acid柠檬酸cycle

1、L1. Citric acid cycleL2. Electron transport and oxidative hosphorylationL3. PhotosynthesisSection L Respiration and EnergyRoleLocationThe cycleEnergy yieldRegulationBiosynthetic pathwaysCitric acid cycleRole The citric acid柠檬酸柠檬酸cycle ,also known as the TCA (tricarboxylic acid三羧酸三羧酸) cycle or Krebs

2、cycle (after its discover in 1993), is used to oxidize the pyruvate formed during the glyco-lytic breakdown of glucose into CO2 and H2O. The cycle is a major energy source in the form of ATP and also produces precursors for many biosynthetic pathways.Location The citric acid cycle occurs within the

3、mitochondria of eukaryotes and the cytosol of prokaryotes.The cycle Step 1 .Oxidation of fuel molecules to acetyl CoA A major source of energy is glucose which is converted by glycolysis into pyruvate. Pyruvate dehydrogenase (a complex of three enzymes and five coenzymes ) then oxidizes the pyruvate

4、 (using NAD+ which is reduced to NADH ) to form acetyl CoA and CO2 since the reaction involves both an oxidation and a loss of CO2, the process is called oxidative decarboxylation.Acetyl-CoA The cycle carries out the oxidation of acetyl groups from acetyl CoA to CO2 with the production of four pairs

5、 of electrons, stored initially in the reduced electron carriers NADH and FADH2 The citric acid cycle Cycle modeEIGHT STAGES The citric acid cycle has eight stages: (1)Production of citrate from oxaloacetate (草草酰乙酸酰乙酸) and acetyl CoA (catalyzed by citrate synthase) COOH CH3 CH2-COOH CO + CO HO-C-COO

6、H CH2 S CH2-COOH COOH CoACitrate formationIsomerization (异构化作用异构化作用)of citrate to isocitrate (catalyzed by aconitase 顺乌头酸酶顺乌头酸酶) CH2-COOH CH2-COOH HO-C-COOH H-C-COOH CH2-COOH HO-CH-COOHIsocitrate formationOxidation of isocitrate to ketoglutarate（-酮酮戊二酸戊二酸, catalyzed by isocitrate dehydrogenase; the

7、reaction requires NAD+ ） COOH COOH CH2 CH2 H-C-COOH CH2 HO-C-H CO COOH COOHNAD+ NADH+H+CO2 ketoglutarate Oxidation of ketoglutarate to succinyl CoA (catalyzed by the ketoglutarate dehydrogenase complex ; the reaction requires a NAD+ )Step 4 Conversion of succinyl CoA to succinyl catalyzed by succiny

8、l CoA synthetase; the reaction requires inorganic phosphate and GDP(orADP)Step5Succinyl-CoA Oxidation of succinyl to fumarate (catalyzed by succinyl dehydrogenase; the reaction involves FAD).Step 6malonate Hydration of fumarae to malate苹果酸苹果酸 (catalyzed by fumarase 延胡索酸酶延胡索酸酶) Oxidation of malate to

9、 oxaloacetate (catalyzed by malate dehydrogenase; the reaction requires NAD+ )Step 8Whole pathwayOxaloacetate formation Oxidation of NADH and FADH2 produced by the citric acid cycle The NADH and FADH2 produced by the citric acid cycle are reoxidized and the energy released is used to synthesize ATP

10、by oxidative phosphorylation Energy yield For each turn of the cycle ,10 ATP molecules are produced , one directly from the cycle and 9 from the re-oxidation of the three NADH and one FADH2 molecules produced by the cycle oxidative phosphorylation.Regulation The citric acid cycle is regulated at the

11、 steps catalyzed by citrate synthase, i s o c i t r a r e d e h y d r o g e n a s e a n d keoglutarate dehydrogenase via feedback inhibition by ATP , citrate, NADH and succinyl CoA , and stimulation of isocitrare dehydrogenade by ADP. Pyruvate dehydrogenase Pyruvate dehydrogenase is inhibited by ace

12、tyl CoA and NADH. In addition, this enzyme is inactivated by phosphorylation, a reaction catalyzed by pyruvate dehydrogenase kinase. Pyruvate dehydrogenase A high ratio of NADH/ NAD+, acetyl CoA/CoA or ATP/ADP stimulates ph-osphorylation of pyruvate dehydro-genase and so inactivates this enzyme. Pyr

13、uvate inhibits the kinase. Removal of the phosphate group (dephospho-rylation) by a phosphatase reactivates pyrucate dehydrogenase.modeBiosynthetic pathways Amino acids, purines 嘌呤嘌呤 and pyrimidines 嘧啶嘧啶, porphyrins 卟啉卟啉, fatty acids and glucose are all synthesized by pathways that use citric acid i

14、ntermediates 中间体中间体 as precursors.Overview Redox potential Electron transport from NADH Formation of H+ gradient Electron transport from FADH2 Electron transport inhibitorsElectron transport and Oxidative phosphorylationOverview7. Oxidative phosphorylation8. ATP synthase as a rotatory engine9. Coupl

15、ing and respiratory control10. Uncouplers11. Reoxidation of cytosolic NADHOverview Electron transport and oxidative phosphorylation re-oxidize NADH and FADH2 and trap the energy released as ATP. In eukaryotes, electron transport and oxidative phosphorylation occur in the inner membrane of mitochondr

16、ia whereas in prokaryotes the process occurs in the plasma membrane . modeRedox potential The oxidation-reduction potential, E, (or redox potential) is a measure of the affinity of a substance for electrons and is measured relative to hydrogen .Redox potential Oxidation-reduced reactions involve the

17、 transfer of electrons. In the oxidation-reduction reactions : NADH+H+1/2O2 = NAD+H2OOxidation formredox potential For biological systems, the standard redox potential for a substance (E0) is measured under standard conditions, at pH7, and is expressed in volts. The standard free energy change of a

18、reaction at pH7, G0, can be readily calculated from the change in redox potential E0 of the substrates and products: G0nFE0 redox potential NADH+H+1/2O2 NAD+H2O E0 = + 1.14V G0 = - nFE0 G0 = - 52.6 kcal mol-1 Electron transport from NADH NADH + H+1/2O2 NAD+H2O G0 = - 52.6kcal mol-1 And the synthesis

19、 of ATP ADP + PI + H+ ATP + H2O G0 = +7.3kcal mol-1 overviewstructureconstituteATPNADFADFeS proteinChainUQCytochromeOxidationcomplexIII and IVElectron transport 1Electron transport 2Electron transport from FADH2Electron transport inhibitors Rotenone 鱼藤酮鱼藤酮 and amytal 阿密妥阿密妥 inhibit electron transpor

20、t at NADH dehydrogen-ase, antimycin 抗霉素抗霉素 A inhibits the cyto-chrome bc1 complex,and cyanide 氰化物氰化物 (CN- ),azide 叠氮物叠氮物 (N-3 ) and carbon monoxide(CO) all inhibit cytochrome oxidase. chainFormation of an H+ gradient The change in redox potential along the chain is a measure of the free energy chang

21、e at each step. At the steps involving NADH dehydrogenase, the cytochrome bc1 complex and cytochrome oxidase, the free energy change is large enough to pump H+ ions across the inner mitochondrial membrane,from the mitochondrial matrix into the intermembrane space, to create an H+ gradient.Therefore,

22、each of these complexes is an H+ pump driven by electron transport.The Chemiosmotic TheoryPeter Mitchell proposed in 1961.PROTON PUMPThe principal features (1)As electrons pass through the electron transport chain, protons are transported from the matrix and released into the intermembrane space. As

23、 a result, an electrical potential and a proton gradient pH arise across the inner membrane-the electrochemical proton gradient.Redox potential (2).Protons, which are present in the intermembrane space in great excess, can pass through the inner membrane and back into the matrix down their concentra

24、tion gradient only through special channels. As protons pass through a channel, each of which contains an ATP synthase activity, ATP synthesis occurs.Fig Oxidative Phosphorylation (1) Conception The process whereby the energy generated by the electron transport chain is conserved by the phosphorylat

25、ion of ADP to yield ATP. Process of Oxidative Phosphorylation I. The Chemical Coupling Hypothesis Key point: A high-energy intermediate generated by the electron transport process is used in a second reaction to drive the formation of ATP from ADP and Pi.Process PiOCH2CHOHCHO + Pi PiOCH2CHOHCOOPi +

26、ADP PiOCH2CHOHCOOH + ATPFailed points The proposed intermediate has never been identified. The hypothesis could not account for several experimental findings, such as uncoupler, entire inner mitochondrial membrane.Evidence supporting the Chemiosmotic Theory uncoupler, such as 2,4,-dinitrophenolIonop

27、hores,e.g. valinomycin and thermogeninFIG 1Experiment 2ATP synthase as a rotatory engine In 1964, Paul Boyer proposed theconformational coupling hypothesis.structureExperiment 3FoF1 In 1970s, Paul Boyer has suggested a Binding-change Mechanism on the basis of detailed kinetic and binding studies of

28、the reactions catalyzed by FoF1 .The principal features (1)The energy-requiring step is the release of ATP from the enzyme (ATP synthase) and binding of ADP and Pi. (2)The enzyme has three equivalent adenine nucleotide binding sites.The proton-motive force causes a cooperative conformational change

29、during ATP synthesis.Three conformation Three conformation L (loose-binding) ADP+Pi bind loosely to L site T (tight-binding) ATP occupies T site O (open) ATP dissociates from itHistory In 1990s, Paul Boyer complemented the mechanism with rotational catalysis.FIGATP synthase ATP synthase behaves as a

30、 rotating molecular machine. The rotor (or revolving) component of the machine consists of subunits , and c12, whereas subunits a, b2, ,3 and 3 comprise the stator (or stationary) component.Inhibitor of oxidative phosphorylation Fo is the portion of the ATP synthase and sensitivity to oligomycin. Ol

31、igomycin inhibits proton passing through ATP synthase and thus oxidative phosphorylation. (1) As electrons pass through the electron transport chain, the electrochemical proton gradient is established by proton pump across the inner membrane of mitochondria. (2) Protons pass through a special channe

32、l in ATP synthase and result in ATP synthesis by rotational catalysis and cooperative conformational changeCoupling and respiratory control Electron transport is normally tightly coupled to ATP synthesis; electrons do not flow through the electron transport chain to oxygen unless ADP is simultaneous

33、ly phosphorylaed to ATP. Figrespiratory control If ADP is available, electron transport proceeds and ATP is made; as the ADP concentration falls ,electron transport slows down. This process, called respiratory control, ensures that electron flow occurs only when ATP synthesis is required. Uncouplers

34、 (1). uncoupling agents (2). Ionic carrier (3). Thermogenin (uncoupling protein)DNPADP transportReoxidation of cytosolic NADHThe glycerol 3-phosphate shuttleMalate-aspartate shuttle Overview Location Light harvesting in green plants Photosystems I and IIvNoncyclic photophosphorylationvCyclic photoph

35、osphorylationvBacterial photosynthesisvThe dark reactionsvThe C4 pathwayL3 PHOTOSYNTHESISOverview Photosynthesis 光合作用光合作用 occurs in green plants ,algae and photosynthetic bacteria. Its role is to trap solar energy and use this to drive the synthesis of carbohydrate from carbon dioxide and water : H2

36、O+CO2 light (CH2O) + O2 Overview light reactions Photosynthesis dark reactionsPhotosynthesis The light reaction: which use light energy to synthesize NADPH and ATP; The dark reactions: that use the NADPH and ATP to synthesize carbohydrate from CO2 and H2O Photosynthesis Location In green plants and

37、algae photosynthesis takes place in chloroplasts. The light reactions occur in the thylakoid类囊体类囊体 membranes and the dark reactions take place in the stroma 基质基质. Fig 2Location In photosynthetic bacteria the light reaction take place in the bacterial plasma membrane ,or in invaginations 内陷内陷 of it (

38、chromatophores 载色体载色体).Light is harvesting in green plants Sunlight is absorbed by chlorophyll molecules. Chlorlphyll is a porphyrin in which nitrogen atoms are coordinated to a magnesium ion. Green plants contain two types of chlorophyll molecules , chlorophyll a and chlorophyll b. The capture of s

39、olar energy occurs in photosystems. Each photosystem consists of an antenna complex and a photosynthetic.Light spectrumpigment藻红蛋白植醇Pigment wavelength藻红蛋白藻青蛋白Light absorbPhotosystems I and II Green plants and algae use two types of photosystem called photosystemI (PSI) and photosystem II (PSII) Plastoquinone 质体醌质体醌 Plastocyanin 质体蓝素质体蓝素 Ferredoxin 铁氧还蛋白铁氧还蛋白PSII and PSIZ schemNoncyclic photophosphorylation The formation of ATP via the joint operation of PSI and PSII is called