Chapter 9~ Cellular Respiration: Harvesting Chemical Energy What s the point? The point is to make! 2006-2007 Principles of Energy Harvest Catabolic pathway Fermentation Cellular Respiration C6H126 + 62 > 6C2 + 6H2 + E ( + heat) 1
respiration Harvesting stored energy Glucose is the model catabolism of glucose to produce glucose + oxygen energy + water + carbon dioxide C 6 H 12 6 + 6 + 6H 2 + 6C + heat CMBUSTIN = making a lot of heat energy by burning fuels in one step uel carbohydrates) C + H 2 + heat RESPIRATIN = making (& some hea by burning fuels in many small steps enzymes glucose 2 C + H 2 + (+ heat How do we harvest energy from fuels? Digest large molecules into smaller ones break bonds & move electrons from one molecule to another as electrons move they carry energy with them that energy is stored in another bond, released as heat or harvested to make loses e- gains e- oxidized reduced + + + e - e - oxidation redox e - reduction How do we move electrons in biology? Moving electrons in living systems electrons cannot move alone in cells electrons move as part of H atom move H = move electrons loses e- gains e- oxidized reduced + + + H oxidation e p H reduction C 6 H 12 6 + 6 6C + 6H 2 + H e - oxidation reduction 2
Coupling oxidation & reduction REDX reactions in respiration release energy as breakdown organic molecules break C C bonds strip off electrons from C H bonds by removing H atoms C 6 H 12 6 C =thefuel has been oxidized electrons attracted to more electronegative atoms in biology, the most electronegative atom? H 2 =oxygen has been reduced couple REDX reactions & use the released energy to synthesize oxidation C 6 H 12 6 + 6 6C + 6H 2 + reduction 2 xidation & reduction xidation adding removing H loss of electrons releases energy exergonic oxidation Reduction removing adding H gain of electrons stores energy endergonic C 6 H 12 6 + 6 6C + 6H 2 + reduction Moving electrons in respiration Electron carriers move electrons by shuttling H atoms around NAD + (reduced) FAD +2 FADH 2 (reduced) NAD + nicotinamide Vitamin B3 niacin P phosphates P H N + C NH 2 + H reduction oxidation adenine ribose sugar carries electrons as a reduced molecule like $$ in the bank P P reducing power! H H C NH N + How efficient! Build once, use many ways 3
xidizing agent in respiration NAD+ (nicotinamide adenine dinucleotide) Removes electrons from food (series of reactions) NAD + is reduced to Enzyme action: dehydrogenase xygen is the eventual e acceptor Electron transport chains Electron carrier molecules (membrane proteins) Shuttles electrons that release energy used to make Sequence of reactions that prevents energy release in 1 explosive step Electron route: food > > electron transport chain > oxygen Cellular respiration: overview (anaerobic) 1.Glycolysis: cytosol; degrades glucose into pyruvate (aerobic) Pyruvate oxidation 2.Kreb s Cycle: mitochondrial matrix; pyruvate into carbon dioxide 3.Electron Transport Chain: inner membrane of mitochondrion; electrons passed to oxygen 4
What s the point? The point is to make! 2006-2007 Glycolysis Breaking down glucose glyco lysis (splitting sugar) glucose pyruvate 6C 2x 3C ancient pathway which harvests energy where energy transfer first evolved transfer energy from organic molecules to still is starting point for ALL cellular respiration but it s inefficient generate only 2 for every 1 glucose occurs in cytosol That s not enough for me! In the cytosol? Why does that make evolutionary sense? Evolutionary perspective Enzymes Prokaryotes of glycolysis are first cells had no organelles well-conserved Anaerobic atmosphere life on Earth first evolved without free oxygen ( ) in atmosphere energy had to be captured from organic molecules in absence of Prokaryotes that evolved glycolysis are ancestors of all modern life ALL cells still utilize glycolysis You mean we re related? Do I have to invite them over for the holidays? 5
verview 10 reactions convert glucose (6C) to 2 pyruvate (3C) produces: 4 & 2 consumes: 2 net yield: 2 & 2 glucose C-C-C-C-C-C enzyme 2 fructose-1,6bp P-C-C-C-C-C-C-P enzyme DHAP P-C-C-C DHAP = dihydroxyacetone phosphate G3P = glyceraldehyde-3-phosphate enzyme enzyme 2 G3P C-C-C-P 2H 2P enzyme i enzyme enzyme enzyme 2P i pyruvate C-C-C ADP 2 2 4 4 NAD + ADP ENERGY INVESTMENT Glycolysis summary endergonic invest some ENERGY PAYFF NET YIELD G3P C-C-C-P -2 4 exergonic harvest a little & a little like $$ in the bank net yield 2 2 Substrate level Phosphorylation In the last steps of glycolysis, where did the P come from to make? the sugar substrate (PEP) 9 P is transferred from PEP to ADP kinase enzyme ADP H 2 Phosphoenolpyruvate (PEP) ADP Pyruvate enolase 10 pyruvate kinase Phosphoenolpyruvate (PEP) Pyruvate H 2 ADP - C C CH 2 - C C CH 3 P I get it! The P i came directly from the substrate! 6
Energy accounting of glycolysis 2 2 ADP glucose pyruvate 6C 2x 3C 4 ADP 4 2 NAD + 2 But glucose has Net gain = 2 + 2 some energy investment ( 2 ) small energy return (4 + 2 ) 1 6C sugar 2 3C sugars so much more to give! All that work! And that s all I get? Is that all there is? Not a lot of energy for 1 billon years + this is how life on Earth survived no = slow growth, slow reproduction only harvest 3.5% of energy stored in glucose more carbons to strip off = more energy to harvest glucose pyruvate 6C 2x 3C Hard way to make a living! But can t stop there! DHAP G3P raw materials products NAD + P i + P i 6 1,3-BPG P i P i 1,3-BPG NAD + + ADP 7 ADP Glycolysis 3-Phosphoglycerate 3-Phosphoglycerate (3PG) (3PG) glucose + 2ADP + 2P i + 2 NAD + 2pyruvate+ 2 + 2 8 Going to run out of NAD + without regenerating NAD +, energy production would stop! another molecule must accept H from so NAD + is freed up for another round 2-Phosphoglycerate 2-Phosphoglycerate (2PG) (2PG) 9 H 2 H 2 Phosphoenolpyruvate Phosphoenolpyruvate (PEP) (PEP) ADP 10 ADP Pyruvate Pyruvate 7
Another molecule must accept H from How is recycled to NAD +? H 2 with oxygen aerobic respiration pyruvate NAD + without oxygen anaerobic respiration fermentation C recycle which path you use depends on who you are acetyl-coa Krebs cycle NAD + lactate lactic acid fermentation acetaldehyde NAD + ethanol alcohol fermentation Fermentation (anaerobic) Bacteria, yeast pyruvate ethanol + C 3C 2C 1C NAD + back to glycolysis beer, wine, bread Animals, some fungi pyruvate lactic acid 3C 3C NAD + back to glycolysis cheese, anaerobic exercise (no ) Alcohol Fermentation pyruvate ethanol + C 3C 2C 1C NAD + back to glycolysis Dead end process at ~12% ethanol, kills yeast can t reverse the Count the carbons! reaction recycle bacteria yeast 8
Lactic Acid Fermentation pyruvate lactic acid 3C 3C NAD + back to glycolysis Reversible process once is available, lactate is converted back to pyruvate by the liver Count the carbons! recycle animals some fungi Pyruvate is a branching point Pyruvate fermentation anaerobic respiration mitochondria Krebs cycle aerobic respiration Glycolysis is only the start Glycolysis glucose pyruvate 6C 2x 3C Pyruvate has more energy to yield 3 more C to strip off (to oxidize) if is available, pyruvate enters mitochondria enzymes of Krebs cycle complete the full oxidation of sugar to C pyruvate C 3C 1C 9
xidation of pyruvate Pyruvate enters mitochondrial matrix 2x pyruvate acetyl CoA + C [ 2 3C 2C 1C] NAD 3 step oxidation process releases 2 C (count the carbons!) reduces 2NAD 2 (moves e ) produces 2acetyl CoA Acetyl CoA enters Krebs cycle Where does the C go? Exhale! Krebs cycle 1937 1953 aka Citric Acid Cycle in mitochondrial matrix 8 step pathway each catalyzed by specific enzyme step wise catabolism of 6C citrate molecule Evolved later than glycolysis does that make evolutionary sense? bacteria 3.5 billion years ago (glycolysis) free 2.7 billion years ago (photosynthesis) eukaryotes 1.5 billion years ago (aerobic respiration = organelles mitochondria) Hans Krebs 1900-1981 Count the carbons! pyruvate 3C 4C 2C acetyl CoA 6C citrate This happens twice for each glucose molecule 4C 4C oxidation of sugars x2 6C 5C C 4C 4C C 10
Count the electron carriers! pyruvate 3C 2C 4C 6C acetyl CoA citrate C This happens twice for each glucose molecule 4C 4C reduction of electron carriers x2 6C 5C C FADH 2 4C 4C C Whassup? So we fully oxidized glucose C 6 H 12 6 C & ended up with 4! What s the point? Electron Carriers = Hydrogen Carriers Krebs cycle produces large quantities of electron carriers FADH 2 go to Electron Transport Chain! What s so important about electron carriers? ADP + P i H+ H+ 11
Energy accounting of Krebs cycle 4 NAD +1 FAD 4 +1FADH 2 2x pyruvate C 3C 3x 1C 1 ADP 1 Net gain = 2 = 8 + 2 FADH 2 Value of Krebs cycle? If the yield is only 2 then how was the Krebs cycle an adaptation? value of & FADH 2 electron carriers & H carriers reduced molecules move electrons reduced molecules move ions to be used in the Electron Transport Chain like $$ in the bank Kreb s Cycle: review If molecular oxygen is present. Each pyruvate is converted into acetyl CoA (begin w/ 2): C2 is released; NAD+ > ; coenzyme A (from B vitamin), makes molecule very reactive From this point, each turn 2 C atoms enter (pyruvate) and 2 exit (carbon dioxide) xaloacetate is regenerated (the cycle ) For each pyruvate that enters: 3 NAD+ reduced to ; 1 FAD+ reduced to FADH2 (riboflavin, B vitamin); 1 molecule 12
accounting so far Glycolysis 2 Kreb s cycle 2 Life takes a lot of energy to run, need to extract more energy than 4! There s got to be a better way! I need a lot more! A working muscle recycles over 10 million s per second There is a better way! Electron Transport Chain series of proteins built into inner mitochondrial membrane along cristae transport proteins & enzymes transport of electrons down ETC linked to pumping of to create gradient yields ~36 from 1 glucose! only in presence of (aerobic respiration) That sounds more like it! Remember the Electron Carriers? Glycolysis glucose G3P Krebs cycle 2 8 2 FADH 2 Time to break open the piggybank! 13
NAD + + H e p H e- + Electron Transport Chain Building proton gradient! C intermembrane space inner mitochondrial membrane Q e H e e FADH 2 FAD H NAD + 2 1 + 2 2 H 2 cytochrome cytochrome c dehydrogenase bc complex oxidase complex mitochondrial matrix What powers the proton ( ) pumps? Electron Transport Chain Intermembrane space Inner mitochondrial membrane Q C dehydrogenase Mitochondrial matrix cytochrome bc complex cytochrome c oxidase complex Stripping H from Electron Carriers Electron carriers pass electrons & to ETC H cleaved off & FADH 2 electrons stripped from H atoms (protons) electrons passed from one electron carrier to next in mitochondrial membrane (ETC) flowing electrons = energy to do work transport proteins in membrane pump (protons) across inner membrane to intermembrane space TA-DA!! Moving electrons do the work! C Q e e NAD + 2 1 + 2 e FADH 2 FAD cytochrome dehydrogenase bc complex cytochrome c oxidase complex ADP H 2 + P i H+ H+ 14
But what pulls the electrons down the ETC? H 2 electrons flow downhill to oxidative phosphorylation Electrons flow downhill Electrons move in steps from carrier to carrier downhill to oxygen each carrier more electronegative controlled oxidation controlled release of energy make instead of fire! We did it! Set up a gradient Allow the protons to flow through synthase Synthesizes ADP + P i proton-motive force ADP + P i Are we there yet? 15
Chemiosmosis The diffusion of ions across a membrane build up of proton gradient just so H+ could flow through synthase enzyme to build Chemiosmosis links the Electron Transport Chain to synthesis So that s the point! Peter Mitchell Proposed chemiosmotic hypothesis revolutionary idea at the time 1961 1978 proton motive force 1920-1992 Pyruvate from cytoplasm Inner mitochondrial membrane Q Intermembrane space Electron transport C system e - 1. Electrons are harvested Acetyl-CoA and carried to the transport system. Krebs cycle C FADH 2 e - e - 2. Electrons provide energy to pump protons across the H 2 membrane. 3. xygen joins 1 with protons to 2 2 form water. + 2 e - Mitochondrial matrix 4. Protons diffuse back in down their concentration gradient, driving the synthesis of. synthase 16
Taking it beyond What is the final electron acceptor in Q e Electron Transport Chain? NAD + 2 cytochrome dehydrogenase bc complex So what happens if unavailable? ETC backs up nothing to pull electrons down chain & FADH 2 can t unload H production ceases cells run out of energy and you die! C e FAD 2 1 + e FADH 2 H 2 cytochrome c oxidase complex What s the point? The point is to make! 2006-2007 Review: Cellular Respiration Glycolysis: 2 (substrate level phosphorylation) Kreb s Cycle: 2 (substrate level phosphorylation) Electron transport & oxidative phosphorylation: 2 (glycolysis) = 6 2 (acetyl CoA) = 6 6 (Kreb s) = 18 2 FADH2 (Kreb s) = 4 38 TTAL /glucose 17
Cellular Respiration ther Metabolites & Control of Respiration 2006-2007 Beyond glucose: ther carbohydrates Glycolysis accepts a wide range of carbohydrates fuels polysaccharides glucose hydrolysis ex. starch, glycogen other 6C sugars glucose modified ex. galactose, fructose amino group = waste product excreted as ammonia, urea, or uric acid Beyond glucose: Proteins proteins amino acids hydrolysis waste H H N C C H H R glycolysis Krebs cycle 2C sugar = carbon skeleton = enters glycolysis or Krebs cycle at different stages 18
Beyond glucose: Fats fats glycerol + fatty acids hydrolysis glycerol (3C) G3P glycolysis fatty acids 2C acetyl acetyl Krebs groups coa cycle 3C glycerol 2C fatty acids enters glycolysis as G3P enter Krebs cycle as acetyl CoA Metabolism Coordination of chemical processes across whole organism digestion catabolism when organism needs energy or needs raw materials synthesis anabolism when organism has enough energy & a supply of raw materials by regulating enzymes feedback mechanisms raw materials stimulate production products inhibit further production C Control of Respiration Feedback Control 2006-2007 19
Feedback Inhibition Regulation & coordination of production final product is inhibitor of earlier step allosteric inhibitor of earlier enzyme no unnecessary accumulation of product production is self limiting A B C D E F G enzyme X enzyme enzyme enzyme enzyme 1 2 3 4 5 enzyme 6 allosteric inhibitor of enzyme 1 Respond to cell s needs Key point of control phosphofructokinase allosteric regulation of enzyme why here? can t turn back step before splitting glucose AMP & ADP stimulate inhibits citrate inhibits Why is this regulation important? Balancing act: availability of raw materials vs. energy demands vs. synthesis A Metabolic economy Basic principles of supply & demand regulate metabolic economy balance the supply of raw materials with the products produced these molecules become feedback regulators they control enzymes at strategic points in glycolysis & Krebs cycle levels of AMP, ADP,» regulation by final products & raw materials levels of intermediates compounds in pathways» regulation of earlier steps in pathways levels of other biomolecules in body» regulates rate of siphoning off to synthesis pathways 20
A Metabolic economy Basic principles of supply & demand regulate metabolic economy balance the supply of raw materials with the products produced these molecules become feedback regulators they control enzymes at strategic points in glycolysis & Krebs cycle levels of AMP, ADP,» regulation by final products & raw materials levels of intermediates compounds in pathways» regulation of earlier steps in pathways levels of other biomolecules in body» regulates rate of siphoning off to synthesis pathways Got the energy Ask Questions!! 2006-2007 21