OXIDATIVE PHOSPHORYLATION AND PHOTOPHOSPHORYLATION The Biochemical Makeup of the Mitochondrion The mitochondrion has an outer membrane that is freely permeable to small molecules and ions The inner membrane is impermeable to most small molecules and ions including H It contains the respiratory electron carriers Complexes I IV ADP ATP translocase ATP synthase FoF1 and other membrane transporters The mitochondrial matrix contains the pyruvate dehydrogenase complex citric acid cycle enzymes fatty acid oxidation enzymes amino acid oxidation enzymes DNA ribosomes many other enzymes ATP ADP Pi Mg2 Ca2 K and many soluble metabolic intermediates There are also ribosomes and porin channels Complex I is called NADH dehydrogenase and has FMN and Fe S as prosthetic groups Complex II is called succinate dehydrogenase and has FAD and Fe S as prosthetic groups Complex III is called ubiquinone cytochrome c oxidoreductase and has hemes and Fe S prosthetic groups Cytochrome c has a heme group Complex IV cytochrome oxidase has hemes CuA and CuB as prosthetic groups The inner membrane creates cristae and has transporters The matrix is inside the inner membrane The Respiratory Electron Transport Chain Coenzyme Q Ubiquinone Q is the fully oxidized form If it accepts a proton it becomes a semiquinone radical If it accepts another proton it becomes ubiquinol QH2 the fully reduced form Ubiquinone has ten units It is a lipid and can be dissolved in the mitochondrial membrane between complexes Ubiquinone transfers just one proton or electron It is similar to plastoquinone in the electron transport chain Prosthetic Groups of Cytochromes B type cytochromes have iron protoporphyrin IX Heme C is found in c type cytochromes Heme A is found in a type cytochromes There is a heme group in the active site of cytochromes Cytochrome a contains heme A and a vinyl group Cytochrome b contains iron protoporphyrin IX and 2 vinyl groups Cytochrome c contains heme C and no vinyl group The conjugated bonds in the cytochromes determine their absorption spectra Absorption Spectra of Cytochrome c absorbs in the 500 and 550 regions Cytochrome c absorbs strongly in the 400 region Reduced cytochrome c also Cytochrome c is characterized by the bend absorbance Cytochrome c s is the highest and cytochrome a s is the lowest The absorbance depends on the associated protein A cytochrome can be named based on the bend absorbance FAD can accept one or two electrons NAD can only transfer 2 electrons Cytochromes accept only one electron Proteins with iron Sulfur Centers complex or simple Ferredoxin of Anabaena has an iron sulfur center Iron sulfur centers can be Iron sulfur centers are networks of iron and sulfur They vary in complexity The Riske iron sulfur coordinates iron through a Histidine and contains Fe3 The Flow of Electrons and Protons through the Respiratory Electron Transport Chain NADH donates its electrons to Complex I Electrons flow from the N side into Complex I to ubiquinol to Complex III and cytochrome c and finally to Complex IV Succinate also donates electrons to Complex II which transfers them to Q forming fumarate 4 H are moved from the N side to the P side by Complex I and Complex III and 2 H are moved by Complex IV The electrons are finally donated to O2 to make H2O Cytochrome c moves between Complex III and Complex IV transferring electrons O2 accepts 2 H to make a molecule of water Four protons are required to reduce O2 This makes 4 waters A separation of charge across the membrane creates the energy used to drive the formation of ATP The Complexes of the Respiratory Chain complexes that can be separated Electron carriers are organized into membrane embedded supramolecular Complex I takes electrons from NADH and transfers them to ubiquinone The Path of Electrons from Different Donors to Ubiquinone NADH H donates its electrons to Complex I forming NAD The electrons are donated to FMN and move to a Fe S cluster through Complex I to Q When succinate donates its electrons they get donated to FAD and then move through Fe S clusters to Q Fumarate is formed in the process FAD can also donate electrons to Q The ubiquinone pool refers to the large number of ubquinone dissolved in the membrane Electrons from glycolysis are transferred through FAD from G3P using G3P dehydrogenase Complex I NADH Ubiquinone Oxidoreductase NADH donates 2 electrons to FMN These electrons move through a series of Fe S centers to N 2 Then they move to Q forming QH2 with the addition of 2 H 4H are moved from the N side to the P side by the membrane arm FMN takes electrons from NADH There are many Fe S clusters that transfer electron between them Reduction potentials go from more negative to more positive Q takes protons from the matrix N side to form QH2 This process is vectorial meaning it only occurs in one direction Complex I pumps protons from the matrix to the P side the intermembrane space Oxidation of one NADH pumps 4 protons across the membrane FeS clusters transport only 1 electron FMN takes 2 electrons but only gives up 1 at a time This reduces ubiquinone to ubiquinol The topology of the complexes their orientation in the membrane is crucial Complex I catalyzes the vectorial transfer of 4 protons per NADH from the matrix N for negative side to the intermembrane space P for positive Structure of Complex II Succinate Dehydrogenase from Porcine heart The heme protects against ROS reactive oxygen species which are bad for the cell FAD is in the substrate binding site which sticks out on the N side There are multiple subunits The cytochrome does not participate There are Fe S centers and a heme b If the heme malfunctions the number of ROS in the cell increases which can cause tumors Heme b shields the system from electrons that end up on the wrong oxygen creating radicals The Cytochrome bc1 Complex III There is a cavern between cytochrome b and heme bH There are two heme bLs Heme c1 and cytochrome c1 stick out on the P side as well as a Rieske iron sulfur protein which has 2Fe and 2S Drugs that block electron transport include myxothiazol which binds at the QP site and antimycin A which binds at the QN site piscicide The Q cycle accommodates the switch between the two electron carrier ubiquinone and the one electron carriers cytochromes b562 b566 c1 and c 4 protons pass through Complex III per pair of electrons passed on from Complex III to 2 cytochrome c Cytochrome bc1 transfers electrons from ubiquinol to cytochrome c The Riske FeS protein