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The water oxidase (oxygen evolving centre, OEC), which catalyses the oxidation of water via
P 680, contains 5 metal centres at its active site. Four of these metal ions (3 Mn 3+/4+ and one
2+
2-
Ca ) form, together with 4 O , a cubane-like cluster (Fig. 16). In Fig. 16, the (assumed)
catalytic process is also shown. During turn-over, the four manganese centres change between
the oxidation states +IV to +III step by step in the 4-electron oxidation of 2 molecules of water.

P 680
Me O Me OH
-
hν e , H +
[P 680 ] + {FeN O}
4 Me Me 6-10 Me 6-10
O OH Me
Plastoquinone/-hydroquinone
Plastochinon/Hydrochinon
- H +

[S(Met)] (Cys)S S N(His)
Fe Fe
S(Met) N N (Cys)S S N(His)
Fe
- (His)N Cu N N (Fe 3+ Fe 2.5+ )
e S(Cys) Rieske-Protein
(His)N N
[P
700 ] + (Cu Cu ) (Fe 3+ Fe 2+ )
1+
2+

Plastocyanin Cytochrome b, c

Figure 14. Simplified representation of the electron transport chain between PSII and PSI.

Dimerisation via
Dimerisierung über R =CH 3:Chl. a

H-bridges in the
H-Brücken in den =CHO: Chl. b

reaction centres of
Reaktionszentren der

PSI and PSII
Photosysteme O

Saturated in
H H R gesättigt im
bacterial chlorophyll
H 2 C = C H O Bakteriochlorophyll
H 3 C C H 2 CH 3 Cytochromes b, c
N N

M g Dimerisation via
Dimerisierung über
H 3 C N N C H 3 H-bridges in the
H-Brücken in den
reaction centres of
Reaktionszentren der
H 2 C PSI and PSII
Photosysteme
H 2 C O Mg
O = C N C = O H O
N H O C H 3
O
( H i s) H Figure 15: Chlorophylls
H 2 9 C 2 0