Oxidative Phosphorylation

Cheatsheet Content

### Overview - **Definition:** Metabolic pathway that uses energy released by the oxidation of nutrients to produce ATP. - **Location:** Occurs in the inner mitochondrial membrane in eukaryotes, and the plasma membrane in prokaryotes. - **Key Components:** Electron Transport Chain (ETC) and ATP Synthase. - **Overall Goal:** Convert the energy from NADH and FADH2 (produced during glycolysis, pyruvate oxidation, and Krebs cycle) into a proton gradient, and then use that gradient to synthesize ATP. ### Electron Transport Chain (ETC) - **Function:** A series of protein complexes that transfer electrons from electron donors (NADH, FADH2) to electron acceptors (ultimately O2), pumping protons across the inner mitochondrial membrane. - **Components:** - **Complex I (NADH Dehydrogenase):** Accepts electrons from NADH, pumps 4 H+ per NADH. - **Complex II (Succinate Dehydrogenase):** Accepts electrons from FADH2 (part of Krebs cycle), does NOT pump protons directly. - **Coenzyme Q (Ubiquinone):** Lipid-soluble carrier, transfers electrons from Complex I & II to Complex III. - **Complex III (Cytochrome bc1 complex):** Accepts electrons from CoQ, pumps 4 H+ per electron pair. - **Cytochrome c:** Small protein carrier, transfers electrons from Complex III to Complex IV. - **Complex IV (Cytochrome c Oxidase):** Accepts electrons from Cytochrome c, transfers them to O2 (final electron acceptor), forming H2O. Pumps 2 H+ per electron pair. - **Electron Flow:** - NADH $\rightarrow$ Complex I $\rightarrow$ CoQ $\rightarrow$ Complex III $\rightarrow$ Cytochrome c $\rightarrow$ Complex IV $\rightarrow$ O2 - FADH2 $\rightarrow$ Complex II $\rightarrow$ CoQ $\rightarrow$ Complex III $\rightarrow$ Cytochrome c $\rightarrow$ Complex IV $\rightarrow$ O2 - **Proton Pumping:** Complexes I, III, and IV pump protons from the mitochondrial matrix into the intermembrane space, creating an electrochemical proton gradient (proton-motive force). ### ATP Synthase (Complex V) - **Function:** Uses the energy of the proton gradient to synthesize ATP from ADP and Pi. - **Structure:** - **F0 subunit:** Embedded in the inner mitochondrial membrane, acts as a proton channel. - **F1 subunit:** Protrudes into the mitochondrial matrix, contains the catalytic sites for ATP synthesis. - **Mechanism (Chemiosmosis):** 1. Protons flow down their electrochemical gradient through the F0 channel into the matrix. 2. This proton flow causes the F0 subunit to rotate. 3. The rotation of F0 induces conformational changes in the F1 subunit. 4. These conformational changes drive the synthesis of ATP from ADP and Pi. - **Yield:** Approximately 2.5 ATP per NADH and 1.5 ATP per FADH2. ### Proton-Motive Force (PMF) - **Definition:** The energy stored in the electrochemical gradient across the inner mitochondrial membrane. - **Components:** - **Electrical potential ($\Delta\Psi$):** Due to the charge difference (more positive in intermembrane space). - **pH gradient ($\Delta pH$):** Due to the concentration difference of protons (lower pH in intermembrane space). - **Role:** Drives ATP synthesis by ATP synthase. ### Inhibitors and Uncouplers - **ETC Inhibitors:** Block electron flow, stopping proton pumping and ATP synthesis. - **Cyanide (CN-), Carbon Monoxide (CO):** Inhibit Complex IV. - **Rotenone, Amytal:** Inhibit Complex I. - **Antimycin A:** Inhibits Complex III. - **ATP Synthase Inhibitors:** Block proton flow through ATP synthase, stopping ATP synthesis. - **Oligomycin:** Binds to F0 subunit, blocking proton channel. - **Uncouplers:** Dissipate the proton gradient without allowing ATP synthesis, leading to heat production. - **Dinitrophenol (DNP):** Lipid-soluble proton carrier, transports protons across the membrane, bypassing ATP synthase. - **Thermogenin (UCP1):** Natural uncoupling protein found in brown adipose tissue, generates heat.