Exam 2 Review: Oxidative Phosphorylation, ATP Synthase, Photophosphorylation, Metabolic Control, and Fatty Acid Oxidation

Oxidative Phosphorylation

Fundamental Concepts

• Reduction Potential and Free Energy: The reduction potential (E0') measures a molecule's tendency to gain electrons. The relationship to free energy is given by the Nernst equation: $\Delta G^\circ' = -nF\Delta E^\circ'$ where n is the number of electrons transferred, F is the Faraday constant, and $\Delta E^\circ'$ is the difference in reduction potential between donor and acceptor.

• Electron Carriers: Some cofactors (e.g., NAD+, FAD, ubiquinone) transfer two electrons, while others (e.g., cytochromes, iron-sulfur clusters) transfer one. Each carrier has distinct oxidized, reduced, and sometimes radical states.

• Radical Electron Transfer: Metals (like Fe in cytochromes) and quinones can participate in single-electron (radical) chemistry, facilitating stepwise electron transfer.

• Proton-Electron Coupling: Electron movement through the respiratory chain is coupled to proton translocation across the mitochondrial membrane, generating a proton gradient.

• Oxygen Reduction: O2 is the terminal electron acceptor, reduced to H2O. The high reduction potential of O2 'pulls' electrons through the chain.

Key Mechanistic Details

• Electron Donors: NADH and FADH2 donate electrons to the respiratory chain at Complex I and II, respectively.

• Electron Flow: Electrons move through a series of cofactors (FMN, Fe-S clusters, ubiquinone, cytochromes) in complexes I-IV. Some complexes pump protons (I, III, IV), others transfer electrons or catalyze redox chemistry.

• Chemiosmotic Theory: Proposed by Peter Mitchell, this theory states that the energy from electron transfer is used to pump protons, creating an electrochemical gradient (proton motive force) used to synthesize ATP.

• Terminal Electron Acceptor: O2 accepts electrons at Complex IV, driving the chain forward due to its high reduction potential.

• Inhibitors: Inhibitors at different points in the chain alter O2 consumption and ATP production, which can be visualized in experimental charts.

Example:

Rotenone inhibits Complex I, decreasing both O2 consumption and ATP synthesis. Cyanide inhibits Complex IV, stopping electron flow and ATP production entirely.

ATP Synthase

Fundamental Concepts

• Coupling Gradient to Mechanical Work: ATP synthase uses the proton gradient to drive rotation of its c-subunit, converting electrochemical energy into mechanical energy.

• Energetic Barriers of ATP Formation: ATP synthesis from ADP and Pi is energetically unfavorable without the proton motive force. The reaction coordinate involves overcoming a significant activation energy.

Structural and Functional Details

• Subunit Functions: The F0 portion forms a proton channel; the c-subunit rotates as protons flow through. The F1 portion (with alpha and beta subunits) catalyzes ATP synthesis.

• Proton Route: Protons enter the F0 channel, bind to the c-subunit, causing rotation, which induces conformational changes in the F1 catalytic sites.

• Energetics: The proton motive force has two components: chemical (ΔpH) and electrical (Δψ). Both contribute to the free energy available for ATP synthesis: $\Delta p = \Delta \psi - \frac{2.3RT}{F} \Delta pH$

• ATP Formation: The beta subunits catalyze ATP synthesis via conformational changes (binding change mechanism), while alpha subunits play a structural role.

Example:

Three protons passing through ATP synthase are required to synthesize one ATP molecule (exact number varies by organism).

Photophosphorylation

Fundamental Concepts

• Photon Coupling: Light energy excites electrons in chlorophyll, raising them to higher energy levels.

• Water Splitting: The oxygen-evolving complex (OEC) in photosystem II splits water, releasing O2, protons, and electrons.

Mechanistic Details

• Architecture: Photosystems contain protein complexes and cofactors (chlorophyll, quinones, Fe-S clusters) embedded in the thylakoid membrane.

• Electron Excitation: Light excites electrons in chlorophyll, which are transferred through a series of carriers. The OEC replenishes lost electrons by oxidizing water.

• Electron Path: Excited electrons move through the electron transport chain, driving proton pumping and ultimately reducing NADP+ to NADPH.

• LHCII Control: Light-harvesting complex II (LHCII) regulates energy transfer and photoprotection.

Example:

In the Z-scheme, electrons flow from water (via PSII) to NADP+ (via PSI), generating both ATP and NADPH for the Calvin cycle.

Metabolic Flux and Control

Enzyme Regulation

• Types of Regulation: Enzyme activity is regulated by allosteric effectors, covalent modification (e.g., phosphorylation), substrate availability, and gene expression.

• Substrate Concentration and Activity: The Michaelis-Menten equation describes the relationship between substrate concentration and enzyme velocity: $v = \frac{V_{max}[S]}{K_m + [S]}$ At [S] = Km, the enzyme operates at 50% Vmax. For 10% and 90% activity, solve for [S] accordingly.

• Pathway Control: Key regulatory enzymes (e.g., AMPK) integrate signals to coordinate metabolic pathways, activating or inhibiting processes as needed.

Example:

AMPK activates catabolic pathways (e.g., glycolysis, fatty acid oxidation) and inhibits anabolic pathways (e.g., fatty acid synthesis) in response to low energy status.

Fatty Acid Oxidation

Fundamental Concepts

• Stepwise Breakdown: Fatty acids are degraded by beta-oxidation, removing two-carbon units as acetyl-CoA in each cycle.

• Reduced Equivalents: Each cycle produces one FADH2 and one NADH, which feed into the electron transport chain.

• Special Cases: Unsaturated and odd-chain fatty acids require additional enzymes for complete oxidation.

Mechanistic and Transport Details

• Beta-Oxidation Steps: 1) Activation (fatty acyl-CoA formation), 2) Dehydrogenation (FADH2 produced), 3) Hydration, 4) Dehydrogenation (NADH produced), 5) Thiolysis (acetyl-CoA release).

• B Carbon Activation: The beta carbon is activated (converted to a thioester/anhydride), making it susceptible to nucleophilic attack and facilitating cleavage.

• Lipid Transport: Lipids are transported via chylomicrons (intestine), VLDL (liver), and free fatty acids bound to albumin (adipocytes).

• Thermodynamics: Each step is energetically favorable, with the overall process yielding significant ATP via subsequent oxidation of acetyl-CoA.

Example:

Odd-chain fatty acids yield propionyl-CoA in the final cycle, which is converted to succinyl-CoA for entry into the citric acid cycle.

Table: Comparison of Electron Carriers in Oxidative Phosphorylation

Carrier	Electron Capacity	Type	Role
NAD+/NADH	2 electrons	Soluble	Initial electron donor (Complex I)
FAD/FADH2	2 electrons	Prosthetic group	Electron donor (Complex II)
Ubiquinone (Q)	1 or 2 electrons	Lipid-soluble	Mobile electron carrier
Cytochromes	1 electron	Heme protein	Electron transfer (Complexes III, IV)
Fe-S Clusters	1 electron	Protein-bound	Electron transfer (Complexes I, II, III)

Additional info: Table entries inferred from standard biochemistry knowledge for completeness.