Oxidative Phosphorylation Results from Cellular Respiration Mitochondrion Structure mitochondria oxidize carbon fuels to produce ATP and H2O this transformation involves electron and proton flow through several large protein complexes the rate of oxidative phosphorylation is determined by the need for ATP (site of citric acid cycle & fatty acid oxidation) (permeable to small molecules) (site of oxidative phosphorylation) Net Reaction: 30 molecules of ATP are produced by the complete oxidation of glucose (C6H12O6)

Sequence of Electron Carriers in the Respiratory Chain ∆G°' = -nF∆E°' where F = 96.48 kJ mol-1 V-1 ∆G°' = the standard free energy change at pH 7 ∆E°'= the standard reduction potential at pH 7 Standard Reduction Potentials of Some Reactions Strong reductants (e.g., NADH) are capable of reducing strong oxidizing agents (e.g., O2)

High Potential Electrons of NADH Enter the Respiratory Chain at NADH-Q Oxidoreductase Structure of NADH-Q oxidoreductase at 22Å Net reaction: NADH + Q + 5H+ (matrix)  NAD+ + QH2 + 4H+ (cytosol) Q is reduced in the membrane segment of the protein NADH is oxidized in the segment that projects into the matrix 2 protons are taken up from the matrix upon reduction of Q

Electrons Flow from Ubiquinol to Cytochrome c Through Q-Cytochrome c Oxidoreductase Net reaction: QH2 + 2Cyt cox + 2H+ (matrix)  Q + 2Cyt cred + 4H+ (cytosol) Q-Cytochrome c Oxidoreductase Structure the iron ions of the cytochrome alternates between the reduced Fe2+ and oxidized Fe3+ forms during electron transport Q-cytochrome c oxidoreductase is a dimer that contains a total of three hemes and one 2Fe-2S iron-sulfur cluster (a.k.a. Rieske center) oxidized form reduced form oxidized form reduced form Q cycle Two electrons of a bound QH2 are transferred, one to cytochrome c and the other to a bound Q to form the semiquinone. The newly formed Q dissociates and is replaced by a second QH2, which gives up its electrons, one to a second molecule of cytochrome c and the other the reduce the semiquinone to QH2. This second electron transfer results in the uptake of two H+ from the matrix.

Cytochrome c Oxidase Catalyzes the Reduction of O2 to H2O Net reaction: 4 Cyt cred + 4 H+ (matrix) + O2  4 Cyt cox + 2H2O Cytochrome c Oxidase Structure Cytochrome oxidase mechanism The cycle begins with all prosthetic groups in their reduced forms. Note that the four hydrogens found in the water molecules originated from the matrix H+ gradient.

Proton Gradient Across the Inner Mitochondrial Membrane Drives ATP Synthesis Peter Mitchell’s chemiosmotic hypothesis: the pH gradient and membrane potential constitute a proton-motive force that is used to drive the synthesis of ATP in other words, the respiratory chain and ATP synthase are biochemically separate systems that are linked by the proton motive force)

Power Transmission by Proton Gradients: A Unifying Theme in Bioenergetics