle

  1. Step 1: Acetyl-CoA and oxaloacetate combine to form citrate (citration).

  2. Steps 3-4: Oxidative decarboxylation occurs, generating 2 NADH.

  3. Step 5: GTP is produced via substrate-level phosphorylation, which can be converted to ATP.

  4. Step 6: Oxidation of succinate to fumarate occurs, producing FADH2.

  5. Step 8: Malate is oxidized back to oxaloacetate, producing NADH, completing the cycle.

Energy Yield from Glycolysis and CAC

  • Total ATP yield from glucose oxidation:

    • Glycolysis: 2 ATP, 2 NADH (contributed to an additional energy yield).

    • Pyruvate Oxidation: 2 NADH

    • Citric Acid Cycle: 6 NADH, 2 FADH2, 2 ATP/GTP

  • Overall total: ~30 to 32 ATP, with the exact number depending on the shuttle systems utilized for NADH transport into mitochondria (malate-aspartate shuttle vs. glycerol phosphate shuttle).

Electron Transport Chain (ETC)

  • Transfers electrons from reduced cofactors (NADH and FADH2) to O2, facilitating oxidative phosphorylation.

  • Major complexes:

    • I: NADH-Q oxidoreductase

    • II: succinate-Q reductase

    • III: cytochrome bc1 complex

    • IV: cytochrome c oxidase

  • Ubiquinone (Coenzyme Q) and cytochrome c act as mobile electron carriers, continually transferring electrons between complexes.

Proton-Motive Force

  • The electrons moving through the chain create a proton gradient across the inner mitochondrial membrane, which drives ATP synthesis via ATP synthase.

  • This process relies on impermeable membranes to maintain the proton gradient essential for ATP production.

Mitochondrial ATP Synthase

  • Complex consists of:

    • F1: catalyzes the hydrolysis of ATP in the mitochondrial matrix.

    • F0: integral membrane part that facilitates proton transport.

  • Proton flow through F0 causes conformational changes that drive ATP synthesis from ADP and inorganic phosphate (Pi).

Inhibitors of Oxidative Phosphorylation

  • 2,4-dinitrophenol (DNP) uncouples oxidative phosphorylation, allowing protons to re-enter the mitochondrial matrix without driving ATP synthesis.

  • ATP synthase inhibitors like oligomycin directly obstruct ATP production by preventing proton flow through the transmembrane channel, significantly impacting energy yield.