Oxidative phosphorylation Study Guide

📖 Core Concepts Oxidative phosphorylation – mitochondrial (or bacterial membrane) pathway that uses electron transport to create a proton‑motive force (Δp) and synthesize ATP. Chemiosmotic coupling – energy from electron flow pumps protons, building Δp (ΔpH + electrical potential). Protons flow back through ATP synthase → ATP. Electron transport chain (ETC) – four inner‑membrane complexes (I‑IV) plus mobile carriers (CoQ, cytochrome c). Electrons travel from NADH/FADH₂ to O₂, releasing energy for proton pumping. Proton‑to‑ATP ratio – ≈ 3–4 H⁺ per ATP synthesized (depends on c‑ring size of ATP synthase). Reactive oxygen species (ROS) – partial reduction of O₂ at Complex III/IV produces superoxide (O₂⁻) and H₂O₂, which can damage macromolecules. 📌 Must Remember NADH → Complex I → pump 4 H⁺; FADH₂ → Complex II (no pump) → Q → Complex III (pump 4 H⁺) → Complex IV (pump 2 H⁺). Total H⁺ pumped per NADH ≈ 10 (4 + 4 + 2); per FADH₂ ≈ 6 (0 + 4 + 2). Q‑cycle in Complex III moves 4 H⁺ per ubiquinol oxidized. Oligomycin blocks FO of ATP synthase → stops proton re‑entry, halts NADH oxidation. Rotenone, piericidin A inhibit Complex I; antimycin A blocks Q‑cytochrome b segment; cyanide/CO inhibit Complex IV. Uncouplers (e.g., dinitrophenol) collapse Δp, stimulate respiration but prevent ATP synthesis. 🔄 Key Processes Electron flow & proton pumping NADH → FMN (Complex I) → Fe‑S clusters → Q → Complex III → cytochrome c → Complex IV → O₂ → H₂O. Each redox step releases energy used to pump protons across the inner membrane. Q‑cycle (Complex III) QH₂ binds, releases 2 e⁻ to cytochrome c and 2 H⁺ to intermembrane space. The semiquinone intermediate picks up 2 e⁻ from matrix Q, completing the cycle and moving a total of 4 H⁺ per QH₂. ATP synthase (Complex V) rotation Proton influx through FO rotates c‑ring → γ‑subunit turns → β‑subunits undergo open → loose → tight → open conformations → ADP + Pᵢ → ATP. ROS generation Incomplete O₂ reduction at ubisemiquinone (Complex III) or heme‑Cu site (Complex IV) yields O₂⁻ → dismutates to H₂O₂. 🔍 Key Comparisons Complex I vs. Complex II – I pumps 4 H⁺ per NADH; II pumps none (entry point for FADH₂). Mitochondrial ΔpH vs. Chloroplast ΔpH – mitochondria rely mainly on electrical potential; chloroplasts rely mainly on ΔpH. Oligomycin vs. Uncoupler – Oligomycin blocks proton re‑entry (stop ATP); uncoupler shuttles protons back (heat, no ATP). Rotenone vs. Antimycin A – Rotenone blocks electron entry at Complex I; Antimycin A blocks electron flow within Complex III (Q‑cytochrome b segment). ⚠️ Common Misunderstandings “Complex II makes ATP” – false; it only feeds electrons to Q, no proton pumping. “All ROS come from Complex IV” – most ROS arise at the ubisemiquinone site of Complex III. “Higher Δp always means more ATP” – excess Δp can back‑pressure pumps, increase ROS, and trigger uncoupling. 🧠 Mental Models / Intuition “Battery + Motor” – ETC = battery charger (pumps protons → stores energy); ATP synthase = motor that converts stored charge into mechanical rotation → ATP. “Waterfall” analogy – electrons flow downhill (high to low redox potential) while dragging protons uphill, building a reservoir (Δp) that later powers a turbine (ATP synthase). 🚩 Exceptions & Edge Cases Prokaryotic ETCs – can use many alternative donors/acceptors; the same Δp principle applies but complexes may be rearranged. Hypoxia‑tolerant ectotherms – keep Complex V from running in reverse, avoid ATP consumption and succinate buildup. Uncoupling proteins (UCPs) – regulated proton leaks in brown adipose tissue produce heat without ATP. 📍 When to Use Which Identify donor: NADH → start at Complex I; FADH₂ or succinate → enter at Complex II. Predict ATP yield: Use 10 H⁺/NADH, 6 H⁺/FADH₂, divide by 3–4 H⁺ per ATP. Choose inhibitor for experiments: Block entry → Rotenone (Complex I) or Antimycin A (Complex III). Halt ATP synthesis → Oligomycin. Collapse Δp → DNP or FCCP (uncoupler). 👀 Patterns to Recognize “Four‑proton” pattern: Complex I, III (Q‑cycle), and IV each contribute 4, 4, and 2 H⁺ respectively per electron pair. “Semiquinone” signal: Presence of ubisemiquinone indicates possible ROS hotspot. “Leakiness” clue: Higher membrane leak → lower Δp, lower ATP, higher heat production (e.g., brown fat). 🗂️ Exam Traps Mis‑counting protons: Students often forget the 2 H⁺ consumed when Q is reduced to QH₂ (adds to gradient). Attributing ATP synthesis to Complex II: Remember it does not pump protons. Confusing inhibitor binding states: CO binds reduced cytochrome c oxidase; cyanide/azide bind oxidized form – exam may swap them. Assuming ROS only rise during hypoxia: In many tolerant ectotherms, ROS are minimal during reoxygenation, contrary to endotherms. --- Keep this sheet handy; the bullet format makes rapid scanning easy right before the exam.