Chemistry Reference
In-Depth Information
which we now know as photosystem II, used solar energy to carry out the thermodynamically demanding and
chemically challenging reaction of water splitting:
4h
V
4e
þ
4H
þ
2H
2
O
!
O
2
þ
This provided an unlimited supply of reducing equivalents to convert CO
2
to carbohydrates, and subsequently
to the other organic molecules of life:
4e
þ
4H
þ
þ
CO
2
/
ð
CH
2
O
Þþ
H
2
O
Prior to this bonanza of hydrogen/electron donors, biology had been restricted to H
2
S, NH
3
, some organic
acids, Fe
2
þ
, and the like, which were in short supply when compared with the unlimited oceans of water on the
surface of our green planet. However, there were two other consequences of the arrival of oxygenic photoauto-
trophs.
2
Firstly, the by-product of the water-splitting reaction, molecular oxygen, transformed our planet from
being anaerobic to aerobic, which increased the efficiency of cellular energy production by around 20-fold, and
most likely drove the subsequent evolution of eukaryotes and multicellular organisms. The second consequence,
which took a bit longer, was the formation of the ozone layer. This provided a shield against harmful UV radiation,
allowing the exploitation of new habitats, including, most importantly, the terrestrial environment.
The combination of X-ray crystallography and a wide range of biochemical, biophysical, and molecular
biological techniques has provided very exciting new results on the molecular properties of PSII. Indeed, as we
will see, we are now very close to understanding the precise chemical mechanism of the water-splitting reaction.
For more information see
Barber, 2008; Barber and Murray, 2008; Ferreira et al., 2004; Goussias et al., 2002;
Photosystem II (PSII), also known as the water-plastoquinone photo-oxidoreductase, is a multienzyme
complex, embedded in the thylakoid membrane of plants, algae, and cyanobacteria, which uses solar energy to
power the oxidation of water to dioxygen by a unique tetra-manganese oxygen-evolving cluster (OEC):
h
V
2Q
þ
2H
2
O
!
O
2
þ
2QH
2
We begin by considering how the light-harvesting system of PSII, which varies widely both between organisms
and as a function of growth conditions, absorbs the solar energy and transfers it to the reaction centre (RC). It became
clear from early structural studies that the PSII core complex of plants and cyanobacteria is dimeric and contains as
light-absorbing pigments only chlorophyll a and
-carotene molecules bound mainly to the proteins CP43 and
CP47. The RC is composed of the proteins D1 and D2, together with all of the redox active cofactors involved in the
energy conversion process. The antenna chlorophylls of the CP47 and CP43 subunits collect the energy of photons
and transfer this to the special pair of chlorophylls, P
D1
and P
D2
(
Figure 16.1
)
in the reaction centre. The excitation of
P(
Barber, 2008
) converts it to a strong reducing agent (P*). P* reduces a nearby pheophytin molecule (Pheo) within
a few picoseconds, forming the radical pair state P
b
$
þ
Pheo
$
. Within a few hundred picoseconds, Pheo
$
reduces
$
þ
PheoQ
A
.P
$
þ
is a powerful oxidant (redox potential
a firmly bound plastoquinone molecule (Q
A
) to produce P
PPheoQ
A
on a nanosecond time scale. Oxidation of
TyrZ is dependent on deprotonation of its phenolic OH group to generate a neutral radical (TyrZ
1 V), and it oxidises a tyrosine residue (TyrZ) to form TyrZ
$
>
$
). In a millisecond
time scale, Q
A
reduces a second plastoquinone (Q
B
) to form TyrZ
PPheoQ
A
Q
B
. At about the same time, the TyrZ
$
extracts an electron from the OEC (Mn
4
Ca) cluster, to which two substrate water molecules are bound. A second
photochemical turnover reduces Q
B
to Q
2
B
. This is protonated to give plastoquinol, QH
2
, which is released into the
lipid bilayer to be re-oxidised by photosystem I. Two further photochemical turnovers provide the four oxidising
equivalents required to oxidise two water molecules and thereby generate dioxygen.
$
2. O
2
producing photosynthetic organisms which require only simple inorganic substances to fulfill their nutritional requirements and CO
2
as
sole carbon source.
