Electron transport chain, oxidative phosphorylation & mitochondrial transport systems Joško Ivica
Electron transport chain & oxidative phosphorylation
collects e - & -H Oxidation of foodstuffs oxidizes e - & -H e - & -H REDUCING EQUIVALENTS Oxidative breakdown of foodstuffs
NAD + -2 e - +2 e - NADH Fig. taken from http://www.textbookofbacteriology.net/metabolism_2.html
FAD = Flavin adenine dinucleotide FMN = Flavin mononucleotide
Oxidative breakdown of foodstuffs reduced coenzymes NADH and FADH 2 generated during TCA cycle, fatty acid oxidation or glycolysis are oxidized by the electron transport chain (respiratory chain)
Mitochondria The system of electron carriers of ETC is located in the inner mitochondrial membrane
OXIDATION-REDUCTION REACTIONS A ox + B red A red + B ox E = standard oxidation-reduction (redox) potential Redox system E NAD + + 2H + + 2e - NADH + H + -0.32 Coenzyme Q ox + 2e - coenzyme Q red +0.10 Cytochrome b (Fe 3+ ) + e - Cytochrome b (Fe 2+ ) +0.12 Cytochrome c (Fe 3+ ) + e - Cytochrome c (Fe 2+ ) +0.22 Cytochrome a (Fe 3+ ) + e - Cytochrome a (Fe 2+ ) +0.29 ½ O 2 + 2H + 2e - 2H 2 O +0.82
ELECTRON TRANSPORT CHAIN Electrons are transferred from one complex to another in steps The final acceptor of electrons is a molecule of oxygen (O 2 )
ELECTRON TRANSPORT CHAIN Electron carriers (ETC complexes) are arranged in order of increasing redox potential (from the most negative to the most positive redox potential) Increasing affinity for electrons, which drives the flow of electrons in this direction! During reduction-oxidation reactions energy is released!
Energy released in reduction-oxidation reactions is used for pumping H + from matrix into intermembrane space Higher concentration of H + in intermembrane space electrochemical potential gradient or proton gradient across the inner mitochondrial membrane Proton gradient provides energy for synthesis of ATP!
protonmotive force = electrochemical potential gradient ΔpH = 0.75-1 ph units ΔΨ = 0.15-2.0 V The energy stored in protonmotive force drives the synthesis of ATP by the movement of protons down the electrochemical gradient through the ATP-synthase Peter Mitchell s chemiosmotic theory
ELECTRON TRANSFER FROM COMPLEX 1, COMPLEX 2 & OTHER FLAVOPROTEINS GLYCEROL 3-PHOSPHATE-DEHYDROGENASE COMPLEX 1 NADH-DEHYDROGENASE NADH SUCCINATE-DEHYDROGENASE FMN = Flavin mononucleotide ETF = Electron-transferring flavoprotein β-oxidation OF FATTY ACIDS ACYL-CoA DEHYDROGENASE
UBIQUINONE = COENZYME Q
Complex 3 (CYTOCHROME b-c 1 COMPLEX): Rieske Fe-S protein, cytochrome b, cytochrome c 1 Complex 4 (CYTOCHROME OXIDASE): Cu, cytochromes a and a 3
ATP SYNTHASE ADP + P i ATP Energy from the proton gradient is used for the release of the newly synthesized ATP Oxidative phosphorylation yields most of ATP produced in cell
ELECTRON TRANSPORT CHAIN PRODUCES THE MAJORITY OF ENERGY = ATP FROM FUEL OXIDATION NADH oxidation (2e-).. 2.5 ATP FADH 2 oxidation (2e-).. 1.5 ATP ATP
SUMMARY - Electron transfer within inner mitochondrial membrane occurs stepwise through a series of four complexes and two mobile carriers to the final acceptor O 2 - As electrons move through complexes 1, 3 and 4, protons are taken up from the matrix and released on the cytosolic side of the membrane - Higher concentration of protons in intermembrane space creates electrochemical potential gradient or proton gradient (ph in matrix is higher and it is negatively charged compared to intermembrane space) - Proton gradient causes proton reflux into the matrix through ATPsynthase that drives the formation of ATP by causing the release of bound ATP from the catalytic site
RESPIRATORY CONTROL Rate of electron transport is tightly coupled to ATP synthesis NAD + NAD + NAD + Higher ATP consumption ( ADP) leads to increased rate of electron transport chain ( NAD + ) ADP, NAD + increased fuel oxidation (glycolysis, TCA, fatty acid oxidation) NAD + muscle contraction active transport biosynthesis ATP ADP + P i
CLINICAL CORRELATION Patient experiences a second myocardial infarction confirmed by an electrocardiogram showing direct evidence of severe ischemia (lack of blood flow) Elevated blood levels of creatine kinase-mb (CK-MB) and troponin I (TnI) Ischemia has caused hypoxia (low levels of oxygen) in the heart muscle, resulting in inadequate generation of ATP (needed for the activity of Na + /K + ATP-ase) This leads to irreversible cell injury (necrosis) and consequently to leakage of cellular proteins into the bloodstream.
CLINICAL CORRELATION Acute inhalation or ingestion of high concentrations of cyanide (CN - ), which is one of the most potent poisons, leads rapidly to convulsions, coma and death It binds to the Fe 3+ of heme a 3 in cytochrome oxidase and prevents electron transport to O 2 Mitochondrial respiration and energy (ATP) production cease leading to cell death.
Mitochondrial transport systems
glycolysis Glycolysis cytosol Reducing equivalents from NADH produced in glycolysis are transported into mitochondrion by shuttle systems MALATE-ASPARTATE SHUTTLE GLYCEROL-PHOSPHATE SHUTTLE
AST AST Mitochondrion MDH malate dehydrogenase G3PDH glyceraldehyde 3-phosphate dehydrogenase AST aspartate aminotransferase
Cytosolic glycerol 3-phosphate dehydrogenase Dihydroxyacetone phosphate Glycerol 3-phosphate Cytosol Mitochondrial glycerol 3-phosphate dehydrogenase Inner mitochondrial membrane Mitochondrion
MITOCHONDRIAL TRANSPORTERS ATP/ADP TRANSLOCASE SYMPORT ANTIPORT
MITOCHONDRIAL TRANSPORTERS UNIPORT
EXPORT OF CITRATE Mitochondrion Malate Citrate CoASH Citrate synthase Acetyl CoA Oxaloacetate
NONSHIVERING THERMOGENESIS brain norepinephrine
SUMMARY ETC represents a series of redox reactions where electrons are eventually passed to the final acceptor oxygen Redox reactions cause H + pumping from mitochondrial matrix into the intermembrane space generating protonmotive force Reflux of H + back into the matrix drives ATP synthesis i.e. release of newly synthesized ATP from the enzyme (ATP synthase) ATP generated by oxidative phosphorylation can be used then in extramitochondrial compartments after being transported by specific mitochondrial transporter