Bioinorganic Chemistry
Photosynthesis
The electrons stored initially on the pheophytin are
passed on to quinones which are reduced to quinols at
the exterior of the thylakoid. Protons are extracted from
the stroma.
The quinols move freely in the membrane and are
oxidized on the inside of the thylakoid to quinonones
again. Protons are released into the lumen.
This process reminds us of the Q-cycle …
Bioinorganic Chemistry
Photosynthesis
… and there are more similarities to be found:
- Cytochrome b6f resembles Complex III. Electrons from plastoquinol are equally
distributed between a 2-heme array which carries them to the stroma side to
reduce plastoquinone, and a Fe2S2 cluster which transfers electrons via
cytochrome f to plastocyanin.
- Plastocyanin is functionally equivalent to cytochrome c and acts as a single
electron shuttle between cytochrome b6f and Photosystem I.
There are also major differences:
- Plastoquinol oxidation by cytochrome b6f at the inner side of the thylakoid differs
from ubiquinol oxidation by Complex III: it cannot be blocked by inactive quinol
analogues. The textbook concludes that the Q-cycle does probably not work
here, however, the plastoquinol is the only reducing output of Photosystem II
and must be re-oxidized somehow.
- Plastocyanin is a copper protein, contains no heme.
Bioinorganic Chemistry
Photosynthesis
Comparison of cytochrome b6f with Complex III
Bioinorganic Chemistry
Photosynthesis
Principal reaction step in water oxidation
R
H
3+
Mn
O
H
e3+
H
O
R
Mn
O
O
+
H
H
H
R
R
O
4+
Mn
O
-
H
H
O
3+
Mn
H
O
H
Bioinorganic Chemistry
Photosynthesis
Babcock‘s proposal
Bioinorganic Chemistry
Photosynthesis
(+ •)
(OH / O•)
Kok state model of water oxidation
Bioinorganic Chemistry
Photosynthesis
Thylakoid redox systems, the more detailed view
Bioinorganic Chemistry
Photosynthesis
Photosystem I
Photosensitive unit similar to photosystem II
- antennae complexes.
- special pair of chlorophylls a which is excited to radical ion pair.
- Absorption maximum at 700 nm.
Different: electron transport chain and potentials
- P700: E°‘=0.45 V (cation), -1.2 V (anion): strongest reducing agent in biology!
- Cation is reduced by plastocyanin, not water.
- Electron from anion is picked up by chlorophyll / Vitamin K, not pheophytin.
- Further electron transport not by quinones but by FeS-clusters.
Bioinorganic Chemistry
Photosynthesis
Output of Photosystem I
- Product is reduced ferredoxin, a very general electron carrier in chloroplasts
- Reduced ferrdoxin is used for two different purposes:
1. Reduction of ferredoxin-NADP reductase, the enzyme that makes
NADPH
2. Cyclic reduction of cytochrome b6f. This increases redox-cycle driven
proton pumping, therefore, ATP yield increases.
ATP and NADPH are utilized to drive the reverse (reductive) pentose phosphate
cycle, which synthesizes sugars from CO2 via glyceral-3-phosphate. This cycle
is also named Calvin cycle after ist discoverer.
Bioinorganic Chemistry
Photosynthesis
Potentials in photosynthesis: the Z scheme
Bioinorganic Chemistry
Photosynthesis
Structural puzzles in Photosystem I and II
The electron transport chains following the
excited special pair in both Photosystems are
doubled
- In Photosystem II, only one of the two chains is
in use, the other is silent.
- In Photosystem I, the question whether both
paths are active is not answered yet.