Biomedical Engineering Reference
In-Depth Information
The reaction center (RCII) of PSII is composed of the heterodimer con-
stituted by the two proteins D1 (PsbA) and D2 (PsbD), which coordinate all
the essential cofactors for its electron transport chain [9].
When the central chlorophylls special pair (P
680
) is excited by a photon
absorbed by the antenna apparatus, a charge separation occurs as an electron
is transferred from chlorophyll to pheophytin, in one of the fastest electron
transfer reaction (2-20 ps) ever observed in nature. The cation P
680
that is
formed on this chlorophyll pair is one of the most oxidizing centers that can
be formed in a living cell. The subsequent electron transfer is to Q
A,
, a plas-
toquinone molecule tightly bound to the D2 protein near the stromal surface.
This electron transfer is slower than the previous one but still fast enough
(
∼
400 ps) to compete with recombination. This step involves a significant loss
of energy that contributes to stabilize the charge separation.
Because of its very high oxidation potential, P
680
is able to extract an
electron from a tyrosine residue of the D1 protein (Tyr161, also named TyrZ)
in about 20 ns; the electron hole at the donor side of PSII ends its run at the
Mn cluster, near the lumenal surface of the membrane. In the mean time, in
the acceptor side at the other membrane surface, the electron standing on Q
A
finds its way to Q
B
, a plastoquinone loosely bound on the D1 protein, also
thanks to the assistance of a non-heme iron, sitting in between the two. This
is the last reaction in the electron transfer chain within PSII reaction center
and its rate is the limiting step (200-300
s).
A second turnover, driven by a further excitation from the antenna, brings
a second electron to Q
B
with the very same reaction sequence. When Q
B
receives the second electron, two protons are up-taken from the stroma and
the plastoquinol leaves its binding site on D1. This is then reoccupied by an
oxidized plastoquinone from the membrane pool. A two-photons-two-electrons
gated mechanism brings about the reduction of a plastoquinone molecule at
the acceptor side of the complex.
In a four-photons-four-electrons gated mechanism, two more photons and
two more electrons injected in the transport chain are required to reduce a
second PQ and oxidize two water molecules with the evolution of one oxygen
molecule. The so-called “oxygen evolving complex” (OEC) sitting in the lu-
menal side of PSII, often indicated also as the “water splitting enzyme,” can
be considered as the heart of the PSII activity, and most of the today research
efforts are directed to elucidate the detailed mechanism for its activity [17].
µ
1.4.2 Photosystem I
In photosystem I (PSI), which is located almost exclusively in stroma lamellae,
the excitation of the chlorophyll special pair P
700
in the reaction center of
PSI (RCI) initiates a series of electron transfer reactions that culminate in
reduction of NADP
+
to NADPH. RCI is activated by the absorption of red
light that promotes it to an excited state (P
700
*) at about
0
.
6V with a
jump of about 1 V with respect to the ground state P
700
(E˚'= +0.4 V).
−
