Biomedical Engineering Reference
In-Depth Information
Although some kind of photoinhibition has been shown to take place also at
the level of PSI [28], the main phenomenon regards PSII, where an inactivation
mechanism inherent to the same electron transfer activity takes place, with low
quantum yield, even at low light irradiance [29]. At increasing light intensities,
inactivation of PSII occurs with higher frequency and in extreme cases may
bring to irreversible oxidative damage of the complex.
In the last decades, considerable efforts were directed to the elucidation
of the molecular mechanisms of this important phenomenon. It is long known
that the D1 protein (PsbA) of the PSII reaction center is rapidly turned
over and that its turnover rate increases with the light intensity [30]. Why
is that so? It has been hypothesized that D1 is the protein subunit, which is
preferentially damaged when the reaction center is over excited, and it needs
to be frequently substituted to maintain the reaction center active. In fact, the
water splitting photochemistry of PSII produces various radicals and active
oxygen species, which cause irreversible damage to PSII. However, damaged
PSII reaction centers do not usually accumulate in the thylakoid membrane
because of a rapid and e
cient repair mechanism. It has been calculated that
this repair mechanism is at least as important as that of DNA, so that the
green kingdom in the biosphere could not survive in its absence.
It is commonly believed that the design of PSII allows protection for
most of its protein and pigment components, with the oxidative damage be-
ing mainly targeted to a single subunit, the reaction center D1 protein. Re-
pair of PSII via turnover of the D1 protein is a complex process (Fig. 1.5)
STACKED THYLAKOID MEMBRANES
INACTIVATION
DAMAGE
LIGHT
P
P
PP
P
PPP
D1
LHC
LHC
LHC
LHC
D1
LHC
D2
D2
D2
D1
LHC
D1
D2
D2
D1
b559
b559
b559
b559
b559
47
43
47
43
1.
43
47
47
2.
43
43
47
33
2
16
33
23
16
16
33
16
33
23
16
33
23
23
PHOSPHORYLATION
MONOMERIZATION
MIGRATION
DIMERIZATION
MIGRATION
7.
psbA
mRNA
3.
D1
P
P
P
D2
D1
D2
DEPHOSPHORYLATION
OF CP43. D2 AND D1
D2
D1
b559
b559
b559
43
D1 PROCESSING
D1 PROTEOLYSIS
47
47
43
47
43
4.
D1 FRAGMENTS
33
16
6.
33
23
16
5.
33
16
23
SYNTHESIS AND INSERTION OF D1
LIGATION OF COFACTORS
23
STROMA-EXPOSED THYLAKOID MEMBRANES
Fig. 1.5.
Repair cycle of photosystem II
