Photosynthesis, Anabolism I Carbohydrates

Photosynthesis e– hole! ⊕ ⊝ e– gained! Photosystem II Evolves Oxygen from Water Photosynthesis Mn4CaO5 Goes through a neutral free radical: Y-O• + H• The water-splitting complex offers single electrons to photosystem II to fill the hole after charge separation. After four electrons are freed from two waters, a single O2 is released. Overall reaction: Oxygen-evolving Complex 2H2O ⇌ O2 + 4e– + 4H+ 1 Photosynthesis Photosystem II Transfers Electrons to Plastoquinones [NOTE on what side of the thylakoid membrane is the water splitting machine…… e– hole! 2H2O ⇌ O2 + 4e– + 4H+ ⊕ Recall that the DEo is enough to provide an electron hole deep enough to dissociate water (E’o +1.45V), but not a reductant with the ability to reduce NADP+ 2 PQH2 4H+ ⊝ e– gained! (PQ, like CoQ, likes e– more than NADH). Plastoquinones are structurally and functionally similar to ubiquinones (CoQ) found in the mitochondria. These lipid-soluble small molecules transport two electrons to the cytochrome b6f complex. Photosynthesis x2 • Structurally and functionally similar to Complex III in the electron-transport chain • Protons are removed from plastoquinones (PQH2) as plastoquinones are oxidized (PQ). • The PQ cycle is similar to the Q cycle in the ETC. • Electrons are funneled to plastocyanin, a single electron carrier, which carries the electron to PS-I. Cytochrome b6f Complex Translocates Protons into the Lumen 2 Photosynthesis Cu2+ ⇌ Cu1+ Plastocyanin (E’o is +0.37) What do we do with all the reduced Plastocyanin? Cytochrome b6f Complex Translocates Protons into the Lumen Photosynthesis e– hole! ⊕ ⊝ e– gained! A1 (QK) A1 (QK) PSII PSI Photosystem I creates the strongest biological reductant known: A0– Photosystem I (P700) (E’o is ~–1.4) 3 Photosynthesis A0– e– hole! ⊕ ⊝ Plastocyanin (E’o is +0.37) e– gained! Shuttling the electron quickly out: A1 (QK) A1 (QK) PSII PSI Photosystem I creates the strongest biological reductant known: A0– Ferredoxin (E’o is –0.4) Photosystem I (P700) (E’o is ~–1.4) Photosynthesis NADPH is produced by the NADPH Reductase in the Stroma Photosystem I created reduced ferrodoxin (Fd) (E’o is –0.4) Fd is a 1-e– carrier, so 2 Fd transfer their electrons, one at a time, to the FAD cofactor in the reductase. The fully reduced FADH2 then transfers 2e– to NADP+ to make each NADPH. 4 –1.7 PS-II –0.32 V +P 700 DEo for Mito (–1.14V) (+0.4) PS-I ⟿ (+1.45) hn ⊝ (–0.4) +0.82 V Need >1.8 V > ⟿ > > > > > > > > > > hn +P 680 ⧛ ⧛ ⧛ ⧛ ⧛ ⧛ (+0.37) > > > > > > > > > > 2x PC 2x Fd >> >> >> > ⟿ ⟿ ⟿ PQ (+0.08) Cytochrome b6f Complex ⧛ ⧛ ⧛ ⧛ ⧛ ⧛ ⧛ NADP+ NADPH QA (–0.05) A0 (–1.4) ⟿ NADH Reductase > > > > > > > > > > > > > > Photosynthesis ⊕ hn 2NADP+ + 2H2O ⇌ O2 + 2NADPH + 4H+ +1.6 Photosynthesis In Plants, the Two Reaction Centers Work Together [Not scaled right] Cyclic Photosynthesis Can Be Utilized to Generate Excess ATP This explains the energetics, but what about the stoichiometry? 5 Photosynthesis 4 photons 1 O2 pHin ≈ 4-5 4 e– + 4 H+ in 4 photons 8 H+ 4 4 e– 4 H+ pHout ≈ 8 ⊕⊕⊕⊕ D! = –0.05V 2 PQH2 ⊝⊝⊝⊝ 4 e– out 4 Organization of 2 NADPH Photosynthetic Machinery [H ] in the Thylakoid Membrane DG = RTln ____ + zF D! + ’ out [H ]in + H+in ⇌ H+out = RT2.3(pHin – pHout) + (+1)(96480)(–0.05) = 5.9(–3) – 4.8 kJ/mol = –17.7 – 4.8 kJ/mol = –22.5 kJ/mol per proton per 12 H+ = 12 x –22.5 kJ/mol = –270 kJ/mol Enough for ~8 ATP Photosynthesis pHin ≈ 4-5 Thylakoid Lumen pHout ≈ 8 Stroma DG’ = RTln [H+]out ____ [H+]in + zF D! = –17.7 kJ/mol – 4.8 kJ/mol pH gradient Membrane potential 6 Photosynthesis Net equation: 2 H2O + 2 NADP+ + 8 photons → O2 + 2 NADPH + 2 H+ CO2 + H2O ⇌ CH2O + O2 DE’o = 1.8 V 4 photons 4 photons 2 NADP+ 2 4 4 NADPH +2H+ 2 2 DE’o = 1.14 V 2 4 8 4 Light-Induced Redox Reactions Cause Acidification of Lumen DE’o = 1.5 V ~3 Photosynthesis Light Energy is Converted to ATP • In contrast to oxidative phosphorylation, protons are pumped into the lumen, building up a large concentration gradient quickly in the small space. • The proton-motive force across the thylakoid membrane drives the synthesis of ATP. • The number of c subunits in F0-rings is larger in chloroplasts (~12-14), so synthesis of ATP costs more H+ per ATP (~4-5) than in oxidative phosphorylation. • For the same light, the ratio of ATPto-NADPH synthesized is 3:2 7 Photosynthesis Summary of light reactions: • Photosynthetic organisms capture energy from light (l=350750 nm) and funnel it via excitons using accessory pigments to reaction centers: P680 & P700, where chemistry happens. • In plants, electrons are freed from H2O, which generates O2 and H+. The electrons popping out of the reaction centers reduce PQ, which moves them through the photosynthetic electron-transport chain (cytb6f), ending up in plastocyanin. • Per oxygen (O2) produced from 4 photons in PS-II, 12 H+ are mobilized into the lumen from the 2H2O and cytb6f (Q-cycle). This produces a proton-motive force sufficient for 3 ATPs • The final electron acceptor for linear photosynthesis is NADP+, which requires two electrons for conversion to NADPH. Per 8 photons (4 each to PS-I & PS-II), 2 NADPH are produced. • When ATP stores are low, the electrons from Fd can be transported to cytochrome b6f and cycled through the second half of photosynthesis. This cyclic photosynthesis resulting in an increased proton gradient and more ATP. ANABOLISM I Carbohydrates Photosynthesis and Carbohydrate Synthesis in Plants 8 Photosynthesis Assimilation of CO2 by Plants • Taking CO2 into biological, more reduced, carbon intermediates is CO2 assimilation. • Question was: What is the first compound made by plants when they assimilated CO2? Calvin and Benson used the 14 C radioisotope to determine the sequence of reactions in CO2 fixation. They exposed Chlorella to 14 CO2, then extracted the organic compounds and separated them by paper chromatography. 3 min 30 min Photosynthesis • Plant cells: can also make 3-C intermediates for further synthesis – made from CO2, H2O, plus ATP and NADPH from photosynthesis • It 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. • It produces 3-phosphoglycerate, which is rapidly converted to glyceraldehyde 3-phosphate (GA3P) & dihydroxyacetone phosphate (DHAP), which are important intermediates for all other compounds. • The net result is the reduction of CO2 with the NADPH that was generated in the light reactions of photosynthesis. 9 Photosynthesis The Three Stages of the Calvin Cycle of CO2 Assimilation 10 enzymes (6) (6) (6) Rubisco (10) (6) (2) (12) (12) (12) (12) (12) chloroplast isoforms (12) – 3-phosphoglycerate kinase (3PGK) (12) – glyceraldehyde 3-phosphate dehydrogenase (GAPDH) Overall: 6 CO2 + 12 NADPH + 10 H2O + 18 ATP → 2 glyceraldehyde 3phosphate (GA3P) + 4 H+ + 12 NADP+ + 18 ADP + 16 Pi *Ribulose 1,5-bisphosphate carboxylase/oxygenase (also RuBisCo) Photosynthesis 10 Photosynthesis First Stage of Calvin Cycle CO2 Fixation Is Catalyzed by Rubisco • Most plentiful, most important, enzyme on Earth (4 tons/yr) – Low turnover # of kcat = 3 s–1 at 25 °C – means a LOT of the enzyme needed! – 50% of plant enzymes are rubisco. • Large (560 KDa; a8b8)(L=56 kDa, S=14 kDa); Mg2+– dependent enzyme; activated by CO2 carbamylation • pH dependence is sharp, optimal activity at pH=8 • Catalyzes the reaction: – ribulose 1,5-bisphosphate + CO2 à 2 3-phosphoglycerate – DG°’ = –35.1 kcal/mol Photosynthesis First Stage of Calvin Cycle b(S) a(L) b(S) a(L) Structure of Rubisco (Form II) There are two distinct forms of rubisco. Form I is found in vascular plants, algae, and cyanobacteria; Form II is confined to certain photosynthetic bacteria. Structure of Rubisco (Form I) 11