Electron Transport and Oxidative Phosphorylation

Biochemistry Fifth Edition - Campbell Farrell Chapter 20: Electron Transport and Oxidative Phosphorylation

Problem Assignment

• Reference for exercises: 6th, 7th, and 8th Edition Problems

• Specific problems: 1-2, 5, 8, 15-19, 22, 25, 30-32, 38-42, 44-47, 49 (b and e).

• Course Code: CHM 351

Introduction to Electron Transport in Metabolism

• In eukaryotic cells, aerobic processes (conversion of pyruvate to acetyl CoA, the citric acid cycle, and electron transport chain) occur in the mitochondria.

• Anaerobic glycolysis occurs in the cytosol.

• The reactions of the electron transport chain transpire in the inner mitochondrial membrane, utilizing oxygen as the final electron acceptor.

• ATP production in mitochondria is a result of oxidative phosphorylation, where ADP is phosphorylated to form ATP.

Oxidative Phosphorylation and Electron Transport

• The production of ATP by oxidative phosphorylation is considered an endergonic process distinct from the electron transport process to oxygen, which is exergonic. These processes are, however, tightly coupled.

• Electrons are passed from carrier to carrier, which leads to the pumping of protons (hydrogen ions) across the inner mitochondrial membrane from the matrix to the intermembrane space.

• The resultant pH gradient (proton gradient) represents stored potential energy, serving as the basis for the coupling mechanism.

• Oxidative phosphorylation accounts for the majority of ATP production linked to the complete oxidation of glucose.

Role of Electron Carriers

• Electrons from NADH and FADH2 derived from glycolysis and the citric acid cycle are transferred to oxygen through a series of oxidation-reduction reactions in the electron transport chain.

• NADH and FADH2 are reoxidized to NAD+ and FAD, enabling their reuse in metabolic pathways.

• Oxygen serves as the ultimate electron acceptor, getting reduced to H2O; this indicates that glucose is fully oxidized to CO2 and water.

• Key carriers in the electron transport chain include:

  • FMN (flavin mononucleotide): A coenzyme differing from FAD by lacking an adenine nucleotide.

  • CoQ (Coenzyme Q or ubiquinone): A mobile carrier in the electron transport pathway.

  • Cytochromes (Cyt): Heme-containing proteins involved in the electron transmission process.

ATP Production Metrics

• Approximately 2.5 moles of ATP are generated per mole of NADH and about 1.5 moles of ATP for each mole of FADH2.

• NADH transfers electrons to CoQ, while FADH2 enters the chain at a different stage which results in less ATP generation due to lesser proton pumping capabilities.

• Electrons from CoQ are then passed to cytochrome proteins before concluding at the oxygen acceptor.

Standard Reduction Potentials

• The flow of electrons in redox reactions is measured in voltage or electromotive force (Eo).

• Eo indicates standard potential; a more negative value implies stronger reducing potential.

• Negative voltage corresponds to an endergonic reaction, whereas positive voltage signifies an exergonic reaction.

• The relationship between Gibbs free energy (Go') and standard potential (Eo') is given by the equation: Go' = -nFEo'

  • Where:

  • n = number of electrons transferred

  • F = Faraday constant = 96,400 J/volt.mole

• Example Calculation: If Eo' = +1.14 volts and 2 electrons are transferred,
  Go' = (-2)(96,400 J/volt mole)(1.14 volts) = -220 kJ/mole (indicating an exergonic process).

Respiratory Complexes Overview

General Description

• The electron transport chain comprises four distinct respiratory complexes isolated from the inner mitochondrial membrane, functioning as multienzyme systems with over 20 subunits.

Complex I

• Name: NADH-CoQ oxidoreductase.

• Catalyzes the initial transfer of electrons from NADH to coenzyme Q (CoQ).

• Features include:

  • Integral part of the inner mitochondrial membrane.

  • Contains iron-sulfur clusters and covalently bound flavoprotein (FMN).

  • Responsible for proton pumping, contributing to the pH (proton) gradient.

  • The standard free energy change (ΔGo') is -81 kJ/mol, indicating it is a strongly exergonic reaction that can drive the phosphorylation of ADP to ATP.

Energy Flow in the Electron Transport Chain

• Electron carriers exist in oxidized or reduced states but exhibit a specific directionality in electron flow across the complexes.

• For example,

  • Reduced NADH donates electrons to CoQ; it is crucial to note that the reverse does not occur.

• Carriers vary; some like NADH transport both electrons and protons, while others like iron-sulfur proteins transport only electrons.

• When NADH reduces iron-sulfur proteins and subsequently passes electrons, hydrogen ions translocate on the opposite side of the membrane, facilitating ATP production.

• CoQ acts as a mobile electron carrier, transporting electrons from Complex I to Complex III.

Complex II

• Name: Succinate-CoQ oxidoreductase.

• Catalyzes transfer of electrons from succinate to CoQ.

• Features include:

  • Acts on succinate from the TCA cycle, converting it to fumarate.

  • Composed of a flavoprotein and iron-sulfur protein, integral to the inner mitochondrial membrane.

• The standard free energy change (ΔGo') for Complex II is -13.5 kJ/mol; although exergonic, it does not generate sufficient energy for ATP production and does not pump hydrogen ions across the membrane.

Complex III

• Name: CoQH2-cytochrome c oxidoreductase (Cytochrome reductase).

• Catalyzes oxidation of reduced CoQ (CoQH2), with electrons transferred to cytochrome c.

• Features include:

  • Composed of cytochrome c1 and iron-sulfur proteins, integral to the inner mitochondrial membrane, facilitating hydrogen ion movement out of the matrix.

  • Cytochromes can carry electrons but not protons, while CoQ can carry two electrons concurrent with cytochromes’ potential to only carry one at a time.

• The interaction of the electron flow through this complex results in a negative ΔGo' sufficient to drive ADP phosphorylation.

Complex IV

• Name: Cytochrome c oxidase.

• Facilitates the final transfer of electrons from cytochrome c to oxygen.

• Features include:

  • Integral part of the inner mitochondrial membrane; involvement of copper ions in the electron transport process is crucial.

  • This stage results in proton pumping contributing to the ATP production.

  • Contains multiple subunits (about 10) essential for its function.

• Electrons transition through Cu2+ before transferring to the molecular oxygen, enabling aerobic respiration.

Summary of Electron Transport Chain Energetics

• The overall flow of electrons moves from NADH to O2.

• NADH captures electrons from various substrates (pyruvate, isocitrate, α-ketoglutarate, and malate).

• The electron transport complex exhibits dynamic properties, with complexes moving laterally within the inner mitochondrial membrane.

• The binding site for oxygen in Complex IV is oriented towards the matrix of the mitochondrion.

Chemiosmotic Coupling

• The electrochemical potential of the proton gradient across the inner mitochondrial membrane is converted to the chemical energy of ATP.

• The key coupling factor is a protein that spans the inner mitochondrial membrane and projects into the matrix, known as ATPase or ATP synthetase.

• The outer mitochondrial membrane is permeable, while the inner membrane is impermeable, leading to specific proton transport channels.

• Protons may only return across the membrane through the ATPase channel/pore (Fo subunit), inducing a conformational change in the F1 subunit that catalyzes ADP phosphorylation to ATP.

Understanding Uncouplers

• Uncouplers prevent ADP phosphorylation without impacting electron transport, achieving gradient dissipation by:

  • Reacting with protons on one side of the membrane.

  • Creating alternate pathways for proton entry into the matrix (through pores or channels).

• Notable examples include brown adipose tissue where energy dissipation as heat serves vital physiological purposes.

  • example of 2,4-DNP as a dietary drug is cautioned due to potential metabolic acceleration in the absence of ATP production.

Applications of Brown Adipose Tissue

• Situations benefiting from energy dissipation as heat include:

  • Cold-induced nonshivering thermogenesis: essential for survival in cold conditions after acclimatization.

  • Diet-induced thermogenesis: prevents obesity despite overeating, allowing energy from food molecules to dissipate as heat and not fat storage.

• Brown adipose tissue (BAT) is characterized by high mitochondrial content. The mitochondrial protein thermogenin acts as a proton channel through the inner membrane, aiding in heat dissipation.

Respiratory Inhibitors

• Inhibitors can obstruct electron transport chain progression, causing blockage prior to the reaction site while reducing compounds before the blockade and oxidizing compounds after it (no electron flow).

• This blockage can be utilized experimentally to discern the sequence and order of components within the electron transport chain and simultaneously impact ATP production as a result of disrupted electron flow.

Shuttle Mechanisms

Glycerol Phosphate Shuttle

• Facilitates transport of NADH from glycolysis that cannot cross the mitochondrial membrane, operational in skeletal muscle and brain.

• Yields approximately 1.5 ATP per NADH as electrons are transferred to FAD at the cost of 1 ATP.

Malate-Aspartate Shuttle

• A more complex shuttle mechanism found in the kidney, liver, and heart.

• Transfers electrons from cytosolic NADH to mitochondrial NADH, yielding 2.5 ATP per NADH. -

• Electrons transferred to malate, which can cross the mitochondrial membrane, are eventually returned to NADH.

ATP Yield from Complete Oxidation of Glucose

• Total ATP yield calculated as:

  1. Glucose to 2 pyruvate yields 2 ATP and 2 NADH.

  2. 2 Pyruvate to 2 acetyl CoA yields 2 additional NADH.

  3. 2 Acetyl CoA through the TCA cycle results in 2 GTP, 6 NADH, and 2 FADH2.

  4. Overall yield:

  • 10 total NADH → 25 ATP

  • 2 FADH2 → 3 ATP

• Total ATP yield from complete oxidation of glucose is 32 ATP.