Electron Transport System and Oxidative Phosphorylation

• Electron Transport Chain and Oxidative Phosphorylation involve ATP synthase using gradient energy to make ATP.
• The Δ G’ o for the Electron Transport Chain is -7.3 kcal/Mol.
• 1 Mol ATP = 507,18 grams.
• Despite having their own genome, most mitochondrial proteins are encoded in the nucleus, made in the cytosol and imported into the mitochondria.
• Uncouplers of oxidative phosphorylation dissociate oxidation in the respiratory chain from phosphorylation.
• Dinitrophenol short-circuits the proton circuit.
• The antibiotic oligomycin completely blocks oxidation and phosphorylation by acting on a step in phosphorylation.
• Dinitrophenol is a hydrophobic molecule that remains in the mitochondrial membrane as a chemical uncoupler for a long time - a very dangerous way to burn fat.
• Atractyloside inhibits oxidative phosphorylation by inhibiting the transporter of ADP into and ATP out of the mitochondrion.
• The uncoupler that has been used most frequently is 2,4-dinitrophenol, but other compounds act in a similar manner.
• These compounds are toxic in vivo, causing respiration to become uncontrolled, since the rate is no longer limited by the concentration of ADP or Pi.
• Mitchell’s chemiosmotic theory postulates that the energy from oxidation of components in the respiratory chain is coupled to the translocation of hydrogen ions (protons, H+) from the inside to the outside of the inner mitochondrial membrane.
• The electrochemical potential difference resulting from the asymmetric distribution of the hydrogen ions is used to drive the mechanism responsible for the formation of ATP.
• The inner mitochondrial membrane lacks an NADH transporter, and NADH produced in the cytosol cannot directly enter the mitochondrial matrix.
• Two electrons of NADH are transported from the cytosol into the matrix using substrate shuttles.
• In the glycerol 3-phosphate shuttle, two electrons are transferred from NADH to dihydroxyacetone phosphate by cytosolic glycerol phosphate dehydrogenase.
• The glycerol 3-phosphate produced is oxidized by the mitochondrial isozyme as FAD is reduced to FADH2.
• Subunit I of Complex IV contains another copper ion (CuB) and two heme groups a and a3.
• On the P side of the membrane, two molecules of QH2 are oxidized to Q to release 2 protons per Q (4 protons per 2 QH2).
• Q reduction takes 2 steps to get to QH2 and uses up 2 protons per Q.
• The heme a3 and CuB form a center that accept electrons from heme a and transfers them to oxygen bound to heme a3.
• In addition it pumps one proton from the N to P side per electron that passes through.
• For every 4 electrons passing through the complex, the enzyme consumes 4 protons from the matrix (N side) in converting oxygen to water.
• Complex III (Pump 2) transfers electrons from QH2 to the Rieske iron-sulfur protein binding at QP near to bL on the P side.
• The complex is large Mr 204,000 and contains 13 subunits of the inner mitochondrial membrane.
• Glycerol 3-phosphate dehydrogenase is a flavoprotein located on the outer surface of the inner mitochondrial membrane and reduces ubiquinone.
• Glycerol 3-phosphate is formed from triacylglycerol breakdown or reduction of dihydroxyacetone phosphate from glycolysis and is used to feed electrons into the system.
• Flavoprotein acyl-CoA dehydrogenase transfers electrons from the substrate to the FAD of the dehydrogenase then to the Electron Transferring Flavoprotein (ETF) which passes electrons to ETF:ubiquinone oxidoreductase.
• Each QH2 donates one electron via the Rieske-Fe-S center to cytochrome c1 and one electron via cytochrome b to a molecule of Q near the N side.
• Complex IV (Pump 3) carries electrons from cytochrome c to molecular oxygen reducing it to water.
• Subunit II of Complex IV contains 2 copper ions complexed with the –SH groups of two cysteine residues (CuA) in a binuclear center that resemble the 2Fe-2S centers of the iron sulfur proteins.
• In the malate-aspartate shuttle, malate is transported into the matrix and two electrons of NADH are transferred to it.
• NADH, FADH2, and NAD+ are coenzymes, that reversibly bind to enzymes.
• FAD is a prosthetic group, that remains tightly bound at the active site of an enzyme.
• The inner mitochondrial membrane can be disrupted into five separate enzyme complexes I, II, III, IV, and V.
• Complexes I to IV each contain part of the electron transport chain, whereas complex V catalyzes ATP synthesis.
• The respiratory system contributes to maintenance of body temperature.
• The adenine nucleotide transporter allows the exchange of ATP and ADP but not AMP.
• The respiratory system as a whole is sufficiently exergonic to be removed from equilibrium, allowing continuous unidirectional flow and constant provision of ATP.
• Such systems are necessary for uptake and output of ionized metabolites while preserving electrical and osmotic equilibrium.