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electron transport

Biochemistry: - reduction of molecular oxygen at the inner mitochondrial membrane with production of ATP - H+ gradient (higher pH with the mitochondrial matrix) provides driving force for F1 ATPase. see figure - oxidative phosphorylation or electron transport in eukaryotes occurs in mitochondria - it takes place in the inner mitochondrial membrane, in contrast to the reactions of the citric acid cycle & fatty acid oxidation which occur in the mitochondrial matrix - in oxidative phosphorylation, the electron transfer potential of NADH or FADH2 is converted into the phosphate-transfer potential of ATP - the driving force of oxidative phosphorylation is the electron transfer potention of NADH or FADH2 relative to molecular oxygen (O2). For NADH this is: 1/2 O2 + 2 H+ + 2 e- -> H2O Eo = +0.82 V NADH -> NAD+ + H+ + 2e- Eo = +0.32 V ---------------------------------------------------------------- 1/2 O2 + NADH + H+ -> H2O + NAD+ E = +1.14 V - the free energy is thus given by: G = nfE = (-2) (23.06) (1.14) = - 52.6 kcal/mole - electrons are transferred from NADH to O2 through a chain of 3 macromolecular complexes: complex I (NADH dehydrogenase), complex III (CoQ-cytochrome C reductase) & complex IV (cytochrome C oxidase) - electron flow within these complexes leads to pumping of protons from the mitochondrial matrix across the inner mitochondrial membrane into the intermembrane space. - electrons are transferred from FADH2 to O2 through complex II (succinate dehydrogenase), complex III & complex IV - electrons are shuttled between the complexes (complex I & complex II) & (complex II & complex III) by reduced coenzyme Q (CoQH2) - cytochrome C shuttles electrons from complex III to complex IV - complex IV catalyzes transfer of electrons from reduced cytochrome C to O2, the final electron acceptor - 4 electrons are transferred to O2 to completely reduce it to H2O with concomitant transfer of H+ from the mitochondrial matrix across the inner mitochondrial membrane into the intermembrane space - complex V utilizes the proton-motive force generated by movement of protons across the inner mitochondrial membrane during electron transport to phosphorylate ATP Mitochondrial matrix NADH or FADH2 F1 ATP ----> -----> ATPase -----> H+ NAD+ FAD ADP ____________________________________________________________________________________ NADH succinate CoQ cyt C cyt C dehydogenase dehydrogenase reductase oxidase F0 Complex I or Complex II Complex III Complex IV Complex V CoQH2 CoQH2 CoQ O2 -----> 2e- -----> 2e- ----> -----> CoQ CoQ CoQH2 H2O _____________________________________________________________________________________ cyt C[Fe+3] cyt C[Fe+3] ----> H+ -----> H+ ----> H+ cyt C[Fe+2] cyt C[Fe+2] Mitochondrial intermembrane space Pathology: - the overall process is hugely inefficient - the free energy for the formation of ATP from ADP is only +7.3 kcal/mole - the 52.6 kcal/mole used from electron transfer to accomplish this leads to an efficiency of about 14% - inefficient transfer of electrons leads to potential hazzards, especially with transfer of electrons to O2 - partial reduction products of O2, peroxides, hydroxyl radical & superoxide are cytotoxic & mutagenic - thus mechanisms have evolved to scavenge these reactive oxygen species produced during oxidative phosphorylation - considerable debate regarding the biological significance these reactive oxygen species surrounds the 'Free Radical Theory of Aging' proposed by Denham Harman in 1956. [2] Expression: - tissues with high rates of oxidative phosphorylation include: - muscle, heart, & brain

Related

combined oxidative phosphorylation deficiency (COXPD) mitochondria

General

molecular pathway

Database Correlations

Kegg map/map00190

References

  1. Stryer Biochemistry WH Freeman & Co, New York, 1988 pg 398-424
  2. Harman D, J Gerontol 11:298-300, 1956
  3. http://www.genome.ad.jp/kegg/pathway/map/map00190.html