The Role of Oxygen in Energy Production PDF

The role of oxygen in making energy available from major nutrients Dr. Kong Kin Weng Department of Molecular Medicine [email protected] 1 Learning Objectives: The cellular uptake of oxygen and nutrients; Mitoc...

The role of oxygen in making energy available from major nutrients Dr. Kong Kin Weng Department of Molecular Medicine [email protected] 1 Learning Objectives: The cellular uptake of oxygen and nutrients; Mitochondrial uptake of nutrient metabolites and oxygen Oxygen binding proteins: the cytochromes Electron transport from nutrient metabolites to mobile carriers Cytochrome Oxidase: oxygen binding and reduction Coupling of oxygen reduction to ATP synthesis 2 Stages in the extraction of energy from fuel molecules I. Digestion, absorption and transport II. Break down of small molecules to key metabolites III. Complete transfer of energy to a usable form by cells 3 Reactions that require Oxygen in Human Biology Oxygen: As electron sink in energy generation, extracting the high potential electron from NADH and FADH2 produced by oxidation of carbohydrate, amino acids and fatty acids. Other reactions that incorporate oxygen or OH groups into organic molecules (eg, biosynthesis of steroids) and also in biodegradation (P450 mono- oxygenases that are involved in the degradation of drugs and toxins). 4 Metabolism of major nutrients and extraction of energy CYTOPLASM Glucose Amino Acids Lactate Pyruvate Amino Acids Acetyl CoA Fatty Acids Oxaloacetate Citrate ATP 3 NADH 2CO2 Synthesis FADH 2 3 NAD + FAD MITOCHONDRIA 5 Role of Mitochondria and Oxygen in ATP Production Oxidative phosphorylation The process in which ATP is formed as a result of transfer of electrons from NADH or FADH2 to O2 (final acceptor) by a series of electron carriers. 6 Oxidative phosphorylation in eukaryotes occurs through protein complexes (electron transport chain and ATP synthase) located in the inner membrane of mitochondrion. 7 The flow of electrons from NADH or FADH2 to O2 through protein complexes (electron transport chain/respiratory chain) leads to the pumping of protons out of the mitochondrial matrix to generate a pH gradient used to generate ATP (Chemiosmostic hypothesis) 8  The Electron Transport Chain (ETC) (also known as the Respiratory chain) 3 oxido-reductase complexes which serve as proton pumps (complexes I, III and IV) NADH-Q reductase (Complex I) Cytochrome reductase (Complex III) Cytochrome oxidase (Complex IV) 2 electron carriers (Ubiquinone/Q and Cytochrome c) 9  The driving force for oxidative phosphorylation is the electron transfer potential of NADH or FADH2 relative to oxygen NADH + ½ O2 + H+ → H2O + NAD+ Go’= -52.6 kcal/mol The energy generated by oxidation of NADH or FADH2 is used to synthesise ATP. Each mole of ATP cost approximately 7.3 kcal to synthesise 10  Complex I (NADH-Q Reductase): Enzyme with 34 polypeptides, with FMN and Fe-S as prosthetic groups The flow of two electrons from NADH to QH2 via NADH-Q reductase results in pumping of 4 protons from the matrix to the cytosol side of the inner membrane. 11 Q: Coenzyme Q or ubiquinone A quinone derivative with long isoprenoid tail. 12 Complex II (Succinate-Q reductase): Q also receives electron from FADH2 Electrons from FADH2 are transferred to Complex II (or succinate dehydrogenase) and then to Q to enter into the electron transport chain. Complex II is not a proton pump. 13 14 Complex III (Cytochrome reductase) and cytochrome C: Electrons flow from ubiquinone (Q) to Complex III then to cytochrome C. 2 protons are pumped out into the cytosol side Cytochrome reductase Cytochrome c is an electron or complex bc1 carrier with a haeme 15  Structure of Haeme (Heme) H O O C - -O C C O CH2 CH2 N N CH2 H H 2C HC Fe CH C N N H3C CH3 N N C HC Fe CH H N N CH3 C C Basic structure of heme H2C H H H3C HC CH2 Heme serves as electron donor or Fe-Protoporphyrin-IX acceptor in cytochromes, and (precursor for haeme and cytochrome c) oxygen binding for haemoglobin and cytochrome oxidase 16  Complex IV (Cytochrome oxidase): Cytochrome c transfer electrons to oxygen Complex IV (or cytochrome a and a3) catalyses the transfer of electrons from cytochrome c to oxygen, with 4 protons pumped out into the cytosol side. 4 Cyt c (Fe2+) + 4H+ + O2 → 4 Cyt c (Fe3+) + 2 H2O https://www.youtube.com/watch?v=xbJ0nbzt5Kw 17 Electron transfer in the respiratory chain can be blocked by specific inhibitors Rotenone and amytal: NADH-Q reductase Antimycin A: cytochrome reductase (cytochromes bc1) CN-, N3- ,CO: cytochrome oxidase (cytochromes a and a3) 18 Chemiosmotic Hypothesis: (Peter Mitchell, 1961) The transfer of electrons through the respiratory chain leads to the pumping of protons from the matrix to the cytosolic side of the inner membrane, generating a proton-motive force. This proton-motive force drives the synthesis of ATP by the ATPase complex. 19  ATP is synthesised by ATP synthase ATP synthase: enzyme complex made of a proton conducting Fo and a catalytic F1 unit. The stalk between Fo and F1 contains several proteins, including one that renders the complex sensitive to F1 oligomycin. Fo 20  The Action of ATP Synthase Proton flux through the synthase results in conformation changes in the enzyme complex → ATP formation. The translocation of three (3) protons through the synthase leads to the formation of one ATP 21 The entry of ADP into mitochondria is coupled to the exit of ATP (ATP-ADP exchange) by the ATP-ADP translocase ATP-ADP exchange cost 1 proton 3 protons to form 1 ATP, 1 proton for transport About 25% of the energy yield from electron transfer by the respiratory chain is consumed for ATP transport. 22 ATP-ADP translocase 23  Energy Yield in Electron Transfer Number of proton pumped per electron pair: 10 NADH-Q reductase (4) cytochrome reductase (2) cytochrome oxidase (4) Synthesis of an ATP: 3 protons Transport of ATP: 1 proton ▪ The flow of a pair of electrons from NADH to O2, => 10 protons are pumped, equivalent to 2.5 cytosolic ATP ▪ For FADH2, 6 protons are pumped = 1.5 cytosolic ATP 24 ATP Yield from the Complete Oxidation of Glucose: 30 ATP Glycolysis: Glucose to 2 pyruvate 2 ATP Glycolysis: 2 NADH in cytosol 3 ATP 2 Pyruvate to 2 acetyl CoA, 2 NADH (in 5 ATP mitochondrion) Krebs Cycle (in mitochondrion) 2GTP 2 ATP Krebs Cycle, 6 NADH (in mitochondrion) 15 ATP Krebs Cycle, 2 FADH2 (in mitochondrion) 3 ATP Total yield of 30 ATP per glucose 25 Two shuttle systems to transport electrons of NADH at the cytosol into mitochondrion Glycerol 3-phosphate shuttle NADH FADH2 Malate aspartate shuttle NADH NADH 26 Uncoupling of Oxidative Phosphorylation The coupling of electron transport and phosphorylation is disrupted by un-couplers such as 2,4 dinitrophenol (DNP). DNP carry protons across the inner membrane. The electron transport (oxidation) still proceeds normally, but ATP formation is reduced because the proton motive force across the membrane is dissipated. The result is increased oxidation and oxygen consumption. 27  Uncoupling and Thermogenesis The dissipation of pH gradient generates heat Uncoupling: a way of generating heat to maintain body temperature in hibernating animals, newborn, and mammals adapted to cold. Brown adipose tissues (rich in mitochondria) specialised in this process of thermogenesis, via the uncoupling protein UCP-1, in the inner membrane. 28  UCP-1: carry protons back across the membrane Thermogenesis is the result of the stimulation of the sympathetic nerve that responds to cold exposure. →Norepinephrine → cAMP → formation of free fatty acids → activates UCP-1 Other uncoupling proteins: UCP 2 to 5 Pharmacological agents that might affect uncoupling proteins → Possible treatment for obesity 29 30  Cyanide poisoning: Inhibition of oxidative phosphorylation by cyanide Clinical pictures of cyanide poisoning: Nausea, vomiting, headache, dizziness, diaphoresis (sweating) Breath: “bitter almonds” Mucous membranes bright red (oxyhemoglobin) Tachypnea, dyspnea, later pulmonary edema Tachycardia, hypertension, arrythmias Lactic acidosis Involuntary muscle contractions: fasciculations; spasms (severe: e.g., opisthotonus) 31 Opisthotonus in a child with tetanus The boy’s head, neck and spinal column enter into a complete ‘arching position – caused by spasm of the axial muscles along the spinal column 32  Role of Oxygen in Releasing Energy: Applied Cyanide poisoning: A medical emergency General support: oxygen GI decontamination: induce vomiting Convert hemoglobin Fe2+ to Fe3+ to form (metHb): metHb can bind CN- strongly, releasing CN- from cytochrome oxidase. e.g., Amyl nitrate, sodium nitrites Thiosulfate: convert CN- to SCN- (catalysed by mitochondrial enzyme: rhodanese) https://www.youtube.com/watch?v=fBXSJGxfnbU https://www.youtube.com/watch?v=VfW64-vT7F8 33  Role of Oxygen in Releasing Energy: At the Frontier Mitochondria: a source of chemical signals for cell damage and death 1. Release of cytochrome C induces activation of caspases in apoptosis (programmed cell death) 2. Deficiencies of ETC enzymes and neurodegenerative disorders: Complex I and Parkinson’s Disease 3. ETC a source of reactive oxygen species: e.g., direct reduction of O2 by Complex I generates O2-·: ageing and metabolic syndromes 34 Summary Optimal extraction of energy from nutrients requires oxygen Oxygen and nutrients are provided by circulation Mitochondria are essential for ATP synthesis “Oxidative Phosphorylation”: the process of mitochondrial ATP synthesis Release of electrons from nutrients drives proton pumping Proton gradient drives ATP synthesis Mobile electron carriers provide electrons to proton pumping complexes (I, III, and IV) Complexes I and II provide link between citric acid cycle and “Electron Transport Chain” 35 References: 1. Devlin, T.M. [Ed.] (2010), Textbook of Biochemistry With Clinical Correlations, 7th Ed., Wiley-Liss Inc, N.Y. 2. Harvey, R.A. and Ferrier, D.R. (2011). Biochemistry 5th Edition, Lippincott Williams & Wilkins, USA. 3. Voet, D. and Voet, J.G. (2011). Biochemistry 4th Edition, Wiley Int. Ed., John Wiley & Sons Inc., USA. 36