Biomedical Engineering Reference
In-Depth Information
that involves reversible phosphorylation of PSII proteins and changes in the
oligomeric structure of the complex [31]. This is combined with the shuttling
of the complex between grana and stroma-exposed thylakoid domains, partial
PSII disassembly, and highly specific proteolysis of the damaged D1 protein.
Replacement of degraded D1 protein with a new copy requires a complex co-
ordination between its degradation, resynthesis, insertion, and assembly into
the PSII core. Although enhanced during higher irradiances, turnover of D1
occurs at all light intensities, and can be easily monitored in pulse-chase ex-
periments, an e
cient tool to study the phenomenon. It is now clear that most
of the regulation of gene expression required for PSII repair cycle is exerted at
the translational and posttranslational levels by the redox conditions in the
thylakoid membrane and in the stroma.
D1 protein together with D2 constitutes the scaffold for all the electron
transfer cofactors within RCII. It is therefore not surprising that its sacrificial
high turnover produces profound effects on the photosynthetic activity as a
whole.
There are intrinsic di
culties in understanding the rational of the pro-
posed model: why should the D1 subunit be preferentially damaged by excess
irradiation? How the specific proteases (belonging to the FtsH family) can
actually recognize the damaged protein, considering that oxidative damage
is probably a random process with no specific targets? The common view
proposes a damage-induced conformational change on the D1 protein that
would trigger its proteolytic degradation and some experimental evidences
are brought about to support this view [32].
Let us take into consideration the main mechanism for oxidative damage
inside the PSII core. When the frequency of excitation of P
680
is higher than
the rate of utilization of the reduction equivalents by PSI or, ultimately by
CO
2
fixing activity (Calvin-Benson cycle), an over-reduction of the plasto-
quinone pool is produced and the electrons coming from RCII cannot find the
physiological exit from the complex. In these conditions, i.e., when the elec-
tron acceptors are fully reduced and cannot receive further electrons, charge
recombination at P
680
produces its triplet form
T
P
680
that is able to activate
oxygen molecules to their singlet state
1
O
2
.
Normally carotenoids take care to avoid oxygen activation in the antenna
by scavenging the chlorophyll triplet states by thermal dissipation. In the reac-
tion center, there are actually carotene molecules, but none of them are close
enough to P
680
for the chlorophyll triplet to be transferred to the carotene
and dissipated. If they were, they would immediately become oxidized by P
680
.
Singlet oxygen is the main responsible of direct or indirect oxidative damage
to both pigments and proteins of PSII but it is not completely clear why the
action of activated form of oxygen should be limited in their target to the D1
subunit.
A possible alternative model comes from experiments on PSII from
cyanobacteria [33]. In this paper, it is shown that, under conditions of strong
over-reduction of the PSII electron acceptors, massive chlorophyll bleaching
