Biology Reference
In-Depth Information
Plants are living organisms that survive
the interaction with light. Therefore, in
plant tissues, photosensitization reactions
and singlet oxygen generation always com-
pete with normal electron transfer reactions
of the energy conversion process in photo-
synthesis. Formation of
1
O
2
in photosystem
II (PSII) of plants was invariably confirmed
by the detection of its characteristic NIR
emission at 1270 nm (Vass
et al
., 1992;
Telfer
et al
., 1994). The generation mecha-
nism involves the reduction of quinone
acceptors and back-electron transfer
between reduced pheophytin and oxidized
P680, leading to the formation of triplet spe-
cies (Durrant
et al
., 1990). However, the
details of
1
O
2
generation are still a matter of
debate and triplets derived from other pho-
tosynthetic reaction centre (RC) pigments
have also been detected (Rinalducci
et al
.,
2004). The formation of
1
O
2
in antenna-
complex trimer proteins has been suggested
to be the result of direct generation of
1
O
2
by
oxygen quenching of triplet chlorophyll
species formed in antenna complexes after
light absorption and intersystem crossing
(Krieger-Liszkay, 2005; Uchoa
et al
., 2008;
Triantaphylides and Havaux, 2009). In the
case of
Rhodobacter sphaeroides
RCs,
Uchoa and coworkers have shown that bac-
teriopheophytin triplets are another possi-
ble source of
1
O
2
(Uchoa
et al
., 2008).
It is becoming clear that understanding
and controlling singlet oxygen generation in
plants may be a key factor for improving
crop yield, because overproduction of
1
O
2
can lead to photo-inhibition of photosyn-
thesis and photo-destruction of the photo-
synthetic RC. Plants have developed
macromolecular supra-structures and a
myriad of antioxidant molecules to decrease
the rate of formation of singlet oxygen by
suppressing triplets and also to directly
suppress singlet oxygen molecules that may
beformed(Uchoa
etal
.,2008;Triantaphylides
and Havaux, 2009). The quantum yield of
singlet oxygen (f
D
, number of times that sin-
glet oxygen molecules are generated per pho-
ton absorbed), from the RCs of
R. sphaeroides
is 0.03 (Uchoa
et al
., 2008), which is consid-
erably smaller than f
D
calculated for PSII
of plants, which was calculated to be 0.2
(Telfer
et al
., 1994). This fact is in agreement
with the smaller tendency of photo-inhibition
in wild-type
R. sphaeroides
compared with
plants and also with carotenoidless strains
of purple bacteria (Uchoa
et al
., 2008).
Carotenoids are especially efficient sup-
pressors of PS triplets and of
1
O
2
. The main
role of carotenoids in photosynthesis is to
quench triplet states that are eventually
formed in the RCs before they photosensitize
1
O
2
formation. Carotenoids may, however,
also suppress
1
O
2
molecules that are formed
in the RCs. Proof of this role for carotenoids
may be obtained by comparing the efficiency
of
1
O
2
generation in different strains of pur-
ple bacteria. In RCs of
R. sphaeroides
, carote-
noids are located within van der Waals
distance of bacteriochlorophylls (
~
3.7 Å) and
at 10 Å of a dimer pair of bacteriochloro-
phylls suppressing triplets and singlet oxy-
gen that are formed.
Rhodopseudomonas
viridis
is a strain of purple bacteria that lacks
carotenoids. Consequently, one could expect
a higher efficiency of singlet oxygen genera-
tion. In fact, Uchoa and coworkers have
measured that the value of singlet oxygen
production F
D
in
R. sphaeroides
(F
D
= 0.03)
is half of the value of F
D
in
R. viridis
(F
D
= 0.06; Uchoa
et al
., 2008).
Not only plants but also humans are
affected by photoinduced
1
O
2
generation
(Lu
et al
., 2000). Skin, hair and eyes are the
most exposed areas and therefore are the tis-
sues most prone to have photodamage
(Fattorusso, 1974; Krishna
et al
., 1991;
Halliwell
et al
., 1992; Chiarelli-Neto
et al
.,
2011). Riboflavin derivatives are widely
spread in living organisms, absorb light in
the UVA spectral region (Speck
et al
., 1975;
Lu
et al
., 2000) and are known to efficiently
produce
1
O
2
(F
D
= 0.5) (Wilkinson
et al
.,
1993; Morita
et al
., 1997; Baier
et al
., 2006).
In fact, flavin co-enzymes FAD, FADH and
FMN, which are of vital importance in cel-
lular metabolism, are considered responsi-
ble for a series of endogenous photodamage
in the skin, which is started by UVA absorp-
tion and generation of
1
O
2
(Berneburg
et al
.,
1999; Kessel, 2000). It has been shown
recently that melanin itself can generate
1
O
2
under visible light exposure, showing the
importance of understanding in more detail
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