Lecture 7 - Energy Metabolism Part 1 PDF

MEDC0034 Energy Metabolism – part 1 Central pathways of ATP production and use. Prof Nathan Davies Macroscopic electrical circuit e- e- Interlocking Battery Motor gears Two Energy C...

MEDC0034 Energy Metabolism – part 1 Central pathways of ATP production and use. Prof Nathan Davies Macroscopic electrical circuit e- e- Interlocking Battery Motor gears Two Energy Coupling 10K chemical transduc device g species of er different reduction Mechanical potential work Weight being lifted 1 Microscopic electrical circuit e- +Pi ADP Food ATP Contains reduced compound s Muscle contraction e- Mechanical O2 Mitochondrion work Electrochemical Oxygen transducer High reduction potential GLUCOSE 2 ADP + 2 2 NAD+ Pi Fructose-1,6 biphosphate 2 ATP 2 2 NADH Pyruvate Anaerobic Anaerobic homolactic Aerobic alcoholic fermentation oxidation fermentation 2 NADH Cytosol 2 NADH 2 NAD+ TCA Cycle 2 NAD+ 2 LACTATE 2 CO2 + 2 EtOH Oxidative phosphorylation MITOCHONDRIA 6 CO2 + 6H2O + ~30 ATP 2 Respiration: An Overview Electrons carried in NADH Electrons Pyruvi carried in c NADH acid and FADH2 Electron Glucos Glycolysi Kreb Transport s Chain e s Cycle Cytoplasm Mitochondrion 3 Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2H20 CoASH + 3 NADH + FADH2 + GTP + 2CO2 + 3H+ 4 INTERMEMBRANE SPACE CRISTAE MATRIX Image from: BIOLOGY by Miller and Levine; Prentice Hall Publishing©2006 5 Watch the movie https:// www.youtube.com/watch?v=rdF3mnyS1p0 How many ATP’s per glucose? 6  Can we view the synthesis of ATP as a simple matter of stoichiometry ? (the fixed ratios of reactants to products in a chemical reaction). A stoichiometric production of ATP does occur at: one step in the citric acid cycle yielding 2 ATPs for each glucose molecule. This step is the conversion of alpha-ketoglutaric acid to succinic acid. at two steps in glycolysis yielding 2 ATPs for each glucose molecule. 7  GLUCOSE 2 ADP + 2 2 NAD+ Pi Fructose-1,6 biphosphate 2 ATP 2 NADH 2 Pyruvate Anaerobic Anaerobic homolactic Aerobic alcoholic fermentation oxidation fermentation 2 NADH Cytosol 2 NADH 2 NAD+ TCA Cycle 2 NAD+ 2 LACTATE 2 CO2 + 2 EtOH Oxidative phosphorylation MITOCHONDRIA 6 CO2 + 6H2O Most of the ATP is generated by the proton gradient that develops across the inner mitochondrial membrane. The number of protons pumped out as electrons drop from NADH through the respiratory chain to oxygen is theoretically large enough to generate, as they return through ATP synthase:- 3 ATPs per electron pair (but only 2 ATPs for each pair donated by FADH2). 8  With 12 pairs of electrons removed from each glucose molecule: 10 by (10x3=3 NAD+ 0); 2 by (2x2=4), this FADH could 2 generate 34 ATPs. + 4 ATPs that are generated in the outside the mitochondria = 38. Intermembrane space Complex III Complex I NADH:UQ reductase UQH :2 cyt c reductase Complex IV Complex II cytochrome c ox idase Succinate:UQ reductase H + H+ H + Cyt c Inner UQ mitochondrial membrane I III IV II Fumarate H2O + NADH + H NAD+ Succinate 12 O + 2H + _ 2 Cyt a IV Matrix N-1 N-3 I S-1 II Rieske III Cu A N-4 S-2 N-5 N-2 S-3 Cyt b Cyt c1 Cyt a3 Cyt b Cu B Metal centres in e- transport chain 9  The energy stored in the proton gradient is used for a number of other mitochondrial functions such as the active transport of a variety of essential molecules and ions through the mitochondrial membranes. NADH is also used as reducing agent for many cellular reactions. So the actual yield of ATP as mitochondria respire varies with conditions and seldom exceeds 30. 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R08109 R08110 R08111 R08112 R08113 R08140 R08141 R08198 R08209 R08210 R08214 R08215 R08240 R08281 R08282 R08306 R08307 R08310 R08382 R08385 10 All of this works in coupled mitochondria Uncouplers 11 Intermembrane space low pH Inner mitochondrial membrane Matrix high pH 12 Alternative energy CH3CH2OH + NAD+ → CH3CHO + NADH + H+ Alcohol Dehydrogenase 13 14  Evidence for Chemiosmotic theory The chemiosmotic theory requires that:- (i)The respiratory chain translocates protons and generates a membrane potential and a pH gradient (ii) The ATP synthetase is a reversible proton translocating enzyme (i.e. it can use a proton gradient to drive ATP synthesis and/or it could use the energy of ATP hydrolysis to catalyse proton translocation. (iii) The mitochondrion has low proton permiability (iv)Specific carriers exist to transport charged metabolites into and out of the mitochondria despite the presence of a membrane potential 15 When might non-efficient metabolism be preferred? In 1924 Otto Warburg observed that cancer cells metabolise glucose in a manner distinct from normal tissue. Warburg discovered that cancer cells ‘ferment’ glucose to lactate even in the presence of sufficient oxygen The Warburg Effect Proliferative tissue Differentiated Tissue /Cancer 16 What are the needs of proliferating tissues? ATP production is only reliant on efficiency when resources are scarce. This is not generally the case for proliferating cells To produce daughter cells, all of the cellular components must be replicated (nucleotides, amino acids, lipids etc) Glucose is used to generate biomass as well as produce ATP. Glucose as a synthetic precurser Example of palmitate – a major constituent of membranes – Synthesis requires:- 7 x ATP; 16 carbons (from 8 x acetyl-CoA); 28 e- from 14 NADPH. – Glucose can potentially provide 36 ATPs (or 30 ATPs + 2 NADPHs). – Or 6 carbons for macromolecular synthesis – Thus to make a 16 carbon fatty acyl chain, 1 glucose can produce 5 times the ATP required, but 7 glucose are needed to make enough NADPH, plus 3 glucose to supply the carbons via acetyl-CoA. 17 Thus: for a cell to proliferate The majority of the available glucose cannot be committed to carbon catabolism for ATP production. Should the ATP/ADP ratio rise substantially – this would impair the flux of glycolytic intermediates, limiting production of acetyl- CoA and NADPH for macromolecular synthesis Proliferating cells The two molecules mainly catabolised are glucose and glutamine – which supply the majority of carbon, nitrogen and free energy necessary for cell division. Cells that convert glucose and glutamine most efficiently will proliferate fastest. The lactate produced can be recycled via the Cori cycle, which then limits the impact on the energy reserves of the whole organism. Tumours can be heterogenous – with some cells using lactate as a fuel for mitochondria. 18  Use of ATP The role of ATP is that of a free energy transmitter rather than a free energy reservoir ATP continually hydrolysed and regenerated [ATP] in a cell is only enough to last for ~1 minute (depending on cell type) At rest an average person consumes and regenerates ~3mol ATP.h-1 Mechano-chemical coupling using ATP Myosin is a force generating ATPase It is a high molecular mass protein (470kDa) comprising 2 heavy and 2 light chains Chains intertwine to form 2 globular heads and 2 long tails Interacts with actin (42kDa) to form actomyosin, which can be seen clearly in skeletal muscle Myofibrils of actomyosin appear as parallel striations as the myosin and actin overlap in bands (striated muscle) A relative movement between myosin and actin can be caused by the addition of ATP 19 Muscle fibre 20  H2O (ii) (i) ADP + ATP Pi Actin moveme nt Myosin https:// www.youtube.com/watch?v=p8iKzWqUU2s Proton translocation & the reactions of complexes I, III, & IV 21  Intermembrane space Complex III Complex I NADH:UQ reductase UQH :2 cyt c reductase Complex II Complex IV cytochrome c oxidase H +Succinate:UQ reductase H+ H + Cyt c Inner UQ mitochondrial membrane I III IV II Fumarate H2O + NADH + H NAD+ Succinate 1 + _2 O 2+ 2H Cyt a IV Matrix N-1 N-3 I II Rieske III Cu A N-4 S-1 S-2 Cyt c1 N-5 N-2 S-3 Cyt b Cyt a3 Cyt b Cu B Metal centres in e- transport chain 22