Dissertation

|1.5 Photosynthesis|
O 2
2 H 2 O
visible
light
OEC
4 x
oxidation center
P680 *
P680
Pheo
PQ
cyt f
P700
*
FeS
FD
FNR
4H + + 2NADP
reduction center
H + (stroma)
PC
H + (lumen) P700
e - e -
tyr
e -
photo system II
visible
light
e - e - e - e - e - e - e -
photo system I
2NADPH +2H +
Figure 10: Z-Scheme of photosynthesis, representing important processes of light absorption,
charge separation and electron transfer, involved in water splitting (see text for details). [19]
electrons from two water molecules (serving as sacrifical donor) and liberates molecular oxygen
and four protons to the lumen. Four single electron transfer steps via a specific tyrosine moiety
(tyr) replenish the photooxidized P680.
2 H 2 O
OEC
→
O 2 + 4 H + + 4 e −
P700 + becomes reoxidized to P700 by the reception of an electron from plastocyanine (PC,
a mobile copper-protein) which had previously been passed down through the redox gradient
originating in PS II. The reduced primary electron acceptor A − 0
passed the electron ultimately
to nicotinamide adenine dinucleotide phosphate (NADP + ). This happens via several other
intermediates, including redox shuttles like phylloquinone (vitamin K), several membrane bound
iron sulfur proteins (FeS) and ferredoxin (Fd), a slightly mobile FeS.
Eventually, the enzyme ferredoxin-NADP reductase (FNR), an oxido-reductase, gathers two of
such electrons and uses one proton for the final transfer step. The reduction of NADP + with
formation of NADPH represents a "fuel production" step in the plant. Importantly, the resulting
NADPH is a hydride carrier molecule which is one of the most widely used energy storage
molecules and reducing agents in cells, e.g. essential for the production of sugars from CO 2 in
the Calvin cycle.
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