Transcript C454_lect6 - University of Wisconsin

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