LE 8-20aAllosteric enzymewith four subunitsRegulatorysite (oneof four)Active formActivatorStabilized active formActive site(one of four)Allosteric activatorstabilizes active form.Non-functionalactive siteInactive formInhibitorStabilized inactive formAllosteric inhibitorstabilizes inactive form.OscillationAllosteric activators and inhibitorsLE 8-21Active siteavailableInitial substrate(threonine)Threoninein active siteEnzyme 1(threoninedeaminase)Enzyme 2Intermediate AIsoleucineused up bycellFeedbackinhibitionActive site ofenzyme 1 can’tbindtheoninepathway offIsoleucinebinds toallostericsiteEnzyme 3Intermediate BEnzyme 4Intermediate CEnzyme 5Intermediate DEnd product(isoleucine)LE 8-22Mitochondria,sites of cellular respiration1 µmLE 9-2ECOSYSTEMLightenergyPhotosynthesisin chloroplastsCellular respirationin mitochondriaOrganicmolecules+ O2CO2 + H2OATPpowers most cellular workHeatenergyCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Principle of Redox• Chemical reactions that transfer electronsbetween reactants are called oxidation-reductionreactions, or redox reactions• In oxidation, a substance loses electrons, or isoxidized• In reduction, a substance gains electrons, or isreduced (the amount of positive charge isreduced)Xe- + Y X + Ye-becomes oxidized(loses electron)becomes reduced(gains electron)Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• The electron donor is called the reducing agent• The electron receptor is called the oxidizing agentCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Some redox reactions do not transfer electrons butchange the electron sharing in covalent bonds• An example is the reaction between methane andoxygenLE 9-3Reactantsbecomes oxidizedbecomes reducedProductsHMethane(reducingagent)Oxygen(oxidizingagent)Carbon dioxide WaterH CHHO OOOC OHHCH42 O2+++CO2Energy2 H2OCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsOxidation of Organic Fuel Molecules DuringCellular Respiration• During cellular respiration, the fuel (such asglucose) is oxidized and oxygen is reduced:C6H12O6 + 6O2 6CO2 + 6H2O + Energybecomes oxidizedbecomes reducedCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsStepwise Energy Harvest via NAD+ and the ElectronTransport Chain• In cellular respiration, glucose and other organicmolecules are broken down in a series of steps• Electrons from organic compounds are usually firsttransferred to NAD+, a coenzyme• As an electron acceptor, NAD+ functions as anoxidizing agent during cellular respiration• Each NADH (the reduced form of NAD+)represents stored energy that is tapped tosynthesize ATPLE 9-4NAD+Nicotinamide(oxidized form)Dehydrogenase2 e– + 2 H+2 e– + H+NADHH+H+Nicotinamide(reduced form)+ 2[H](from food)+Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• NADH passes the electrons to the electrontransport chain• Unlike an uncontrolled reaction, the electrontransport chain passes electrons in a series ofsteps instead of one explosive reaction• Oxygen pulls electrons down the chain in anenergy-yielding tumble• The energy yielded is used to regenerate ATPLE 9-52 H+ + 2 e– 2 H(from food via NADH)Controlledrelease ofenergy forsynthesis ofATPATPATPATP2 H+2 e– H2O+1/2 O21/2 O2H2+1/2 O2H2OExplosiverelease ofheat and lightenergyCellular respirationUncontrolled reactionFree energy, GFree energy, GElectron transport chainCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsThe Stages of Cellular Respiration: A Preview• Cellular respiration has three stages:– Glycolysis (breaks down glucose into twomolecules of pyruvate)– The citric acid cycle (completes the breakdown ofglucose)– Oxidative phosphorylation (accounts for most ofthe ATP synthesis)• The process that generates most of the ATP is calledoxidative phosphorylation because it is powered byredox reactionsLE 9-6_1MitochondrionGlycolysisPyruvateGlucoseCytosolATPSubstrate-levelphosphorylationLE 9-6_2MitochondrionGlycolysisPyruvateGlucoseCytosolATPSubstrate-levelphosphorylationATPSubstrate-levelphosphorylationCitricacidcycleLE 9-6_3MitochondrionGlycolysisPyruvateGlucoseCytosolATPSubstrate-levelphosphorylationATPSubstrate-levelphosphorylationCitricacidcycleATPOxidativephosphorylationOxidativephosphorylation:electron transportandchemiosmosisElectronscarriedvia NADHElectrons carriedvia NADH andFADH2Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings• Oxidative phosphorylation accounts for almost90% of the ATP generated by cellular respiration• A small amount of ATP is formed in glycolysis andthe citric acid cycle by substrate-levelphosphorylationLE 9-7EnzymeADPPSubstrateProductEnzymeATP+Copyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 9.2: Glycolysis harvests energy byoxidizing glucose to pyruvate• Glycolysis (“splitting of sugar”) breaks downglucose into two molecules of pyruvate• Glycolysis occurs in the cytoplasm and has twomajor phases:– Energy investment phase– Energy payoff phaseLE 9-8Energy investment phaseGlucose2 ATPused2 ADP + 2 P4 ADP + 4 P4 ATPformed2 NAD+ + 4 e– + 4 H+Energy payoff phase+ 2 H+2 NADH2 Pyruvate + 2 H2O2 Pyruvate + 2 H2O2 ATP2 NADH + 2 H+Glucose4 ATP formed – 2 ATP used2 NAD+ + 4 e– + 4 H+NetGlycolysisCitricacidcycleOxidativephosphorylationATPATPATPLE 9-9a_1GlucoseATPADPHexokinaseATP ATPATPGlycolysisOxidationphosphorylationCitricacidcycleGlucose-6-phosphateLE 9-9a_2GlucoseATPADPHexokinaseATP ATPATPGlycolysisOxidationphosphorylationCitricacidcycleGlucose-6-phosphatePhosphoglucoisomerasePhosphofructokinaseFructose-6-phosphateATPADPFructose-1, 6-bisphosphateAldolaseIsomeraseDihydroxyacetonephosphateGlyceraldehyde-3-phosphateLE 9-9b_12 NAD+Triose phosphatedehydrogenase+ 2 H+NADH21, 3-Bisphosphoglycerate2 ADP2 ATPPhosphoglycerokinasePhosphoglyceromutase2-Phosphoglycerate3-PhosphoglycerateLE 9-9b_22 NAD+Triose phosphatedehydrogenase+ 2 H+NADH21, 3-Bisphosphoglycerate2 ADP2 ATPPhosphoglycerokinasePhosphoglyceromutase2-Phosphoglycerate3-Phosphoglycerate2 ADP2 ATPPyruvate kinase2 H2OEnolasePhosphoenolpyruvatePyruvateCopyright © 2005 Pearson Education, Inc. publishing as Benjamin CummingsConcept 9.3: The citric acid cycle completes theenergy-yielding oxidation of organic molecules• Before the citric acid cycle can begin, pyruvatemust be converted to