The site of water oxidation is photosystem II, or the water-oxidation
complex. To oxidize water to molecular oxygen, the protein performs a
four-electron process that is coupled with the transfer of four protons:
2H 2 O → O 2 + 4H
+
+ 4e
−
(20.5)
A series of experiments was performed in the 1960s
by Pierre Joliot and later Bessel Kok using transient
optical spectroscopy. These results demonstrated clearly
that oxygen evolved only after the fourth light flash,
giving rise to the proposed mechanism of the S-state
model (Figure 20.11). For full oxygen evolution, photosystem II was shown to require the presence not only
of manganese but also calcium.
Recently, the three dimensional structure of photosystem II has been reported (Figure 20.12). Whereas
the structure reveals the overall organization of the
CHAPTER 20
PHOTOSYNTHESIS
431
S4
2H2O
H
ϩ
350 μs
100 μs
30 μs
Y Z и
ϩ
P680 и
ϩ
P680 и
ϩ
P680 и
ϩ
P680 и
ϩ
P680
Light excitation
Light excitation
Light excitation
Light excitation
P680
P680
P680
Y Z и
ϩ
YZ и
ϩ
YZ и
ϩ
YZ
YZ
YZ
YZ
H
ϩ
H
ϩ
H
ϩ
O2
S3
S2
S1
S0
1 ms
4
8
12
16
20
24
Normalized oxygen evolution
Flash number
Figure 20.11 Flash photolysis revealed oscillations in oxygen production that gave rise to the
concept of S states of photosystem II. Modified from Britt (1996).
9781405124362_4_020.qxd 4/29/08 14:08 Page 431
complex. To oxidize water to molecular oxygen, the protein performs a
four-electron process that is coupled with the transfer of four protons:
2H 2 O → O 2 + 4H
+
+ 4e
−
(20.5)
A series of experiments was performed in the 1960s
by Pierre Joliot and later Bessel Kok using transient
optical spectroscopy. These results demonstrated clearly
that oxygen evolved only after the fourth light flash,
giving rise to the proposed mechanism of the S-state
model (Figure 20.11). For full oxygen evolution, photosystem II was shown to require the presence not only
of manganese but also calcium.
Recently, the three dimensional structure of photosystem II has been reported (Figure 20.12). Whereas
the structure reveals the overall organization of the
CHAPTER 20
PHOTOSYNTHESIS
431
S4
2H2O
H
ϩ
350 μs
100 μs
30 μs
Y Z и
ϩ
P680 и
ϩ
P680 и
ϩ
P680 и
ϩ
P680 и
ϩ
P680
Light excitation
Light excitation
Light excitation
Light excitation
P680
P680
P680
Y Z и
ϩ
YZ и
ϩ
YZ и
ϩ
YZ
YZ
YZ
YZ
H
ϩ
H
ϩ
H
ϩ
O2
S3
S2
S1
S0
1 ms
4
8
12
16
20
24
Normalized oxygen evolution
Flash number
Figure 20.11 Flash photolysis revealed oscillations in oxygen production that gave rise to the
concept of S states of photosystem II. Modified from Britt (1996).
9781405124362_4_020.qxd 4/29/08 14:08 Page 431