21 electron transport

Electron Transport Chain: Overview

Introduction to Electron Transport

• Electron Carriers: Electrons (e-) are stored in electron carriers such as NADH and FADH2.

• Passage of Electrons: These electrons are passed down a series of redox centers located in the inner mitochondrial membrane.

• Final Destination: The ultimate destination for the electrons is molecular oxygen (O2).

• Energy Utilization: The energy released from electron transfer is employed to pump protons (H+) across the inner membrane, thereby establishing a proton gradient.

• ATP Synthesis: This proton gradient is subsequently utilized by ATP synthase to form adenosine triphosphate (ATP).

Components of the Electron Transport Chain

1. Complexes and Carriers: The electron transport chain consists of four multi-subunit complexes (I, II, III, and IV) and several mobile electron carriers.

2. Overall Reaction (assuming NADH):

   • $\text{NADH} + \text{H}^+ + \frac{1}{2}\text{O}<em>2 \rightarrow \text{NAD}^+ + \text{H}</em>2\text{O} \ \Delta G'° = -220 \text{ kJ/mol (NADH)}$

3. Mechanism: The electron transport chain operates through a series of redox reactions.

Redox Reactions and Potentials

Redox Half-Reactions

• Hypothetical Reaction: For example,

  • $\text{X} + 2\text{H}^+ + 2e^- \rightarrow \text{XH}_2$

• Redox Pair: The species XH2 and X form a redox pair.

Standard Reduction Potential (E°)

• Definition: Every redox pair has a characteristic standard reduction potential, E°, representing the tendency of X to attract electrons.

• Philosophical Analogy: E° is analogous to phosphoryl transfer potential for activated phosphate compounds.

• Conversion in Oxidative Phosphorylation: Oxidative phosphorylation is the process that converts electron transfer potential to phosphoryl transfer potential.

Understanding E°

• Measurement: E° is expressed in volts and is measured by assessing the voltage between a half-cell (having the redox pair) at 1 M concentration and a reference half-cell (1 M H+ and H2 at 1 bar).

• Electronics of Electron Flow: If electrons flow toward X, it indicates that X is hungrier for electrons than H+, resulting in a positive E°. Conversely, if electrons flow toward H+, then X is less hungry for electrons than H+, yielding a negative E°.

Biochemical Standard Reduction Potential

• pH = 7: The biochemical standard reduction potential (E°') can be tabulated for rapid reference.

• Gibbs Free Energy Relation: The change in Gibbs free energy (ΔG°') for redox reactions can be derived using the equation:

  • $\Delta G°' = -nF\Delta E°' = -nF(E°'<em>{acceptor} - E°'</em>{donor})$

• Example: For the reaction

  • $\text{NAD}^+ + \text{H}^+ + 2e^- \rightarrow \text{NADH}$, where E°' = -0.32 V.

Calculating Gibbs Free Energy

• Example Calculation: For lactate formation

  • $\text{Pyruvate} + 2\text{H}^+ + 2e^- \rightarrow \text{lactate}\ \ E°' = -0.19 V$.

• ΔG°' Calculation:
  \Delta G°' = -nF\Delta E°' = -2 \times 96.5\text{ kJ/mol·V} \times (-0.19V - (-0.32V))
  $\Delta G°' = -25.1\text{ kJ/mol}$, where n = number of electrons and F = Faraday constant (96,485 J/mol·V).

Complexes of the Electron Transport Chain

Overview of Complexes

1. Complex I: 46 Subunits

2. Complex II: 4 Subunits

3. Complex III: 11 Subunits

4. Complex IV: 13 Subunits

Mobile Electron Carriers

• Types:

  • Coenzyme Q (lipid-soluble)

  • Cytochrome c (membrane-associated but soluble)

Categories of Electron Carriers

• Flavin Mono- and Dinucleotides

• Iron–Sulfur Clusters

• Ubiquinone / Coenzyme Q

• Cytochromes

Complex I Functionality

• NADH Contribution: Complex I utilizes the energy of electrons from NADH to actively pump protons across the inner mitochondrial membrane.

• Complex Reaction:

  • $\text{NADH} + Q + 5\text{H}^+<em>{matrix} \rightarrow \text{NAD}^+ + \text{QH}</em>2 + 4\text{H}^+_{cytoplasm}$

• Flavin Compounds: Flavin mononucleotide (FMN) and FAD serve as initial electron acceptors in complexes I and II, transitioning from 2e– hydride transfers to 1e– transitions through FeS clusters to Q.

• Flavoproteins: These proteins associate with FAD/FMN and can be tightly or covalently bound to their respective enzymes.

Iron-Sulfur Clusters

• Functionality: Iron-sulfur clusters in the respiratory chain facilitate low-potential single electron transfers, as they can alternate between Fe2+ and Fe3+ oxidation states.

• Environmental Influence: The protein environment surrounding the iron-sulfur proteins modifies the reduction potential.

• Electron Transport Mechanism: Multiple iron-sulfur clusters can form electronic complexes (or wires), enabling efficient electron transport over larger distances.

Coenzyme Q (Ubiquinone)

Role in Electron Transport

• Characteristics: Ubiquinone (coenzyme Q) is lipid-soluble and carries electrons between membrane protein complexes. It can carry two electrons and participate in one-electron or two-electron transfers.

• Radicals: When transporting one electron, it forms a semiubiquinone radical.

Complex I Transfer Mechanism

• Transfer Description: Complex I transfers electrons one at a time to coenzyme Q.

• Similarity to Antiporters: Some subunits of complex I exhibit homology with Na+/H+ antiporters, displaying potential pathways for proton transport across the membrane.

Complex II (Succinate Dehydrogenase)

Enzymatic Action

• Complex II utilizes FAD to extract two electrons from succinate in the citric acid cycle and transfers these to coenzyme Q.

• Succinate Affinity: The high affinity of succinate for these electrons necessitates a stronger oxidizing agent for their extraction, meaning electrons entering the electron transport chain through this complex carry less energy.

• Effect on ATP Production: Due to their lower energy state, electrons from complex II contribute less to ATP generation compared to those from complex I.

Glycerophosphate Shuttle Overview

• Functionality: This shuttle mechanism transfers electrons from cytosolic NADH into the electron transport chain, albeit with some energy loss, effectively restoring NAD+ for glycolysis.

Complex III

QH2–Cytochrome c Reductase Functionality

• Complex Details: Comprising 11 subunits, complex III employs heme groups and iron-sulfur clusters to facilitate electron transfer via the Q cycle.

• Translocation: In the Q cycle mechanism, complex III transfers two electrons from QH2 to cytochrome c one at a time, while translocating 4 protons in the process.

• Cytosol-Matrix Dynamics: The Q pool is referred to as a mix of ubiquinone (Q) and ubiquinol (QH2) found within the membrane compartment.

Reaction Overview

• General Reaction:

  • $2\text{QH}<em>2 + 1\text{Q} \rightarrow 2\text{Q} + 1\text{QH}</em>2$

  • $4\text{H}^+<em>{matrix} \rightarrow 4\text{H}^+</em>{cytoplasm}$

  • $2\text{cyt } c<em>{ox} \rightarrow 2\text{cyt } c</em>{red}$

• Interaction Mechanisms: Both electrostatic and hydrophobic forces assist in retaining cytochrome c (a soluble yet membrane-associated carrier) at the mitochondrial membrane.

Complex IV (Cytochrome c Oxidase)

Mechanism and Energy Transfer

• Functionality: Cytochrome c oxidase utilizes energy from electron transfer to pump protons out of the mitochondrial matrix.

• Reduction of Oxygen: It employs a sequence of four electrons transferred from cytochrome c to reduce molecular oxygen (O2) to two molecules of water (H2O).

• Reaction Example:

  • $4\text{cyt } c<em>{red} + \text{O}</em>2 + 4\text{H}^+<em>{mat} \rightarrow 4\text{cyt } c</em>{ox} + 2\text{H}_2\text{O} + 4\text{H}^+$

• Complex Structure: This complex consists of 13 subunits in mammals.

Energy Capture and Inhibition

Summary of Energy Dynamics

• Final Reaction Index: Reduction of oxygen by NADH again showcases that considerable energy is available to phosphorylate adenosine diphosphate (ADP).

• Overall Reaction (Reiterated):

  • $\text{NADH} + \text{H}^+ + \frac{1}{2}\text{O}<em>2 \rightarrow \text{NAD}^+ + \text{H}</em>2\text{O}$

Investigative Methods and Inhibitors

• Inhibitor Analysis: By analyzing accumulated intermediates in the presence of inhibitors, scientists have mapped out the sequence of events in the electron transport chain.

• Experimental Setup: These experiments involved the isolation of functional mitochondria or fragments in vitro.

Specific Inhibitors

1. Antimycin A: Inhibits complex III at the Q cycle, notable for its high toxicity.

2. Rotenone: Inhibits electron transfer from complex I to coenzyme Q, causing NADH reduction and oxidation of Q, cyt b, cyt c1, cyt c, cyt a, and cyt a3.

Conclusion and Implications

Key Takeaways

• Reduction Potential Understanding: Students should grasp the meaning of reduction potential and its correlation with ΔG.

• Sequential Mechanism: NADH is oxidized in a stepwise fashion through three large protein complexes, ultimately transferring electrons to oxygen.

• Proton Translocation: Complexes I, III, and IV facilitate proton translocation from the mitochondrial matrix to the cytoplasmic side of the inner mitochondrial membrane.

• Role of Carriers: FMN/FAD, iron-sulfur clusters, coenzyme Q, copper ions, and heme groups in cytochromes serve as the electron carriers throughout the process.

• Energy Storage: Energy derived from oxidation is conserved in the form of a proton gradient.

• Q Cycle Dynamics: The Q cycle allows a two-electron carrier to transfer electrons one at a time, effectively increasing the efficiency of electron transfer.

• Mobile E− Carriers: Q and cytochrome c are highlighted as mobile electron carriers facilitating transfers between complexes.

Resources for Further Study

• Instructor Q&A: Available on Wednesdays from 4-5pm at Health Sciences Building I132.

• Reader/Grader Office Hours:

  • Sanjay: Wednesdays 9:30-10:20am at Health Sciences J375.

  • Sidhant: Fridays 1:30-2:20pm at Health Sciences J375.