Biomedical Engineering Reference
In-Depth Information
3
In Between Photosynthesis
and Photoinhibition: The Fundamental Role
of Carotenoids and Carotenoid-Binding
Proteins in Photoprotection
G. Bonente, L. Dall'Osto, and R. Bassi
3.1 When Light Becomes Dangerous
for a Photosynthetic Organism
During operation of oxygenic photosynthesis, highly reactive molecules such
as excited chlorophylls are placed in an environment, the chloroplast. This is
probably the very spot where oxygen concentration is the highest on earth.
There are two major sources of reactive oxygen species (ROS) in the pho-
tosynthetic apparatus. The first source is electron transport, where electrons
are extracted from water and transported to
NADP
+
. The powerhouses of
photosynthetic electron transport are the PSI and PSII reaction centers, which
transfer one electron at time. Now, relatively stable oxygen forms are, respec-
tively, the most oxidized one (
O
2
) and the most reduced one (
H
2
O
). All the
other intermediate redox states are highly reactive and are, in fact, called
ROS. It can easily be understood that during a multistep electron transport
chain, the occasions in which
O
2
can be exposed to reduction by a single
electron thus yielding superoxide are easily produced. Moreover, in PSII, the
site where electrons are extracted one by one from water, intermediate redox
states may be produced.
The second major source of ROS is the process of light energy absorp-
tion by chlorophylls and transfer of the excitation energy between the many
chlorophylls forming the antenna system. Plants are particularly prone to
photo-oxidative damage, and for the same reasons they are effective at photo-
synthesis, namely because the primary pigment, chlorophyll (Chl) is a very
e
cient sensitizer. In fact, chlorophyll has a long living singlet state (5 ns) thus
allowing for intersystem crossing and formation of triplet chlorophyll-excited
states, which can react with
O
2
, a triplet in its ground state, to yield
1
O
2
.
This reactive species as well as others deriving from its reaction with water
and organic molecules cause oxidative damage to occur in proteins, lipid,
and pigments, leading to photoinhibition of photosynthesis and, ultimately,
to photobleaching of pigments.
