Lecture 6 - Photosynthesis: the Light Reactions
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Lecture 6 - Photosynthesis: The Light Reactions Chem 454: Regulatory Mechanisms in Biochemistry University of Wisconsin-Eau Claire 1 Introduction Photosynthesis represents for the biosphere Text the major source of Free energy (1017 kcal/year stored) Carbon (1010 tons/year assimilated) Oxygen CO2 + H2O Light (CH2O) + O2 carbohydrate 2 2 Overview Analogous to the combination of oxidative phosphorylation and the citric acid cycle. 3 3 1. Chloroplasts Like oxidative phosphorylation, Text photosynthesis is compartmentalized 4 4 1 Mitochondria Mitochondria are bound by a double membrane 5 5 2. Electron Transfer Light absorption by Text chlorophyll induces electron transfer 6 6 2. Electron Transfer Absorption of light leads to photoinduced Text charge separation 7 7 2. Electron Transfer Absorption of light leads to photoinduced Text charge separation 8 8 2.1 Bacteria vs. Green Plants Plants have two photosystems Photosystem I 13 polypeptides chains 60 Chlorophylls 3 4Fe-4S centers 1 Quinone Photosystem II 10 polypeptide chains 30 Chlorophylls 1 Non-heme Fe 4 Mn+2 (Mn+2, Mn+3, Mn+4 , Mn+5) 9 9 2.1 Bacteria vs. Green Plants Bacteria have a single system 4 Bacteriochlorophylls b 2 Bacteriopheophytin b 2 Quinones 1 Fe2+ 10 10 2.1 Bacteria vs. Green Plants Bacteria have a single system 11 11 2.2 The Bacterial Photosynthetic Reaction Center 12 12 2.3 Electron Transfer The rate of electron transfer is dependent up two factors: Distance Driving Force 13 13 3.3 Q-Cytochrome c Oxidoreductase The Q cycle: QH + 2 Cyt c + + + 2 ox 2 H matrix Q + 2 Cyt cred + 4 H cytosol 14 14 3. Two Photosystems of Plants In green plants there are two photosynthetic Text reaction centers. 15 15 3.1 Photosystem II The core of PSII is similar to the bacterial system. 16 16 3.1 Photosystem II 2 Q + 2 H2O O2 + 2 QH2 17 17 3.1 Photosystem II Four photons are required to generate one oxygen molecule 18 18 3.1 Photosystem II The 4 Manganese center is where the electrons are extracted from the water. 19 19 3.1 Photosystem II An equivalent of 4 H+ are moved across the thylakoid membrane by PSII 20 20 3.2 Cytochrome bf The cytochrome bf complex transfers the electrons from plastoquinone to platocyanin: 21 21 3.2 Cytochrome bf 2+ + + 2 QH2 + 2 Pc(Cu ) Q + 2 Pc(Cu ) + 2 H thylakoid lumen 22 22 3.3 Q-Cytochrome c Oxidoreductase The Q cycle: QH + 2 Cyt c + + + 2 ox 2 H matrix Q + 2 Cyt cred + 4 H cytosol 23 23 3.3 Photosystem I Though larger, the core of PSI is also similar to the bacterial system. 24 24 3.3 Photosystem I The receptor of the the electrons from PSI is the small one-electron redox protein, Ferredoxin. 25 25 3.3 Photosystem I + 2+ Pc(Cu ) + Fdox Pc(Cu ) + Fdred 26 26 3.3 Photosystem I The Z-scheme of photosynthesis 27 27 3.4 Ferredoxin-NADP+ Reductase Ferredoxin-NADP+ reductase 28 28 3.4 Ferredoxin-NADP+ Reductase Ferredoxin-NADP+ reductase contains the coenzyme FAD. 29 29 3.4 Ferredoxin-NADP+ Reductase Ferredoxin-NADP+ reductase contains the coenzyme FAD. 30 30 4. The Proton Gradient PSII, Cytochrome bf and ferredoxin-NADP+ Text reductase each contribute to the proton gradient. 31 31 4. The Proton Gradient André Jagendorf's Text demostration 1966, was one of the earliest pieces of evidence to support Peter Mitchell's chemiosmotic hypothesis. 32 32 4.1 ATP Synthase The ATP synthase closely resembles the ATP synthase of mitochondria 33 33 4.1 ATP Synthase Comparing photosynthesis to oxidative phosphorylation: 34 34 4.2 Flow of Electrons Through Photosystem I Normally photosynthesis produces both ATP and NADPH. When there is not NADP+ available, cyclic flow of electrons through PSI can be used to produce ATP without reducing NADP+ 35 35 4.2 Flow of Electrons Through Photosystem I Cyclic flow of electrons through PSI: 36 36 4.3 Stoichiometry of Photosynthesis + + 2 H2O + 2 NADP + 10 H stroma + O2 + 2 NADPH + 12 H lumen 12 protons are expected to be used for one turn of the ATP synthase, therefore, producing 3 AtP's from 3ADP's and 3Pi's 37 37.