Lecture 7 Calvin Cycle PDF

Photophosphorylation ATP, NADPH FOR PLANTS ATP Hphotosynthesis eencreted Both in CO2 NADPH + H+...

Photophosphorylation ATP, NADPH FOR PLANTS ATP Hphotosynthesis eencreted Both in CO2 NADPH + H+ ATP f I HCOH I 1 R Ingen-Housz (1779) “The discovery that plants have a power of correcting bad air shows that the air, spoiled and rendered noxious to animals by their breathing in it, serves to plants as a kind of nourishment” 2  Elucidation of pathway of CO2 fixation 1. Radioactive isotopes – 11C (Ruben), short half-life 14C (Calvin), stable 2. 2D – paper chromatography Questions: 1. Initial stable product of PS? 2. Acceptor molecule for CO2? 3. How is acceptor synthesised? 4. Path of C to sugar? (Sucrose NOT glucose) 3 Benson and Calvin – Scenedsmus, Chlorella Radioactive carbon added during PS. Algae killed at time intervals in alcohol to prevent further reaction 4 14C pulse-chase *****CO +X ****A *B C 2 CO2 + X **A *B *C CO2 + X *A *B **C 5 2D paper chromatography, autoradiography Phosphoglyceric acid (PGA) = first product C3 pathway of photosynthesis 6 7 8  What is acceptor molecule? CO2 + X PGA (3C) suggests 2C?? Massini & Calvin: X = 5C acceptor = Ribulose bisphosphate (RuBP) Light essential for synthesis of RuBP and utilisation of PGA. Require ATP, NADPH from Photophosphorylation 9  Wilson and Calvin (lower pp CO2 in light) Acceptor : synthesis continues in light then – depletion of remaining intermediates PGA : no longer synthesised from RuBP (no CO2) PGA still metabolised in light 1. RuBP regenerated by same (NADPH, ATP) pathway that uses PGA 2. RuBP carboxylase fixes CO2 (Quayle) 10 The Calvin Cycle: C3 photosynthesis ) ?? 11  The Three Stages of the Calvin Cycle RuBP carboxylase CGAD) Gyp GAP dehydrogenase 3-PGA kinase 12  Three Stages of the Calvin Cycle 3 RuBP + 3 CO2 2) 63-P6A 1. CO2 fixation: 3 ribulose 1,5-bisphosphate + 3 CO2 → 6 x 3-phosphoglycerate – catalyzed by RuBP Carboxylase Reduction 2. 3-PGA reduced to GAP (triose phosphate) using NADPH and ATP from - - photosynthesis 3. Regeneration of ribulose 1,5-bisphosphate requires additional ATP Overall: 3 CO2 + 6 NADPH + 6H+ + 9 ATP → glyceraldehyde 3-phosphate (G3P) + 3H2O + 6 NADP+ + 9 ADP + 8 Pi - - - Glycolysis 13  Stage 1 : Ribulose bisphosphate carboxylase Bispin af Hydrolysis 11 T H 5 & - - RuBB 14  Stage 2: 3-phosphoglycerate (PGA) reduced to glyceraldehyde 3-phosphate(GAP) all 1 :1 ratio 6 PGA + 6 ATP + 6 NADPH + 6 H+ → 6 GAP+ 6 ADP + 6 NADP+ + 6 Pi 3-phosphoglycerate kinase (PGA kinase) PGA + ATP - 1,3 BisPGA + ADP glyceraldehyde 3-phosphate dehydrogenase (GAPDH) 1,3- BisPGA + NADPH GAP + NADP+ + Pi “Reversal of glycolysis”. Driven forward by high conc. of NADPH and ATP in chloroplast stroma > AFTER PHOTOSYNTHESIS - 15  Stage 3: Regeneration of 5/6 triose phosphate (GAP/DHAP) to RuBP 16 Stoichiometry of CO2 Assimilation in the Calvin Cycle 9 ATP > G3P 3CO2 6 NADPH - + + Fixation of three CO2 molecules yields one glyceraldehyde 3- phosphate (triose phosphate) for use in anabolic processes. Nine ATP molecules and six NADPH molecules are consumed. 17  Triose phosphates (Ga3P, DHAP) are used in PGA carbohydrate biosynthesis and oxidation storage mobileepe catabolism: glycolysis 63D GAP anabolism: starch and DHAP sucrose synthesis (not glucose) DHAP exported to cytosol in exchange for Pi triosephosphate translocator 18  Fate of the Pi from ATP Hydrolysis in Stage 2 Eight of the nine Pi combine with ADP to regenerate ATP. Ninth incorporated into GAP/DHAP. – Pi and triosephosphate counter-exchange across chloroplast antiport Critical in maintaining Pi content of CP - DHAP GNADPH + 9ATP 8 restured , IP: in antiporter 19 ↳ only Triosephosphate translocator (TPT) - energy export from chloroplast to cytosol spor 1 ↓j - PHAP ISOMERS 20 DHAP &63P Regulation of Calvin Cycle Rubis 20 RuBP Carboxylase – later lecture SBPase – slowest step, possibly “rate limiting” Light activation – thioredoxin 21  Light activation of Calvin Cycle enzymes Target enzymes: – ribulose 5-phosphate kinase PRK – fructose 1,6-bisphosphatase FBPase – sedoheptulose 1,7-bisphosphatase SBPase – glyceraldehyde 3-phosphate dehydrogenase NADP-GAPDH If oxidized (Cys residues in Cys-Cys disulfide form) → inactive Illuminated chloroplasts, PS I reduces ferredoxin (Fd). Electrons from Fd diverted to thioredoxin (TR) (ca. 10kDa) Ferredoxin Thioredoxin Reductase (FTR) : reduces disulfide bonds to Cys-SH. Coordinates Calvin Cycle with photochemical reactions 22 Light activation PS of Calvin Cycle Enzymes oxidized relat reg For enzyme 23 Assimilation of CO2 into Biomass Next lecture: O2 The problem of oxygen 24