Post on 11-Mar-2020
Chem 454: Regulatory Mechanisms in BiochemistryUniversity of Wisconsin-Eau Claire
Lecture 6 - Photosynthesis:The Light Reactions
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TextPhotosynthesis represents for the biosphere the major source of
Free energy (1017 kcal/year stored)
Carbon (1010 tons/year assimilated)
Oxygen
Introduction
CO2 + H2O (CH2O) + O2Light
carbohydrate
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Analogous to the combination of oxidative phosphorylation and the citric acid cycle.
Overview
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TextLike oxidative phosphorylation, photosynthesis is compartmentalized
1. Chloroplasts
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Mitochondria are bound by a double membrane
1 Mitochondria
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TextLight absorption by chlorophyll induces electron transfer
2. Electron Transfer
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TextAbsorption of light leads to photoinduced charge separation
2. Electron Transfer
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TextAbsorption of light leads to photoinduced charge separation
2. Electron Transfer
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Plants have two photosystemsPhotosystem I13 polypeptides chains
60 Chlorophylls
3 4Fe-4S centers
1 Quinone
Photosystem II10 polypeptide chains
30 Chlorophylls
1 Non-heme Fe
4 Mn+2 (Mn+2, Mn+3, Mn+4 , Mn+5)
2.1 Bacteria vs. Green Plants
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Bacteria have a single system4 Bacteriochlorophylls b2 Bacteriopheophytin b2 Quinones1 Fe2+
2.1 Bacteria vs. Green Plants
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Bacteria have a single system
2.1 Bacteria vs. Green Plants
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2.2 The Bacterial Photosynthetic Reaction Center
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The rate of electron transfer is dependent up two factors:
2.3 Electron Transfer
Distance Driving Force
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3.3 Q-Cytochrome c Oxidoreductase
The Q cycle:
QH2 + 2 Cyt cox 2 H+matrix Q + 2 Cyt cred + 4 H+cytosol+
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TextIn green plants there are two photosynthetic reaction centers.
3. Two Photosystems of Plants
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The core of PSII is similar to the bacterial system.
3.1 Photosystem II
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3.1 Photosystem II
2 Q + 2 H2O O2 + 2 QH2
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3.1 Photosystem II
Four photons are required to generate one oxygen molecule
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3.1 Photosystem II
The 4 Manganese center is where the electrons are extracted from the water.
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3.1 Photosystem II
An equivalent of 4 H+ are moved across the thylakoid membrane by PSII
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The cytochrome bf complex transfers the electrons from plastoquinone to platocyanin:
3.2 Cytochrome bf
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3.2 Cytochrome bf
2 QH2 + 2 Pc(Cu2+) Q + 2 Pc(Cu+) 2 H+thylakoid lumen+
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3.3 Q-Cytochrome c Oxidoreductase
The Q cycle:
QH2 + 2 Cyt cox 2 H+matrix Q + 2 Cyt cred + 4 H+cytosol+
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Though larger, the core of PSI is also similar to the bacterial system.
3.3 Photosystem I
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3.3 Photosystem I
The receptor of the the electrons from PSI is the small one-electron redox protein, Ferredoxin.
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3.3 Photosystem I
Pc(Cu+) + Fdox Pc(Cu2+) + Fdred
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The Z-scheme of photosynthesis
3.3 Photosystem I
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Ferredoxin-NADP+ reductase
3.4 Ferredoxin-NADP+ Reductase
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Ferredoxin-NADP+ reductase contains the coenzyme FAD.
3.4 Ferredoxin-NADP+ Reductase
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Ferredoxin-NADP+ reductase contains the coenzyme FAD.
3.4 Ferredoxin-NADP+ Reductase
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TextPSII, Cytochrome bf and ferredoxin-NADP+ reductase each contribute to the proton gradient.
4. The Proton Gradient
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TextAndré Jagendorf's demostration
1966, was one of the earliest pieces of evidence to support Peter Mitchell's chemiosmotic hypothesis.
4. The Proton Gradient
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The ATP synthase closely resembles the ATP synthase of mitochondria
4.1 ATP Synthase
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Comparing photosynthesis to oxidative phosphorylation:
4.1 ATP Synthase
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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+
4.2 Flow of Electrons Through Photosystem I
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Cyclic flow of electrons through PSI:
4.2 Flow of Electrons Through Photosystem I
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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
4.3 Stoichiometry of Photosynthesis
2 H2O + 2 NADP+ + 10 H+stroma
O2 + 2 NADPH + 12 H+lumen