Electron transport, ATP synthesis and allosteric enzymes

Electron Transport Chain and Reduction Potential

  • Electron transport relies on universal electron acceptors like NAD^+ and FAD, which pass electrons to carriers in the inner mitochondrial membrane.

  • Electron flow is driven by reduction potential, a molecule's affinity for electrons. Electrons move from donors with lower reduction potential to acceptors with higher reduction potential.

  • The change in free energy \Delta G is proportional to the change in reduction potential \Delta E, described by the equation: \Delta G = -nF\Delta E.

  • Positive \Delta E indicates a spontaneous redox reaction.

Example: Alcoholic Fermentation

  • In alcoholic fermentation, acetaldehyde reacts with NADH to form ethanol and NAD^+. The spontaneity can be determined by calculating \Delta E'^{\circ} using standard reduction potentials.

  • \Delta E'^{\circ} = (reduction potential of acetaldehyde) - (reduction potential of NADH).

  • Free energy change: \Delta G'^{\circ} = -nF\Delta E'^{\circ}. A negative \Delta G'^{\circ} confirms spontaneity.

Non-Standard Conditions

  • Reduction potential under non-standard conditions is adjusted using the formula:

    • E = E'^{\circ} + \frac{RT}{nF} \ln{\frac{\text{[electron acceptor]}}{\text{[electron donor]}}}

Types of Electron Carriers

  • Main types of electron carriers:

    1. Ubiquinone (Coenzyme Q): Diffuses in the inner mitochondrial membrane and accepts electrons and protons.

    2. Cytochromes: Proteins with iron heme groups (A, B, and C).

    3. Iron-Sulfur Centers: Transfer one electron at a time.

Electron Flow in the Electron Transport Chain

  • Electron carriers are ranked by reduction potential to predict electron flow (e.g., NADH to Coenzyme Q to Cytochrome B).

Experimental Validation of Electron Transport Chain Sequence

  • Disrupting the electron transport chain with drugs leads to:

    • Upstream components being reduced.

    • Downstream components being oxidized.

  • Examples:

    • Rotenone: Downstream oxidized.

    • Antimycin A: Coenzyme Q and Cytochrome B reduced; downstream oxidized.

    • Cyanide: All carriers reduced.

  • This helps define the sequence of electron carriers.

Complexes I-IV

  • Complex I (NADH-Ubiquinone Oxidoreductase):

    • Transfers electrons from NADH to coenzyme Q, coupled with the extraction of four protons from the mitochondrial matrix into the intermembrane space.

  • Complex II (Succinate Dehydrogenase):

    • Catalyzes fumarate synthesis from succinate in the citric acid cycle, transferring electrons to FAD to form FADH_2, then to coenzyme Q.

  • Complex III (Ubiquinol-Cytochrome c Oxidoreductase):

    • Transfers electrons from ubiquinol (QH_2) to cytochrome c via the Q cycle, also transporting protons across the inner membrane.

  • Complex IV (Cytochrome c Oxidase):

    • Transfers electrons from cytochrome c to oxygen, forming water, and transports protons across the membrane.

Electron and Proton Flow

  • The exergonic transfer of electrons yields ATP by creating a proton gradient.

Net Equation

  • NADH + H+ + 1/2 O2 → NAD^+ + H2O

  • Energy is stored in a proton gradient with both chemical and electrical potential energy, described by:

    • \Delta G = 2. 3RT(\Delta pH) + F(\Delta \Psi)

Coupling Experiments

  • Oxygen consumption and ATP synthesis are measured to study the coupling between electron transport and ATP synthesis.

  • Uncoupling molecules disrupt the proton gradient, demonstrating the proton-motive force directly drives ATP synthesis.

ATPase

  • ATP synthase (F0 and F1 subunits) uses the proton motive force to synthesize ATP.

ATP Synthesis

  • ATP synthesis is energetically unfavorable and driven by conformational changes from the proton motive force.

Other Processes Reducing the Proton Motive Force

  • Adenine translocase and phosphate translocase reduce the proton motive force.

Operation of ATPase CF0

  • Rotation of the c-ring is ratcheted, driving the rotation of the gamma chain and sequential conformational changes needed to synthesize ATP.

Allosteric Regulation

  • Allosteric enzymes like ATCase are key players in homeostasis, regulated by effectors that bind reversibly and non-covalently.

Allosteric Regulation/PFK-1

  • PFK-1 regulates liver glycolysis, responding to cell energy levels. ATP inhibits, while AMP and fructose 2,6-bisphosphate stimulate it.

Allosteric Control in the Liver

  • Glucose metabolism is controlled by hexokinase isoforms and allosteric regulation to maintain energy balance.

Gluconeogenesis

  • Gluconeogenesis bypasses irreversible steps of glycolysis:

    1. Pyruvate to phosphoenolpyruvate (PEP) requires ATP and GTP.

    2. Fructose 1,6-bisphosphate to fructose 6-phosphate.

    3. Glucose 6-phosphate to glucose.

Gluconeogenesis Regulation

  • Regulated by allosteric effectors and hormones, with reciprocal control between glycolysis and gluconeogenesis.