Dark Reaction Topics CO2 Fixation Calvin Cycle Starch and Sucrose Biosynthesis C3 vs C4 Plants

Dark Reaction Glucose Oxidation Exergonic Endergonic Dark Reaction C6H12O6 + 6O2 6CO2 + 6H2O Go’= -2,860 kJ/mol Dark Reaction 6CO2 + 6H2O C6H12O6 + 6O2 Go’ = + 2,860 kJ/mol

CO2 Fixation (One Enzyme) CH2OP C=O C-OH Enediol  electrons CH2OP C-OH CO2 CH2OP C- C=O C-OH HO- CO2 Ribulose 1,5bisPO4 H2O CH2OP C-CO2 HO- H .. CH2OP C-CO2 HO- CH2OP C-CO2 C-OH HO- H H+ 2, 3PGA C-OH CH2OP HO- + H COO- C-OH CH2OP H or RUBISCO Ribulose 1,5 bisPO4 carboxylase

The Calvin Cycle in Photosynthesis Rule: The Calvin cycle is the mechanism for turning CO2 into glucose Rule: The cycle fixes 6 CO2 to 6 ribulose 1,5-bisPO4 acceptor molecules, makes glucose with 6 of the carbons, and returning the rest to continue the cycle. Rule: The driving force is to synthesize the 5 carbon product ribulose 1,5-bisPO4

Calvin Cycle Strategy 6CO2 Glucose 6CO2 + 6RuBP G6P + 6RuBP (Ideal) 6CO2 Glucose (Real) 6CO2 + 6RuBP G6P + 6RuBP 6C 30C 6C 30C (Mechanism) 6CO2 + 6RuBP 12, 3PGA 6C 12, 3PGA G6P + 2F6P + 4G3P + 2DHAP 2E4P + 2SHBP 4Xu5P + 2Ru5P Intermediates 20C 10C Final

Carbon Balance in Calvin Cycle 12 3-PGA 36c 36c ATP ADP 36c 12 1,3 bisPO4 glycerate NADPH + H+ NADP+ 24c 6c 12 glyceraldehyde-3-PO4 DHAP Calvin Calvin 3 Fructose 1,6bisPO4 18c 12c 2 Fructose-6-PO4 Calvin 12c Glucose-6-PO4 6c

TK and TA Reactions C C=O CH2OP C CH2OP HO C=O TK CHO C CH2OP Ru5P F6P G3P Xu5P CH2OP C=O C COH HO Pi C CH2OP HO C=O TK CHO C CH2OP CHO C CH2OP + CH2OP C=O CH2OH TA CHO C CH2OP Xu5P R5P Ru5P DHAP E4P S1,7BP G3P

Finishing Touches with 5 Carbon Rule: In the Calvin cycle 5C sugars are never donor or acceptor substrates for transketolases or transaldolases C-OH HO-C CH2OP CH2OH C=O C-OH CH2OP CH2OH C=O C-OH CH2OP C=O ATP Epimerase Kinase Xu5P Ru5P C-OH CH2OP CHO C-OH CH2OP CH2OH C=O RuBP Isomerase ATP 30 carbons are returned to CO2 fixation R5P Ru5P

3C 3C 3C 3C 3C 6C 4C 7C 5C 5C 5C G3P DHAP G3P G3P DHAP E4P SBP F6P Xu5P R5P Xu5P

Synthesis of Sucrose and Starch Cytosol F6P  G6P  G1P UTP PPi ATP PPi ADP-glucose UDP-glucose fructose-6-PO4 (glucose)n ADP + (glucose)n+1 UDP + Sucrose-6-PO4 Stroma H2O Pi Starch (amylose) Sucrose

Thank goodness for photosynthesis

Control of the Calvin Cycle ΔGo’ ΔG Phosphoribokinase -21.8 -15.9 Ribulose bisPO4 carboxylase -35.1 -41.0 Phosphoglycerate kinase + +18.0 -6.7 glyceraldehyde-3-PO4 dehydrogenase Triose phosphate isomerase -7.5 -0.8 Aldolase -21.8 -1.7 Fructose bisphosphatase -14.2 -27.2 Transketolase +6.3 -3.8 Aldolase -23.4 -0.8 Sedoheptulose bisphosphatase -14.2 -29.7 Transketolase +0.4 -5.9 Phosphopentose epimerase +0.8 -0.4 Ribose phosphate isomerase +2.1 -0.4 Each controlling enzyme is itself regulated by light either through pH or Mg+2

Does the Calvin cycle occur in the dark? Control of Flux in Plants Glyceraldehyde-3-PO4 Pi Glyceraldehyde-3-PO4 dehydrogenase NADP+ NADPH 1,3-Bisphosphoglycerate Does the Calvin cycle occur in the dark? Phosphoglycerate kinase ADP ATP 3-PGA NO Calvin Cycle

PS1*RED PS1OX FdOX FdRED FdRRED FdROX Binds tightly to CH2OPO3 HO-C-CO2 H-C-OH Binds tightly to RUBISCO when there is no light Therefore CO2 fixation cannot occur in the dark 2-Carboxyarabitol-1-PO4 Active* Thioredoxin SH S PS1*RED PS1OX FdOX FdRED FdRRED FdROX SH 1,7 BisPtase SH S Inactive Sedoheptulose 1,7-bisphosphatase is activated by light Text p555