Tema 13 (4).pdf

Energy transductions: Oxidative phosphorylation and photophosphorylation Mitochondrial electron transport chain Oxidative phosphorylation. Respiratory control. Uncoupling agents. Photosynthetic transport chain and photophosphorylation....

Energy transductions: Oxidative phosphorylation and photophosphorylation Mitochondrial electron transport chain Oxidative phosphorylation. Respiratory control. Uncoupling agents. Photosynthetic transport chain and photophosphorylation. Carbohydrate biosynthesis (Calvin cycle). Regulation of photosynthesis. M 1  RESPIRATION STAGE 3 : OXIDATIVE PHOSPHORYLATION Generates a lot of ATP M 2  RESPIRATION STAGE 3 : OXIDATIVE PHOSPHORYLATION Generates a lot of ATP Electron transport chain M 3  RESPIRATION STAGE 3 : OXIDATIVE PHOSPHORYLATION Generates a lot of ATP CHEMIOSMOTIC MODEL FOR ATP SYNTHESIS Electron transport sets up a proton-motive force Energy of proton-motive force drives synthesis of ATP M 4  STAGE 3 : OXIDATIVE PHOSPHORYLATION SUMMARY OF ELECTRON TRANSPORT Complex I → Complex IV 1NADH + 11H+(N) + ½O2 ——> NAD+ + 10H+(P) + H2O Complex II → Complex IV FADH2 + 6H+(N) + ½O2 ——> FAD + 6H+(P) + H2O Difference in number of protons transported reflects differences in ATP synthesized. Cadena de transporte de electrones (Video): M https://www.youtube.com/watch?v=LQmTKxI4Wn4 5  MITOCHONDRIAL ATP SYNTHASE COMPLEX Contains two functional units: ❑ F1 ✓ Soluble complex in the matrix ✓ Individually catalyzes the hydrolysis of ATP ❑ F0 ✓ Integral membrane complex ✓ Transports protons from IMS to matrix, dissipating the proton gradient ✓ Energy transferred to F1 to catalyze phosphorylation of ADP ATP sintasa en acción: https://www.youtube.com/watch?v=kXpzp4RDGJI M 6  HOW TO DETERMINE THE COMPLEXES ORDER? M 7  CONSEQUENTLY, ELECTRON TRANSPORT IS COUPLED TO ATP SYNTHESIS As described, ATP synthesis requires electron transport But electron transport also requires ATP synthesis FADH2 FAD+ Succinate Fumarate M 8  UNCOUPLING AGENTS OF OXIDATIVE PHOSPHORYLATION UNCOUPLES OF OXIDATIVE PHOSPHORILATION Agents that dissipate the proton gradient across the inner mitochondrial membrane. ✓ Prevent ATP synthesis ✓ Permit electron transport Energy is released as heat Examples ✓ 2,4-Dinitrophenol ✓ Uncoupling protein I (Thermogenin), compound in the mitochondria of brown adipose tissue M 9  REGULATION OF OXIDATIVE PHOSPHORYLATION Primarily regulated by substrate availability: NADH and ADP/Pi ✓ Due to coupling both substrates required for electron transport and ATP synthesis Inhibitor of F1 (IF1) ✓ Prevents hydrolysis of ATP during low oxygen ✓ Inhibition of Oxidative Phosphorylation leads to accumulation of NADH ✓Causes feedback inhibition cascade up to PFK-1 in glycolysis M 10 M 11  PHOTOPHOSPHORYLATION In photosynthetic organisms light causes charge separation between a pair chlorophyll molecules Energy of the oxidized and reduced chlorophyll molecules is used to drive synthesis of ATP Water is the source of electrons that are passed via a chain of protein transporters to the ultimate electron acceptor, NADP+ Oxygen is the byproduct of water oxidation M 12  ORGANIZATION OF LIGHT-ABSORBING MOLECULES IN CHLOROPLASTS. LIGHT HARVESTING COMPLEX (LHC) M 13  IN PLANTS, TWO REACTION CENTERS ACT IN TANDEM Energy of the oxidized and reduced chlorophyll molecules is used to drive synthesis of ATP Water is the source of electrons that are passed via a chain of protein transporters to the ultimate electron acceptor, NADP+ M Oxygen is the byproduct of water oxidation 14  CYTOCHROME B6F COMPLEX LINKS PS II AND I AND TRANSLOCATES PROTONS INTO THE LUMEN M 15  Organization of phosynthetic machinery in the thylakoid membrane M 16  LIGHT-INDUCED REDOX REACTIONS CAUSE ACIDIFICATION OF LUMEN The proton-motive force across the thylakoid membrane drives the synthesis of ATP M 17  CO2 ASSIMILATION Occurs in the stroma of chloroplasts via a cyclic process known as the Calvin cycle Key intermediate ribulose 1,5-bisphosphate is constantly regenerated using energy of ATP Produces 3-phosphoglycerate, then glyceraldehyde 3-phosphate (G3P) in eq with dihydroxyacetone phosphate (DHAP) The net result is the reduction of CO2 with NADPH that was generated in the light reactions of photosynthesis M 18  THREE STAGES OF THE CALVIN CYCLE CO2 Fixaton: 3 Ribulose 1,5-bisphosphate + 3 CO2 → 6 molecules of 3-phosphoglycerate ✓ Catalyzed by rubisco 3-phosphoglycerate reduced to glyceraldehyde 3-phosphate using NADPH and ATP from photosynthesis ✓ Catalyzed by 3-phosphoglycerate kinase and glyceraldehyde 3-phosphate dehydrogenase Regeneration of ribulose 1,5-bisphosphate ✓ Complicated series of reactions M 19  ASSIMILATION OF CO2 INTO BIOMASS The light-driven synthesis of ATP and NADPH provides energy and reducing power for the fixation of CO2 into trioses, from which all the carbon-containing compounds of the plant cell are synthesized. Ciclo de Calvin M 20  STAGE 1 (CO2 FIXATION) IS CATALYZED BY RUBISCO About Rubisco ✓ Rubisco = Ribulose 1,5-bisphosphate carboxylase/oxygenase (also RuBisCo) ✓ Large Mg2+enzyme ✓ Catalyzes ▪ Ribulose 1,5-bisphosphate + CO2 → 3-phosphoglycerate ✓ As carboxylase: incorporates inorganic CO2 into organic ribulose 1,5- bisphosphate (first step of Calvin Cycle) ✓ Makes new C-C bond! ✓ Also cleaves 6-C intermediate into two 3-phosphoglycerates M 21  STAGE 2: 3-PHOSPHOGLYCERATE REDUCED TO GLYCERALDEHYDE 3-PHOSPHATE Six 3-phosphoglycerate + 6 glyceraldehyde 3-phosphate 6 ATP + 6 NADPH + 6 H+ + 6 ADP + 6 NADP+ + 6 Pi Requires NADPH and ATP from photosynthesis ~Reversal of glycolysis reactions (except NADPH used rather than NADH) Uses enzymes 3-phosphoglycerate kinase and glyceraldehyde 3-phosphate dehydrogenase Driven forward by high concentration of NADPH and ATP in the chloroplast stroma FATES OF GLYCERALDEHYDE 3-PHOSPHATE Five of the six are recycled to ribulose 1,5-bisphosphate. Remaining one: can be converted to starch in the chloroplast can be converted to sucrose in the cytosol for export M 22  STAGE 2: CONVERSION OF 3-PHOSPHOGLYCERATE TO GLYCERALDEHYDE 3-PHOSPHATE Most of the glyceraldehyde 3-phosphate is recycled to ribulose 1,5-bisphosphate A small fraction of the “extra” glyceraldehyde 3- phosphate may be used immediately as a source of energy, but most is converted to sucrose for transport or is stored in the chloroplast as starch. For this purpose, glyceraldehyde 3-phosphate condenses with dihydroxyacetone phosphate in the stroma to form fructose 1,6-bisphosphate, a precursor of starch. In other situations the glyceraldehyde 3-phosphate is converted to dihydroxyacetone phosphate, which leaves the chloroplast via a specific transporter and, in the cytosol, can be degraded glycolytically to provide energy or used to form fructose 6-phosphate and hence sucrose M 23  STAGE 3: REGENERATION OF RIBULOSE 1,5-BISPHOSPHATE Complicated series of reactions with 3-C, 4-C, 5-C, 6-C and 7-C intermediates Generates pentose phosphate pathway intermediates STAGE 3: REGENERATION OF RIBULOSE 1,5- BISPHOSPHATE AND INTERCONVERSION OF TRIOSE PHOSPHATES WITH PENTOSE PHOSPHATE INTERMEDIATES M 24  STOICHIOMETRY OF CO2 ASSIMILATION IN THE CALVIN CYCLE Fixation of three CO2 molecules yields one glyceraldehyde 3-phosphate Nine ATP molecules and six NADPH molecules are consumed M The light rxs of photosynthesis produce ATP and NADPH at ~this same ratio (3/2) 25  O2 e- e- GLÚCIDOS NADH + H+ (FADH2) e- CO2 H2O H2O e- e- GLÚCIDOS NADPH2 e- CO2 O2 M 26  SUMMARY ATP and NADPH from light reactions are needed in order to assimilate CO2 into carbohydrates Assimilations of three CO2 molecules via the Calvin cycle lead to the formation of one molecule of 3-phosphoglycerate 3-Phosphoglycerate is a precursor for the synthesis of larger carbohydrates such as fructose and starch The key enzyme of the Calvin cycle, rubisco, fixes carbon dioxide into carbohydrates M 27