In addition to the proteins already mentioned, several others were found. They have a low molecu-
lar mass between 3 and 8 kDa, one transmembrane helix, and unknown function.
E. Electron and Proton Transport Within Photosystem II
Photosystem II functions as a water-plastoquinone oxidoreductase. The primary electron donor of PSII is
P680. Following excitation, P680 transfers an electron to pheophytin and is subsequently reduced by
TyrZ. The P680/P680 midpoint potential is unusually oxidizing (1.2 Ev) and can drive the oxidation of
water (0.8 Ev). In contrast to the primary electron donor Chls of all other reaction center types, only the
midpoint potential of the PSII primary donor Chl(s) is shifted in a positive direction relative to free Chl.
It is evident that the structural organization of P680 and its interactions with the D1 and D2 proteins de-
termine the unusual redox properties of P680. The structural organization of P680 is not known; however,
P680 has been reported to have properties of a Chl monomer, a Chl dimer, as well as a chlorin multimer
(reviewed in Ref. 10).
Most of the data suggest that P680 is a Chl dimer [10,43]. However, the absence of an appreciable
red shift in the Qyabsorbance band as expected for a chlorophyll dimer and the Stark effect and hole burn-
ing measurements do not show features predicted for a chlorophyll dimer (reviewed in Ref. 10). To ac-
count for these conflicting observations, Schelvis et al. [43] proposed that P680 is a chlorophyll dimer
with monomeric properties. The monomeric properties are attributed to an antiparallel or asymmetric ori-
entation of the chlorophyll QYtransition moments.
Alternatively, P680 has been proposed to be a multimer of excitonically interacting chlorins (in-
cluding the chlorophyll spectral pair, chlorophyll monomers, and pheophytin) [44]. Following optical
excitation, the P680 excited state rapidly equilibrates with most of the pigments in the reaction center
and charge separation occurs from the equilibrated state. Bleaching of the pheophytin QXtransition
also occurs within 300 fsec after excitation of PSII reaction centers [45]. These observations, in con-
junction with theoretical predictions of the dipole-dipole coupling strengths between the accessory
chlorophyll monomers, pheophytins, and the ChlSP, suggest that all three chlorin groups interact exci-
tonically [45]. Significantly, the multimer model is not particularly dependent on the spatial organiza-
tion of the pigments as long as the chlorins have overlapping QYtransitions and are sufficiently close
to allow rapid (100 fsec) excited state equilibration. This results in a spatially heterogeneous excited
state.
When P680 absorbs a photon, it donates eto the first stable electron acceptor, pheophytin (Phe).
The state P680Pheis referred to as the primary radical pair. It has an electrochemical potential of 1.7
Ev [46]. For efficient charge separation the back reaction with P680must be limited, which is achieved
mostly by rapid transfer of eto the second electron acceptor QA. This is a plastoquinone molecule, which
is tightly bound to the D2 protein and functions as a one-electron carrier and does not normally undergo
protonation. In contrast, the second plastoquinone, D1-bound QB, can accept two electrons and two pro-
tons. Therefore, for the complete reduction of QBtwo primary charge separations take place at P680. In
its fully reduced state QB, this (already plastoquinol) molecule is released from the binding site on the D1
protein into the lipid matrix of the membrane [6].
The described two-electron acceptor site processes contrast with a four-electron gate on the oxi-
dizing (donor) site of PSII. Here four turnovers of primary charge separation are needed to create the
four oxidizing equivalents required for the conversion of two molecules of water to a molecule of oxy-
gen [41]. The model of water splitting (Figure 3) is known as the S-state model [47]. However, the ex-
act location and organization of the catalytic center of the complex are still not known [6]. The pre-
ferred model, based on data from x-ray and resonance techniques, is that the cluster is composed of two
di--oxobridged dinuclear Mn units linked via an oxybridge and a pair of amino acids providing car-
boxalato bridges [48]. The reaction scheme of S-state transitions suggests that D-Tyr161 (YZ) in the ox-
idized state is capable of extracting protons as well as electrons from water molecules bound to the Mn
cluster [41,49]. The abstraction of 4Hand 4eby four turnovers of YZtherefore maintains elec-
troneutrality while at the same time accumulates the oxidizing potential to create dioxygen on the fourth
turnover as required by the S-state model [6,47]. The extracted protons are released in the thylakoid lu-
men when the electrons are transferred via YZto P680 to compensate for electron deficiency, which is
a result of QBreduction [6].
PHOTOSYNTHETIC MEMBRANES IN HIGHER PLANTS 285