# Lecture 16: Oxidative Phosphorylation ## Biochemistry --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the organization and components of the electron transport chain (Complexes I-IV) 2. Explain the chemiosmotic hypothesis (Mitchell's theory) 3. Describe the structure and mechanism of ATP synthase (Complex V) 4. Calculate the ATP yield from NADH and FADH2 oxidation 5. Describe the roles of the malate-aspartate and glycerol-3-phosphate shuttles 6. Explain how uncouplers, inhibitors, and regulatory mechanisms affect oxidative phosphorylation --- ## Lecture Content ### I. Overview Oxidative phosphorylation is the process by which ATP is synthesized using energy from the transfer of electrons through the electron transport chain (ETC) to molecular oxygen. It takes place at the **inner mitochondrial membrane** and is responsible for approximately 90% of ATP production in aerobic cells. Two processes are coupled: **electron transport**, in which electrons from NADH and FADH2 flow through Complexes I-IV to O2, and **ATP synthesis**, in which the proton gradient generated by electron transport drives ATP synthase. ### II. Mitochondrial Structure The **outer membrane** is permeable to small molecules and ions through porins (VDAC). The **intermembrane space** has a composition similar to the cytosol and is where protons accumulate. The **inner membrane** is highly impermeable and folded into cristae to increase surface area; it houses the ETC complexes, ATP synthase, and cardiolipin (important for Complex III and IV function). The **matrix** contains TCA cycle enzymes, PDC, mitochondrial DNA, and ribosomes. ### III. Electron Carriers **NADH** donates 2 electrons as a hydride to Complex I (E^0' = -0.32 V). **FADH2** donates 2 electrons to Complex II (E^0' = -0.22 V). **Ubiquinone** (Coenzyme Q) is a lipid-soluble carrier that shuttles electrons from Complexes I and II to Complex III, accepting one or two electrons to form semiquinone or ubiquinol. **Cytochrome c** is a small, water-soluble protein in the intermembrane space that carries one electron via its Fe3+/Fe2+ couple from Complex III to Complex IV. **Iron-sulfur clusters** in Complexes I, II, and III transfer one electron at a time. **Heme groups** in cytochromes (types b, c1, c, a, a3) also transfer single electrons. ### IV. The Electron Transport Chain **Complex I (NADH Dehydrogenase)** transfers electrons from NADH through FMN and iron-sulfur clusters to ubiquinone, **pumping 4 H+** from the matrix to the intermembrane space per NADH. It is the largest complex with approximately 45 subunits in mammals and is inhibited by rotenone, barbiturates, and piericidin A. **Complex II (Succinate Dehydrogenase)** transfers electrons from FADH2 (generated from succinate) through iron-sulfur clusters to ubiquinone. It **does not pump protons** because the energy released is insufficient. It is the same enzyme as TCA cycle step 6, and malonate is a competitive inhibitor. **Complex III (Cytochrome bc1 Complex)** transfers electrons from ubiquinol through iron-sulfur clusters and cytochrome c1 to cytochrome c via the **Q cycle**, a mechanism that efficiently transfers electrons and pumps protons. It **pumps 4 H+ per pair of electrons** and is inhibited by antimycin A. **Complex IV (Cytochrome c Oxidase)** transfers electrons from cytochrome c through cytochrome a and the CuA/CuB centers to O2, producing water. It **pumps 2 H+ per pair of electrons** (plus 2 H+ are consumed from the matrix side in water formation). Inhibitors include cyanide, carbon monoxide, hydrogen sulfide, and azide, all of which bind to the O2-binding site on cytochrome a3. Cyanide poisoning is treated with nitrites (forming methemoglobin that binds CN-) and thiosulfate (converting CN- to thiocyanate). A diagram of the electron transport chain embedded in the inner mitochondrial membrane. Complexes I through IV are shown as distinct structures spanning the membrane. Electron flow is indicated by arrows: NADH donates to Complex I, FADH2 donates via Complex II, ubiquinone (CoQ) shuttles between Complexes I/II and III, cytochrome c shuttles between III and IV, and O2 is the final electron acceptor at Complex IV. Proton pumping is shown for Complexes I (4H+), III (4H+), and IV (2H+), with protons moving from the matrix to the intermembrane space. The proton gradient (high H+ in IMS, low in matrix) is indicated, with ATP synthase (Complex V) using this gradient to synthesize ATP. Specific inhibitors are labeled at their sites of action: rotenone at Complex I, antimycin A at Complex III, cyanide at Complex IV, and oligomycin at ATP synthase. ### V. The Chemiosmotic Hypothesis Proposed by Peter Mitchell in 1961 (Nobel Prize 1978), the chemiosmotic hypothesis explains that energy from electron transport pumps protons from the matrix to the intermembrane space, creating a **proton-motive force (pmf)** with two components: a **chemical gradient** (delta-pH, with higher H+ in the IMS) and an **electrical gradient** (delta-psi, approximately 180 mV, positive in the IMS and negative in the matrix). The pmf drives protons back through ATP synthase, coupling proton flow to ATP synthesis. Evidence includes the requirement for an intact inner membrane, the observation that uncouplers dissipate the gradient and stop ATP synthesis while electron transport continues, and experiments showing that an artificially created pH gradient drives ATP synthesis. ### VI. ATP Synthase (Complex V / F1F0-ATPase) The **F0 subunit** is membrane-embedded and forms the proton channel. Its c-ring rotates as protons flow through, and the a subunit provides the proton half-channels. The **F1 subunit** projects into the matrix and contains the catalytic sites. Its alpha3-beta3 hexamer has three catalytic beta subunits. The gamma subunit (central stalk) rotates with the c-ring, while the delta and epsilon subunits form the stator that prevents the alpha3-beta3 hexamer from rotating. #### Binding Change Mechanism (Paul Boyer, Nobel Prize 1997) Each beta subunit cycles through three conformational states: **O (Open)**, which has low affinity and releases ATP; **L (Loose)**, which binds ADP + Pi loosely; and **T (Tight)**, which catalyzes ATP formation through tight binding that makes the reaction thermodynamically favorable. Gamma subunit rotation, driven by proton flow, drives these conformational changes. One full 360-degree rotation produces **3 ATP**, and approximately 10 protons flow through F0 per rotation (in humans, with approximately 10 c subunits), yielding approximately 3.3 H+ per ATP. **Oligomycin** blocks the proton channel of F0, stopping both ATP synthesis and proton flow. Electron transport also stops because the proton gradient cannot be dissipated. ### VII. ATP/NADH Yield Per NADH, electrons enter at Complex I with 10 H+ pumped total (4 + 4 + 2), yielding approximately **2.5 ATP** per NADH. Per FADH2, electrons enter at Complex II with 6 H+ pumped (0 + 4 + 2), yielding approximately **1.5 ATP** per FADH2. One additional H+ is consumed by the adenine nucleotide translocase and phosphate carrier per ATP exported to the cytoplasm. ### VIII. Shuttle Systems for Cytoplasmic NADH NADH cannot cross the inner mitochondrial membrane, so two shuttle systems transfer reducing equivalents. The **malate-aspartate shuttle** (active in heart, liver, kidneys) transfers cytoplasmic NADH reducing equivalents to produce matrix NADH, yielding the full approximately 2.5 ATP per NADH. The **glycerol-3-phosphate shuttle** (active in brain and skeletal muscle) transfers electrons to mitochondrial FAD via an inner membrane enzyme, producing FADH2 and yielding only approximately 1.5 ATP per cytoplasmic NADH. ### IX. Uncouplers and Regulatory Mechanisms #### Uncoupling Uncouplers dissipate the proton gradient by carrying protons across the inner membrane without passing through ATP synthase. Electron transport continues or even accelerates, but no ATP is produced and the energy is released as **heat**. **2,4-Dinitrophenol (DNP)** is a lipid-soluble weak acid once used as a dangerous weight-loss drug. **FCCP/CCCP** are laboratory uncouplers. **Thermogenin (UCP1)** is a natural uncoupling protein in brown adipose tissue that generates heat for non-shivering thermogenesis in newborns and hibernating animals; it is activated by fatty acids and inhibited by purine nucleotides. #### Respiratory Control The rate of oxidative phosphorylation is tightly coupled to ATP demand. When ADP is plentiful, electron transport and ATP synthesis proceed rapidly (state 3 respiration). When ADP is scarce, electron transport slows because the proton gradient builds up and opposes further pumping (state 4 respiration). The respiratory control ratio (rate of O2 consumption with ADP divided by rate without ADP) quantifies this coupling. A detailed diagram of ATP synthase structure and mechanism. Panel A: Cross-section of the complete ATP synthase showing the F0 portion (c-ring, a subunit) embedded in the inner membrane and the F1 portion (alpha3-beta3 hexamer, gamma central stalk, delta and epsilon subunits) projecting into the matrix. Proton flow through the a subunit drives rotation of the c-ring and gamma subunit (rotation arrows indicated). Panel B: The binding change mechanism viewed from below the F1 head, showing the three beta subunits in their three conformational states (O = open/empty, L = loose/ADP+Pi bound, T = tight/ATP formed). Arrows show how 120-degree rotation of gamma converts each beta to the next state. Panel C: The overall chemiosmotic coupling showing the proton gradient across the membrane driving ATP synthase, with the ATP-ADP translocase exporting ATP and importing ADP. ---