binds 15 chlorophylls aand 3 -carotenes [14,26]. It is thought that these pigments form a core light-har-
vesting system for the reaction center. However, the luminal loops probably play some role in water ox-
idation reactions [6].
Despite the similarities, CP43 differs from CP47 in two aspects:
- The CP43 N-terminal threonine can be reversibly phosphorylated [11].
- The CP43 association with RC is weaker and it can be removed from the isolated core to yield
a CP47-RC complex [28]. - LHCII Proteins
At least six LHCs are energetically connected with PSII—LHCII a, b, c, d. Their proteins are encoded by
the nuclear gene family lhcIIb [29]. The major LHCII, LCHIIb, is a trimeric complex, binding approxi-
mately 65% of Chl a, and 40% of Chl bis in LHCIIb [30,31]. It plays a crucial role in capturing light en-
ergy for photosynthesis and in regulation of energy flow within the photosynthetic apparatus. Two pop-
ulations of LHCIIb were identified, containing a mixture of polypeptides originally designed as
“LHCII-27” and LHCII-25” and subsequently identified as the lhcb1andlhcb2gene products [32]. The
proximal antenna consists almost exclusively of Lhcb1 and its associated pigments, while the peripheral
antenna contains both Lhcb1 and Lhcb2 [33]. Changes in LHCIIb content in response to the environment
reflect varying quantities of the peripheral antenna per PSII. There are no changes in the size of the prox-
imal antenna [34]. Both Lhcb1 and Lhcb2 proteins bind chlorophyll and xanthophyll chromophores. It is
commonly believed that in pigment binding and the light-harvesting function the various Lhcb1 and
Lhcb2 protein constituents of LHCIIb are structurally and functionally equivalent [35]. However, Walter
and Horton [36] found that Lhcb2 protein, isolated from low light–grown plants, specifically binds at least
one additional chlorophyll acompared with Lhcb1 and alters energy transfer characteristics. Therefore,
the differences in the functioning of LHCIIb from high and low light–grown plants are a direct conse-
quence of the changes in polypeptide composition.
The other LHC proteins, a, c, and d (also known as CP29, CP26, and CP24), bind about 5% of PSII
chlorophylls and probably link LHCIIb with the inner antenna [31].
D. Subunits of Water Oxidation Complex
Another important group of proteins are those involved in water oxidation processes. Despite the fact that
ligands for Mn binding have been identified with the D1 and D2 proteins, other proteins are also likely to
be involved. It was already mentioned that CP47 and CP43 proteins have very large hydrophilic loops ex-
posed on the luminal surface [26]. There is every reason to believe that one or both of these loops main-
tain the structural integrity of the Mn cluster. In addition, PSII contains a 33-kDa protein that, together
with a few smaller proteins, acts to stabilize the Mn cluster.
- PsbO Protein
The product of the psbOgene, often referred to as the 33-kDa manganese-stabilizing protein, actually has
a molecular mass of about 26.5 kDa [37]. It is a hydrophilic protein with a high content of -sheets [38].
Cross-linking studies indicate that it is closely localized to the luminal loop of CP47 [39] and to the PsbE
and PsbI proteins [40]. The 33-kDa protein does not bind Mn directly but the tetramanganese cluster it-
self is a nonequilibrium system. Therefore, it requires a suitable protein matrix for its stabilization and the
PsbO protein plays a crucial role in stabilization and maintenance of an optimal environment for water
oxidation [41].
- Other Proteins
Three other proteins were found to be in close contact with PsbO; these are the PsbP, PsbQ, and PsbR ex-
trinsic proteins, localized in the thylakoid lumen. The PsbP and PsbQ proteins have molecular masses of
about 23 and 17 kDa, respectively. Their functions seem to be to optimize the Ca^2 and Cllevels needed
for water oxidation. Both proteins were located with the PsbO protein [6,12]. The PsbR protein was also
found in the vicinity of the water splitting site. Its molecular mass is 10.2 kDa [42] but its function is
unclear.
284 DENEV AND MINKOV