Chapter 19 Oxidative Phosphorylation and Photophosphorylation Oxidative phosphorylation is when the energy of oxidation drives the synthesis of ATP Photophosphorylation is the means by which photosynthetic organisms capture the energy of sunlight and harness it to make ATP In eukaryotes oxidative phosphorylation occurs in mitochondria and photophosphorylation occurs in chloroplasts Oxidative phosphorylation involves the reduction of O2 to H2O with electrons donated by NADH and FADH2 o Occurs equally well in light or darkness Photophosphorylation involves the oxidation of H2O to O2 with NADP as the ultimate electron acceptor o Dependent on the presence of light Chemiosmotic theory reservoir for the energy extracted from biological oxidation reactions transmembrane differences in proton concentration are the Oxidative phosphorylation and photophosphorylation are mechanistically similar in three respects bound carriers o Both processes involve the flow of electrons through a chain of membrane o The free energy made available by this downhill exergonic electron flow is coupled to the uphill transport of protons across a proton impermeable membrane conserving the free energy of fuel oxidation as a transmembrane electrochemical potential o The transmembrane flow of protons down their concentration gradient through specific protein channels provides the free energy for synthesis of ATP catalyzed by a membrane protein complex that couples proton flow to phosphorylation of ADP OXIDATIVE PHOSPHORYLATION 19 1 Electron Transfer Reactions in Mitochondria Mitochondria have two membranes o The outer mitochondrial membrane is permeable to small molecules and ions which move through transmembrane channels formed by a family of integral membrane proteins called porins o The inner membrane is impermeable to most small molecules and ions including protons transporters The only species that cross this membrane do so through specific Bears the components of the respiratory chain and ATP synthase Segregates the intermediates and enzymes of cytosolic metabolic pathways from those of metabolic processes occurring in the matrix o The mitochondrial matrix enclosed by the inner membrane contains the pyruvate dehydrogenase complex and the enzymes of the citric acid cycle the fatty acid oxidation pathway and the pathways of amino acid oxidation Electrons Are Funneled to Universal Electron Acceptors Oxidative phosphorylation begins with the entry of electrons into the respiratory chain o Most of these electrons arise from the action of dehydrogenases that funnel electrons into NAD NADP FAD or FMN Nicotinamide nucleotide linked dehydrogenases oxidize substrates catalyze reversible reactions that NAD linked dehydrogenases remove two hydrogens from their substrates o One is transferred as a hydride ion to NAD o The other is released as H in the medium NADH and NADPH are water soluble electron carriers that associate reversibly with dehydrogenases o NADH carries electrons from catabolic reactions to their point of entry into the respiratory chain the NADH dehydrogenase complex o NADPH generally supplies electrons to anabolic reactions o Neither can cross the inner mitochondrial membrane but they can shuttle their electrons across Flavoproteins nucleotide FMN or FAD contain a very tightly sometimes covalently bound flavin o The oxidized flavin nucleotide can accept either one electron or two FADH2 o Associated with proteins Electrons Pass through a Series of Membrane Bound Carriers The mitochondrial respiratory chain consists of a series of sequentially acting electron carriers most of which are integral proteins with prosthetic groups capable of accepting and donating either one or two electrons Three types of electron transfers occur o Direct transfer o Transfer as a hydrogen atom o Transfer as a hydride ion two electrons Reducing equivalent reduction reaction a single electron equivalent transferred in an oxidation Three other types of electron carrying molecules function in the respiratory chain o A hydrophobic quinone ubiquinone o Two different types of iron containing proteins cytochromes and iron sulfur proteins Ubiquinone isoprenoid side chain coenzyme Q or Q a lipid soluble benzoquinone with a long o Can accept one electron to become the semiquinone radical o Can accept two electrons to form ubiquinol o Freely diffusible within the lipid bilayer of the inner mitochondrial membrane Cytochromes their iron containing heme prosthetic groups proteins with characteristic strong absorption of visible light due to Mitochondria contain three classes of cytochromes o a o b o c o Distinguished by differences in their light absorption spectra The heme cofactors of a and b cytochromes are tightly bound to their associated proteins o The hemes of c cytochromes are covalently attached through Cys residues The cytochromes of type a and b and some of c are integral proteins of the inner proteins in which the iron is present in association with mitochondrial membrane Iron sulfur proteins inorganic sulfur atoms or with the sulfur atoms of Cys residues in the protein or both Rieske iron sulfur proteins than two Cys residues one Fe atom is coordinated to two His residues rather All iron sulfur proteins participate in one electron transfers in which one iron is oxidized or reduced In the overall reaction catalyzed by the mitochondrial respiratory chain electrons move from NADH succinate or some other primary electron donor through flavoproteins ubiquinone iron sulfur proteins and cytochromes and finally to O2 Electrons tend to flow from carriers of lower reduction potentials to carriers with higher potentials Electron Carriers Function in Multienzyme Complexes There are four complexes into which electron carriers are organized in the inner mitochondrial membrane o Complexes I and II catalyze electron transfer to ubiquinone from NADH Complex I and succinate Complex II o Complex III carries electrons from reduced ubiquinone to cytochrome c o Complex IV transfers electrons from cytochrome c to O2 Complex I NADH to Ubiquinone o Complex I large enzyme composed of 42 different polypeptide chains NADH ubiquinone oxidoreductase or NADH dehydrogenase a Contains an FMN containing flavoprotein and at least 6 iron sulfur centers Catalyzes two simultaneous and obligately coupled processes The exergonic transfer to ubiquinone of a hydride ion from NADH and a proton from the matrix The endergonic transfer of four protons from the matrix to