Page 163 - Algae

146 Algae: Anatomy, Biochemistry, and Biotechnology

Electron Transport: The Z-Scheme

The fate of the released electrons is determined by the sequential arrangement of all the components
of PSII and PSI, which are connected by a pool of plastoquinones, the cytochrome b 6 f complex, and
the soluble proteins cytochrome c 6 and plastocyanin cooperating in series. The electrons from PSII
þ
are finally transferred to the stromal side of PSI and used to reduce NADP to NADPH, which is
þ
catalyzed by ferredoxin-NADP oxidoreductase (FNR). In this process, water acts as electron
donor to the oxidized P 680 in PSII, and dioxygen (O 2 ) evolves as a by-product.
Photosystem II uses light energy to drive two chemical reactions: the oxidation of water and the
reduction of plastoquinone. Photochemistry in PSII is initiated by charge separation between P 680
þ
2
and pheophytin, creating the redox couple P 680 /Pheo . The primary charge separation reaction
takes only a few picoseconds. Subsequent electron transfer steps prevent the separated charges
from recombining by transferring the electron from pheophytin to a plastoquinone molecule
2
within 200 ps. The electron on Q A is then transferred to Q B -site. As already stated, plastoquinone
at the Q B -site differs from plastoquinone at the Q A -site in that it works as a two-electron acceptor
and becomes fully reduced and protonated after two photochemical turnovers of the reaction center.
The full reduction of plastoquinone at the Q B -site requires the addition of two electrons and two
protons. The reduced plastoquinone (plastoquinol, Q B H 2 ) then unbinds from the reaction center
and diffuses in the hydrophobic core of the membrane, after which an oxidized plastoquinone
molecule finds its way to the Q B -binding site and the process is repeated. Because the Q B -site is
near the outer aqueous phase, the protons added to plastoquinone during its reduction are taken
from the outside of the membrane. Electrons are passed from Q B H 2 to a membrane-bound cyto-
chrome b 6 f, concomitant with the release of two protons to the luminal side of the membrane.
The cytochrome b 6 f then transfers one electron to a mobile carrier in the thylakoid lumen, either
plastocyanin or cytochrome c 6 . This mobile carrier serves an electron donor to PSI reaction
center, the P 700 . Upon photon absorption by PSI a charge separation occurs with the electron
fed into a bound chain of redox sites; a chlorophyll a (A 0 ), a quinone acceptor (A 1 ) and then a
bound Fe–S cluster, and then two Fe–S cluster in ferredoxin, a soluble mobile carrier on the
stromal side. Two ferredoxin molecules can reduce NADP þ to NADPH, via the flavoprotein
þ
ferredoxin-NADP oxidoreductase. NADPH is used as redox currency for many biosynthesis reac-
tions such as CO 2 fixation. The energy conserved in a mole of NADPH is about 52.5 kcal/mol,
whereas in an ATP hole is 7.3 kcal/mol.
The photochemical reaction triggered by P 700 is a redox process. In its ground state, P 700 has a
redox potential of 0.45 eV and can take up an electron from a suitable donor, hence it can perform
an oxidizing action. In its excited state it possesses a redox potential of more than 21.0 eV and can
þ
perform a reducing action donating an electron to an acceptor, and becoming P 700 . The couple P 700 /
þ
P 700 is thus a light-dependent redox enzyme and possesses the capability to reduce the most electron-
negative redox system of the chloroplast, the ferredoxin-NADP þ oxidoreductase (redox
potential ¼ 20.42 eV). In contrast, P 700 in its ground state (redox potential ¼ 0.45 eV) is not
able to oxidize, that is, to take electrons from water that has a higher redox potential (0.82 eV).
The transfer of electrons from water is driven by the P 680 at PSII, which in its ground state has a
sufficiently positive redox potential (1.22 eV) to oxidize water. On its excited state, P 680 at PSII
reaches a redox potential of about –0.60 eV that is enough to donate electron to a plastoquinone
þ
(redox potential ¼ 0 eV) and then via cytochrome b 6 f complex to P 700 at PSI so that it can
return to P 700 and be excited once again. This reaction pathway is called the “Z-scheme of photo-
synthesis,” because the redox diagram from P 680 to P 700 looks like a big “Z” (Figure 3.4).
From this scheme it is evident that only approximately one third of the energy absorbed by the
two primary electron donors P 680 and P 700 is turned into chemical form. A 680 nm photon has an
energy of 1.82 eV, a 700 nm photon has an energy of 1.77 eV (total ¼ 3.59 eV) that is three times
more than sufficient to change the potential of an electron by 1.24 eV, from the redox potential of
þ
the water (0.82 eV) to that of ferredoxin-NADP oxidoreductase (20.42 eV).