Biomedical Engineering Reference
In-Depth Information
This generates a reductant strong enough to reduce ferredoxin, a soluble iron-
sulfur protein able to donate electrons to NADP
+
by the action of the stromal
soluble enzyme ferredoxin-NADP
+
reductase. P
700
formed is rereduced by an
electron carried by the reduced plastocyanine (Fig. 1.4). Therefore, PSI which
is situated mainly in the nonstacked, stroma lamellae regions of the thylakoid
membrane, functions as a plastocyanine-ferredoxin reductase. The core of the
RCI is composed of two major subunits, PsaA and PsaB, which contain all
the pigments of the complex (including most of the antenna chlorophylls and
carotenoids) and all the electron carriers. Upon absorption of a photon, charge
separation occurs and an electron is transferred to a primary acceptor A
o
(chlorophyll
a
monomer) from which it proceeds to the intermediate acceptor
A
1
(phylloquinone) and subsequently to the second intermediate acceptor A
2
also indicated as F
x
. This is a (4Fe-4S) cluster, as well as the subsequent
electron acceptors F
A
and F
B
, which transfer the electron to the ferredoxin
bound in its docking site on the stromal surface of the PSI complex. P700, A
o
,
A
1
,andF
X
are associated with the PSI intrinsic core subunits PsaA and PsaB,
while the terminal electron acceptors F
A
and F
B
are bound to an extrinsic
low molecular mass subunit PsaC [11]. It should be noticed that the electron
transfer kinetics within RCI is very fast and not completely resolved yet.
1.4.3 Cytochrome b
6
f
The electron transfer bridge between the two photosystems is realized by
the cytochrome b
6
f complex. This is assembled as a functional dimer with a
twofold symmetry axis, each monomer being composed of four subunits: the
cytochrome b
6
carrying two b-type heme groups classified as high and low
potential, the cytochrome f carrying a single c-type heme group, the Rieske
iron-sulphur protein carrying a (Fe
2
-S
2
) cluster and the subunit IV [13].
The reduced plastoquinoles, freely diffusing inside the lipid bilayer, bind
to the complex and electrons are transferred through cytochrome b
6
ftothe
acceptor, plastocyanin. The transfer of two electrons from plastoquinol to the
single electron carrier plastocyanin (summarized in the reaction below)
QH
2
+ 2plastocyanin(Cu
2+
)
Q + 2plastocyanin(Cu
+
)+2H
+
(1.4)
is realized through a complex series of redox reactions, which involves the
semiquinone form Q
−
, named “Q cycle,” with the effect of transferring 4 H
+
instead of only two from the stroma to the lumen. This increases the e
ciency
of converting reduction potential energy into transmembrane proton gradient.
Reduced plastocyanin moves in the lumen and reaches the PSI where it will
donate its electron to P
700
(see earlier).
1.4.4 ATP Synthase
The last complex in the photosynthetic apparatus is the ATP synthase (F-
ATPase). This kind of enzyme is ubiquitous in energy-transducing membranes,
