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Photosynthesis

Explore the fundamental processes of photosynthesis, including light absorption, electron transfer, and carbon fixation. Learn how these reactions contribute to energy generation and the production of sugars in plants and photosynthetic bacteria.

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Photosynthesis

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  1. Photosynthesis Andy HowardBiochemistry Lectures, Spring 201916 April 2019

  2. Electrons and light • The light reactions of photosynthesis capture light energy to drive anabolism and ATP synthesis • The dark reactions after RuBisCO harness the reductive potential of NADPH to interconvert various 3-7 carbon sugars • We’ve seen some of those reactions in the PPP and Entner-Doudoroff pathway Photosynthesis

  3. Light reactions of photosynthesis PS II & PSI OEC NADPH RuBisCO and carbon fixation Calvin Cycle Reactions Bookkeeping Sucrose & Starch Other C-fixation paths Geometry & control What we’ll discuss Photosynthesis

  4. Photosynthesis (CF&M Ch. 22) • Definition:harvesting of light to generate energy • Happens in plants, photosynthetic bacteria • Photosynthetic reactions offer source for carbon fixation as well as energy in photoautotrophs • In higher plants these events happen in the thylakoid disks of chloroplasts Photosynthesis

  5. Overview of photosynthesis In plants &complex photosynthetic bacteria there are 2 connected photosystems, PS II and PS I. Each has photon-absorbing pair of chlorophyll molecule + other chlorophylls that transfer energy PS II is primarily an ATP source PS I is primarily an NADPH source CF&M §22.2 Photosynthesis

  6. Light reactions • Electrons are promoted from the ground state to excited states upon absorption of a photon by a chromophore Drawing courtesy JohnsonCounty Community College Photosynthesis

  7. Chlorophyll • Heme-like chromophore with Mg2+ in the center • Absorbs strongly in red and blue; therefore it appears green www.steve.gb.com/science/photosynthesis.html Photosynthesis

  8. Special pair • Two out of the large collection of chlorophyll molecules within a single photosystem that are responsible for giving up electrons rather than just getting electrons excited into higher-energy states • Pair of P680 molecules in photosystem II are the special pair in that case; P700 in photosystem I • Other chlorophyll molecules and antenna molecules absorb photons too; these transfer energy to the special pair Photosynthesis

  9. Photosystem II • Beginning of sequence of energy-generating pathways in the chloroplast or the bacterial membrane • Involves P680, a chlorophyll positioned so that its absorption max is at 680nm • Absorption maximum depends on chromophore’s specific structure and on modulation by neighboring protein species Photosynthesis

  10. Electron translocation in photosystem II • Two protons move across the thylakoid membrane for each electron promoted and transferred—plus two protons associated with the conversion of QH2 back to Q • This provide proton pumping capability like that in mitochondria • Difference: gradient is dependent only on pH difference, not electrical potential Photosynthesis

  11. Oxygen is produced Thermosynechococcus PSIIEC 1.10.3.9, 734 KDaPDB 5B66, 1.85Å Oxygen evolving complex (OEC): part of PS II The oxidation of one molecule of water to ½O2 evolves one molecule of NADPH This is the source of most of the oxygen in our atmosphere and the dissolved oxygen in the oceans, seas, and rivers Contributes to proton gradient also Photosynthesis

  12. Summary of PSII reactions plastoquinone Poplar plastocyanin10.6kDa monomerPDB 4DP9, 1Å PSII: 2P680 + 2photons 2P680+ + 2e-PQ + 2e- + 2H+in PQH2 OEC: H2O ½O2 + 2H+out + 2e-2P680+ + 2e-2P680 Cyt bf: 2PQH2 + 2 plastocyanin (Cu2+) 2PQ + 2plastocyanin (Cu+) + 4H+out + 2e- Photosynthesis

  13. Ferredoxin Fe2S2 PS I reactions E.arvense FdII11kDa monomerPDB 1WRI, 1.2Å PSI: 2P700 + 2photons 2P700+ + 2e-2Fdox + 2e- 2Fdred2plastocyanin (Cu+) + 2P700+ 2plastocyanin(Cu2+) + 2P700 FNR: 2 Fdred + H+ + NADP+ 2Fdox + NADPH Photosynthesis

  14. Photosystem I • P700 is primary photon acceptor • Similar translocations of protons • Net reduction of NADP • Non-cyclic: we need to re-oxidize NADPH, and that occurs as the NADPH gets used in anabolic reactions Photosynthesis

  15. Summary of PS I reactions Maize root FNREC 1.18.1.2 36 kDa monomerPDB 3LO8, 1.1Å PSI: 2P700 + 2photons  2P700+ + 2e-2Fdox + 2e- 2Fdred2plastocyanin (Cu+) + 2P700+ 2plastocyanin(Cu2+) + 2P700 Ferredoxin-NADP+ Reductase (FNR):2 Fdred + H+ + NADP+2Fdox + NADPH Photosynthesis

  16. Net light reactions (cf CFM 22.3) • NADPH produced; ~8 protons pass across • Energy is, in principle, available from both sources, but NADPH is employed in anabolic reactions rather than as a source of ATP • Net ATP production per photon: unclear. Probably about 2. • Chemical inventory:H2O + 4photons + 4H+in + NADP+ + H+½O2 + 6H+out + NADPH Photosynthesis

  17. Bookkeeping for light reactions Net ATP production per photon: unclear. Probably about 2. Chemical inventory (Table 15.3 at end):H2O + 4photons + 4H+in + NADP+ + H+½O2 + 6H+out + NADPH Photosynthesis

  18. -carotene Other light-absorbing pigments • Chlorophyll in its various forms is not the only light-absorbing pigment in plants and photosynthetic bacteria • Accessory pigments / accessory proteins involved in resonant energy transfers to chlorophyll • Examples: carotenoids (particularly -carotene), phycoerythrin, phycocyanin Photosynthesis

  19. Accessory pigments in phycobilisomes • Water absorbs red light strongly • Shorter wavelengths (higher energies) are more penetrating • Aquatic plants need accessory pigments that absorb in the range that’s available • Energy absorbed by phycobiliproteins is transferred ultimately to ChlA by Förster resonances Photosynthesis

  20. Dark reactions (CF&M 22.5) • Series of ordinary chemical reactions • Powered by reducing power in NADPH • Anabolic, so we’re making complex molecules • Some common features with pentose phosphate pathway • We’ll begin with RuBisCO and then move on to the sugar transformations Photosynthesis

  21. Dark reactions: overview • RuBisCO: condenses one molecule of CO2 with one molecule of ribulose 1,5-bisphosphate (RuBP) to form 2 molecules of 3-phosphoglycerate • Several reductions and interconversions starting with phosphoglycerate • Pathway is cyclic in that RuBP is regenerated for additional reactions Photosynthesis

  22. Ribulose bisphosphate RuBisCO reaction • Condensation of ribulose 1,5-bisphosphate (RuBP) with CO2 to produce two molecules of 3-phosphoglycerate • Enzyme is ribulose1,5-bisphosphate carboxylase / oxygenase 3-phosphoglycerate Photosynthesis

  23. Oxygenase reaction • Unwanted (?) side-reaction: • RuBP + O23-phosphoglycerate +2-phosphoglycolate • No net carbon incorporation Photosynthesis

  24. RuBisCO structure • L8S8 stoichiometryin higher plants:Mol.Wt. L=55kDa;Mol. Wt. S=12 kDa • TIM barrels in both Rice RuBisCOEC 4.1.1.39L8S8; L2S2 shownPDB 4WDD, 1.35Å Photosynthesis

  25. Large & small subunits All (?) catalytic activity in L (large) subunit L coded for by chloroplast gene S by nuclear genome Does S play a controlling role? Photosynthesis

  26. The unwanted (?) side-reaction of RuBisCO • Secondary reaction isribulose 1,5-bisphosphate + O2 3-phosphoglycerate +2-phosphoglycolate • Uses up oxygen rather than CO2 • No net carbon incorporation into organic molecules 2-phospho-glycolate Photosynthesis

  27. RuBisCO regulation • Plant growth closely associated with carboxylation / oxygenation ratio:Carboxylation high means fast growth • Easy way to alter that: grow plants in high CO2 • Difficult to do that without animal toxicity! • Expensive to put your cornfield in a plastic bubble(but not impossible) Photosynthesis

  28. Could you win genetically? • Attempts to engineer proteins that don’t do oxygenation(or even that have improved CO2/O2 activity ratios) have failed • There are some plants whose RuBisCO has a better SC/O than that of others • Maybe O2 and CO2 bind in precisely the same way! Photosynthesis

  29. Subsequent dark reactions, I • Pair of 3-phosphoglycerate molecules enter reductive pathway toward bigger sugars: the Calvin cycle • Almost all of these reactions are found in other pathways: • Glycolysis (but backward) • Gluconeogenesis • Pentose phosphate pathway (backward) • Cycle: ribulosebisphosphate will be recreated Photosynthesis

  30. Subsequent dark reactions • Three glycolysis / gluconeogenesis rxns: • GAPDH reaction:1,3-bisP-glycerate + NADPH + H+glyceraldehyde-3-phosphate + NADP + Pi • TIM required to convert GlycAld3P to DHAP • Aldolase makes fructose 1,6-bisphosphate Photosynthesis

  31. Recycling ribulose bisphosphate Some RuBP will later be recycled back in to provide input to subsequent condensations with CO2 Photosynthesis

  32. Calvin cycle: first reaction • Begins with ATP-dependent phosphorylation of 3-phosphoglycerate to make 1,3-bisphosphoglycerate via phosphoglycerate kinase • Same reaction found in gluconeogenesis • Enzyme is 3-layer sandwich Thermus thermophilus PG Kinase: 86 kDa dimer PDB 1V6S Photosynthesis

  33. 2nd Calvin-cyclereaction: GAPDH • NADPH-dependent reduction of 1,3-bisphosphoglycerate to glyceraldehyde 3-phosphate • As in gluconeogenesis, reverse of glycolytic reaction • GAPDH: typical NAD(P) dependent oxidoreductase Spinach GAPDH297 kDa octamerdimer + monomer shownPDB 1RM4 Photosynthesis

  34. The fates ofglyceraldehyde-3-phosphate • The pathway divides three ways at this metabolite • One equivalent toward fructose 1,6-bisphosphate and gluconeogenesis • Two head toward pentose phosphate pathway, where a second bifurcation happens Photosynthesis

  35. C3 to C6 • TIM converts 1 molecule of glyceraldehyde 3-phosphate to DHAP • Glyc-3-P and DHAP condense: form fructose 1,6-bisphosphate in standard aldolase reaction • Fructose 1,6-bisphosphatase removes the 1-phosphate to make fructose 6-phosphate • All of this happens in gluconeogenesis Photosynthesis

  36. Photorespiration • 2-phosphoglycolate is the productof the RuBisCO oxygenation reaction • 2-P-glycolate is decarboxylated:2 2-P-glycolate  CO2 + 3-P-glycerate +Piin a pathway that involves shuttles among the chloroplast, peroxisome, and mitochondrion • The 3-P-glycerate can re-enter the Calvin cycle, but at the cost of some carbon and O2 • This lossy pathway is known as photorespiration Photosynthesis

  37. Transketolase • As we saw in the PPP, fructose-6-P can react with glyceraldehyde-3-P in a transketolase reaction to form xylulose-5-phosphate and erythrose-4-phosphate • K6 + A3  A4 + K5 • Typical TPP binding structure Maize transketolaseEC 2.2.1.1675 kDa 9-mer; trimer shownPDB 1ITZ, 2.3Å Photosynthesis

  38. Fates of DHAP • Can participate in F-6-P production • Can condense with erythrose-4-P in an aldolase reaction to form sedoheptulose 1,7-bisphosphate (K3 + A4  K7) • This can be dephosphorylated at the 1-position to form sedoheptulose 7-P via sedoheptulose 1,7-bisphosphatase Photosynthesis

  39. The final Glyceraldehyde 3-P • It can condense with sedoheptulose 7-phosphate in another transketolase reaction to form xylulose-5-phosphate and ribose-5-phosphate:K7 + A3  A5 + K5 • The ribose-5-phosphate is an endpoint but it can also be isomerized to ribulose-5-phosphate • Xylulose-5-phosphate can be epimerized to form ribulose-5-phosphate too Photosynthesis

  40. Activation ofribulose-5-phosphate • Phosphoribulokinase uses ATP as a phosphate source to convert ribulose-5-phosphate to RuBP • Enzyme is similar to adenylate kinase PDB 1A7J32 kDa monomer Rhodobacter sphaeroides Photosynthesis

  41. What is unique here? • Not much • Last reaction is specificto Calvin cycle • Others are found in gluconeogenesis or pentose phosphate pathway or both • In this direction these reactions require the NADPH and ATP derived from the light reactions of photosynthesis Melvin Calvinphoto courtesyNobelprize.org Photosynthesis

  42. Bookkeeping for dark reactions • Numbers given in standard textbooks assume 3 input RuBP molecules per run of the cycle • This makes it easy to divide up the Glyceraldehyde 3-P later • Net reaction is:3 CO2 + 9ATP + 6 NADPH + 5 H2O  glyceraldehyde 3-P + 9ADP + 8 Pi + 6 NADP+ Photosynthesis

  43. Cost of making Acetyl CoA • We get back 2 NADH, 2 ATP when we convert glyceraldehyde 3-P to acetyl CoA • Therefore acetyl CoA costs 9-2 = 7 ATPand 6-2=4 NAD(P)H • At 2.5 ATP per NAD, that total is 7 + 2.5 * 4 = 17 ATP required per acetyl CoA • When we oxidize acetyl CoA we get 10 ATP (see TCA-cycle lecture), so we’re 10/17 = 59% efficient Photosynthesis

  44. Carbohydrate storage in plants • Glyc3P is converted to glucose-6-P or glucose by gluconeogenesis • Glycogen is storage polysaccharide in bacteria, algae, some plants • Other plants make starch (amylose or amylopectin) from glucose-6-P • Pathway begins with conversion of glucose-6-P to glucose-1-P, catalyzed by phosphoglucomutase Photosynthesis

  45. Starch synthesis • Glucose 1-P activated with ATP, not UTP • -D-glucose 1-P + ATP  ADP-glucose + PPi • Reaction driven to the right by hydrolysis of PPi • ADP glucose is added to growing starch molecule with release of ADP:ADP-glucose + (Starch)n ADP + (Starch)n+1 • Branching in amylopectin as in glycogen(Yao et al (2004) Plant Physiol.136:3515) Photosynthesis

  46. Diurnal variations in starch • Starch synthesis in daylight:ATP is readily available • Starch degradation at night Photosynthesis

  47. Starch phosphorylase Corynebacterium callunae Starch phosphorylase350 kDa tetramer PDB 2C4M, 1.9Å Starch phosphorylase cleaves starch to produce glucose-1-phosphate; glucose-1-P to triose phosphates by glycolysis Enzyme similar to glycogen phosphorylase PLP-dependent Photosynthesis

  48. Alternative path for night-time starch degradation • Starch to dextrins via amylase • Dextrins are oligosaccharidesbeginning with a -1,6 link • Dextrins eventuallydegraded to glucose • Glucose is phosphorylated by hexokinase • Enzyme:  sheet domain + TIM barrel Barley amylase45 kDa monomerEC 3.2.1.1PDB 1HT6, 1.5Å Photosynthesis

  49. Sucrose: mobile carbohydrate • Synthesized in chloroplast-containing cells; exported to vascular system so otherplant parts can use it • Two fructose 6-phosphatemolecules are starting points Photosynthesis

  50. Sucrose biosynthesis • One fructose-6-P is converted to Glucose-1-P (via glucose 6-P) and thence to UDP-glucose • That condenses with the other Fructose-6-P with the help of sucrose 6-P synthase to form sucrose 6-P • That gets dephosphorylated to make sucrose Photosynthesis

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