Mitochondrial Electron Transport and ATP Synthesis

Endosymbiosis Theory: Mitochondria originated from a symbiotic relationship between host cells and prokaryotes.
Membrane Specialized Structure: - Outer Membrane: Freely permeable to small molecules. - Inner Membrane: Highly impermeable, requiring specific carriers; folded into cristae to increase surface area for electron transport chain (ETC) proteins.
Function: The mitochondrion catalyzes the conversion of energy from NADH oxidation to the phosphorylation of ADP, forming ATP via oxidative phosphorylation.

Objective: Transfer electrons from NADH and FADH₂ to the final electron acceptor, oxygen ( $O_2$ ), to form water ( $H_2O$ ).
Net Reaction: $NADH + 1/2 O_2 + H^+ \rightarrow NAD^+ + H_2O$ .
Stepwise Release: Energy is released gradually through four complexes to prevent explosive energy loss as heat, allowing for efficient harness into a proton gradient.
Mobile Carriers: - Coenzyme Q (Ubiquinone): A small, lipid-soluble benzoquinone that carries electrons between Complex I/II and Complex III within the lipid bilayer. - Cytochrome c: A water-soluble protein on the outer surface of the inner membrane that transfers electrons from Complex III to Complex IV using electrostatic interactions via Lysine residues.

Complex I (NADH Dehydrogenase / NADH-Coenzyme Q Reductase): - Contains at least $46$ proteins, FMN, and Fe-S clusters. - Transfers electrons from NADH to Coenzyme Q. - Action: Pumps $4\,H^+$ from the matrix to the intermembrane space.
Complex II (Succinate Dehydrogenase / Succinate-Coenzyme Q Reductase): - Part of the TCA cycle; converts succinate to fumarate. - Transfers electrons from FADH₂ to Coenzyme Q via Fe-S centers. - Action: Does NOT pump protons.
Complex III (Cytochrome c Reductase / Coenzyme Q - Cytochrome c Reductase): - Composed of cytochrome b, cytochrome c1, and the Fe-S Rieske protein. - Action: Pumps $4\,H^+$ and transfers electrons to Cytochrome c.
Complex IV (Cytochrome c Oxidase): - Contains cytochrome a, cytochrome a3, and two Cu complexes. - Reduces $O_2$ to $2\,H_2O$ ( $O_2 + 4H^+ + 4e^- \rightarrow 2\,H_2O$ ). - Action: Pumps $2\,H^+$ across the membrane.

Structure: - $F_1$ Catalytic Subunit: Located in the matrix; stoichiometry $\alpha_3\beta_3\gamma\delta\epsilon$ . - $F_o$ Complex: Integral membrane proteins mediating proton transport.
Mechanism: Driven by the proton-motive force (an electrochemical gradient consisting of both concentration and electrical potential components).
Binding Change Mechanism (Paul Boyer): - Proton flow through $F_o$ causes rotation of the shaft relative to the $\beta$ subunits. - Three Conformations: Loose (binds ADP + Pi), Tight (forms ATP), and Open (releases ATP).
Requirement: Approximately $3\,H^+$ are required to synthesize one molecule of ATP.

Respiratory Control: Respiration is dependent on the availability of ADP; without ADP, protons cannot flow back, stopping the ETC.
Uncouplers (e.g., 2,4-dinitrophenol): Lipid-soluble weak acids that dissipate the proton gradient by carrying $H^+$ back into the matrix. They allow respiration to continue without ATP synthesis, releasing energy as heat.
Thermogenin (Uncoupling Protein): Found in brown adipose tissue; used by newborns and hibernating mammals for non-shivering thermogenesis.
Inhibitors: - Statins: Inhibit Coenzyme Q10 synthesis because it shares the same precursor (mevalonate) as cholesterol. - Cyanide: Blocks electron transfer in Complex IV.

Complex I Deficiency: The most frequent cause of oxidative phosphorylation disorders.
Symptoms: Includes progressive encephalomyopathies, cardiomyopathy, and metabolic acidosis.
Genetics: Mitochondrial diseases are often maternally transmitted. The severity of the disease in offspring depends on the Bottleneck Effect, where the number of mutant mitochondria in the mature egg varies randomly.
Prevalence: Estimated at $1$ in $2,500$ to $4,000$ births.