Bài giảng Biochemistry 2/e - Chapter 22: Photosynthesis

Chapter 22Photosynthesisto accompanyBiochemistry, 2/ebyReginald Garrett and Charles GrishamAll rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Outline22.2 The Photoreactivity of Chlorophyll22.4 The Z Scheme of Photosynthesis22.7 Light-Driven ATP Synthesis - Photophosphorylation 22.8 Carbon Dioxide Fixation 22.9 The Calvin-Benson Cycle 22.10 Regulation of Carbon Dioxide Fixation 22.12 The C-4 Pathway of CO2 Fixation The Sun - Ultimate Energy1.5 x 1022 kJ falls on the earth each day1% is absorbed by photosynthetic organisms and transformed into chemical energy6CO2 + 6H2O  C6H12O6 + 6O2 1011 tons (!) of CO2 are fixed globally per year Formation of sugar from CO2 and water requires energy Sunlight is the energy source! PhotosynthesisGeneral Aspects Photosynthesis occurs in thylakoid membranes of chloroplasts - structures involving paired folds (lamellae) that stack to form "grana" The soluble portion of the chloroplast is the "stroma" The interior of the thylakoid vesicles is the "thylakoid space" or "thylakoid lumen" Chloroplasts possess DNA, RNA and ribosomes Photosynthesis Consists of Both Light Reactions and Dark ReactionsThe light reactions capture light energy and convert it to chemical energy in the form of reducing potential (NADPH) and ATP with evolution of oxygen The dark reactions use NADPH and ATP to drive the endergonic process of hexose sugar formation from CO2 in a series of reactions in the stroma Water is the electron donorfor Photosynthetic NADP+ Reduction Equations 22.2 and 22.3 describe the light and dark reactions in green plants, respectively! Equation 22.4 provides a more general version Photosynthetic bacteria use H2S, isopropanol or other oxidizable substrates The O2 we depend upon depends in turn on large amounts of photosynthesis on the earth!ChlorophyllPhotoreactive, isoprene-based pigment A planar, conjugated ring system - similar to porphyrins Mg in place of iron in the center Long chain phytol group confers membrane solubility Aromaticity makes chlorophyll an efficient absorber of light The Photosynthetic UnitMany chlorophylls but only a single reaction center The "unit" consists of several hundred light-capturing chlorophylls plus a pair of special chlorophylls in the "reaction center" Light is captured by one of the "antenna chlorophylls" and routed from one to the other until it reaches the reaction center See Figure 22.9 Eukaryotic PhotosystemsPSI (P700) and PSII (P680) All chlorophyll is part of either LHC, PSI or PSII PSI absorbs at 700 nm PSII absorbs at 680 nm Chloroplasts given light at 680 and 700 nm simultaneously yield more O2 than the sum of amounts when each is used alone.What does each photosystem do?See Figure 22.11 PSII oxidizes water (termed “photolysis") PSI reduces NADP+ ATP is generated by establishment of a proton gradient as electrons flow from PSII to PSI The Z SchemeAn arrangement of the electron carriers as a chain according to their standard reduction potentials PQ = plastoquinone PC = plastocyanin "F"s = ferredoxins Ao = a special chlorophyll a A1 = a special PSI quinone Cytochrome b6/cytochrome f complex is a proton pumpOxygen evolution by PSIIrequires accumulation of four oxidizing equivalents PSII (P680) cycles through five oxidation states 1 e- is removed in each of four steps Fifth step involves H2O oxidized to O2 + 4H+ Structures of Reaction CentersR. viridis is a model! Membrane proteins (as always) are resistant to crystallization (and X-ray diffraction studies) Deisenhofer, Michel and Huber solved R.viridis structure in 1984 (Nobel Prize same year!) Four peptides: L, M, H and cytochrome No electron transfer appears to occur through M See Figures 22.16, 22.18 The Quantum YieldAmount of O2 evolved per photon Four photons per reaction center - 8 total - drive the evolution of 1 O2, reduction of 2 NADP+, and the phosphorylation of 2 and 2/3 ATP PhotophosphorylationLight-Driven ATP Synthesis Electron transfer through the proteins of the Z scheme drives the generation of a proton gradient across the thylakoid membrane Protons pumped into the lumen of the thylakoids flow back out, driving the synthesis of ATP CF1-CFo ATP synthase is similar to the mitochondrial ATP synthase Cyclic PhotophosphorylationATP without NADPH! The photo-excited electron removed from P700 returns to P700 in a pathway indicated by the dashed line in Figure 22.12 Cyclic photophosphorylation depends only on PSI, not on PSII Carbon Dioxide FixationA unique ability of plants, algae, etc. Melvin Calvin at Berkeley in 1945 showed that Chlorella could take up 14CO2 and produce 3-phosphoglycerate What was actually happening was that CO2 was combining with a 5-C sugar to form a 6-C intermediate This breaks down to two 3-P glycerates Ribulose-1,5-BisphosphateThe CO2 Acceptor Fixation is accomplished by ribulose bisphosphate carboxylase (oxygenase), aka rubiscoProbably the world's most abundant protein Study the mechanism in Figure 22.24 Rubisco is activated when carbamylated (CO2 added to Lys-201) and with Mg boundRuBP (substrate!) is inhibitor and must be released from inactive rubisco by rubisco activase. Carbamylation and Mg then activate. The Calvin-Benson Cycleaka The Calvin Cycle The set of reactions that transform 3-P- glycerate into hexose sugar The only net CO2 fixation pathway in nature A disguised gluconeogenesis pathway! With some pentose phosphate pathway reactions thrown in.... See Figure 22.25 Regulation of CO2 FixationActivities of Calvin cycle enzymes (in the stroma!) are coordinated with photosynthesis Three effects: Light-induced pH changes Light-induced generation of reducing power (reduced ferredoxin and NADPH)Light-induced Mg2+ Efflux from Thylakoids The C-4 Pathway for CO2 Fixationaka the Hatch-Slack Pathway Not an alternative to Calvin cycle, nor even a net CO2 fixation pathway Rather, it is a CO2 delivery system, which carries CO2 from the O2-rich leaf surface to interior cells where O2 won't compete in the rubisco reaction Oxaloacetate and malate are the CO2 transporters Read about Crassulacean acid metabolism