BioPHYSICAL chemistry

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, photo-
system II was shown to require the presence not only
of manganese but also calcium.
Recently, the three dimensional structure of photo-
system II has been reported (Figure 20.12). Whereas
the structure reveals the overall organization of the

CHAPTER 20 PHOTOSYNTHESIS 431

S 4

2H 2 O

H

350 μs

100 μs

30 μs

YZ

P680

P680

P680

P680

P680

Light excitation

Light excitation

Light excitation

Light excitation

P680

P680

P680

YZ

YZ

YZ

YZ

YZ

YZ

YZ

H

H

H

O 2

S 3

S 2 S^1

S 0

1 ms

4 8 12 16 20 24

Normalized oxygen evolution

Flash number

Figure 20.11Flash photolysis revealed oscillations in oxygen production that gave rise to the
concept of S states of photosystem II. Modified from Britt (1996).