transport:
Tyr161 is Yz.
His198 binds P680.
Tyr126, Tyr147, Ala150, and Glu130—probably pheophytin.
Tyr254, Phe255, and Gly256 probably interact with QB.
Asp170, Glu189, Gls165, Ala344, His109, His332, and His377 bind tetramanganese cluster.
His215 and His272 probably bind nonheme iron [6,12,13].The tight involvement of D1 proteins in primary photochemistry makes them a major target of pho-
toinduced damage [14,15]. This damage leads to photoinhibition and reduction of photosynthetic effi-
ciency. The damaged D1 is a subject of proteolytic degradation [16]. The major cleavage site on D1 is on
the stromal side of the thylakoid membrane [17]. However, the D1 protein is the protein in PSII with the
highest turnover rate [18]. This phenomenon might be linked to the requirement to repair PSII after it has
been damaged by photoinhibitory light [19].
- PsbD-D2 Protein
The D2 protein is homologous to the D1 protein, although it has a higher molecular mass of about 39.5
kDa. Like D1, the D2 protein consists of five transmembrane helices and has similar surface helices
[6,13]. N-terminal threonine can also undergo reversible phosphorylation [6,11].
The D2 protein binds lesser cofactors involved in primary electron transport, although it does con-
tain inactive cofactors [6]. The D2 protein binds via His198 the P680 and the QA, probably by Thr218,
Phe253, and Trp254 [13]. The His215 and His269 probably form ligands for the nonheme iron and Glu69
forms a ligand for the Mn cluster [7].
- PsbE and PSbF-Cytochrome b 559 Proteins
The PsbE and PsbF proteins are the andsubunits of cytochrome b 559. The proteins have molecular
masses of 9.3 and 4.4 kDa, respectively [20]. Hydropathy plots revealed single transmembrane helices
that each of the proteins forms [21]. The two proteins are closely associated with D1 and D2 proteins and
probably form a heterodimer that binds a heme via the single histidine residue contained in their se-
quences [6,22].
There have been many speculations about the function of Cyt b 559. It was found that Cyt b 559 is more
closely associated with D1 than with the D2 protein, and it was proposed that Cyt b 559 may have a func-
tion in an electron transfer cycle around PSII (including QB, and ChlZ) that protects PSII from accumula-
tion of long-lived P680states and consequently from D1 protein degradation [10].
- PsbI and PsbL Proteins
Both PsbI and PsbL are small proteins with molecular masses of 4.2 and 4.4 kDa, respectively. Their
amino acid sequences are highly conservative and reveal single transmembrane helices. They are located
very close to the D1, D2 proteins but their functions are not clear [23,24]. Perhaps PsbI protein binds
chlorophyll [20], and PsbL seems to be required for the normal function of the QAsite [25].
C. Subunits of the PSII Light-Harvesting Apparatus
Each reaction center is connected to a large set of chlorophyll proteins, which together form the light-har-
vesting apparatus. It consists of the inner antenna and light-harvesting complex (LHCII). The inner an-
tenna is built up from CP47 (CPa-1, psbB) and CP43 (CPa-2, psbC) proteins, about 10–15 chlorophyll
molecules, and carotenoids (-carotene and lutein) [26].
- PsbB-CP47 and PsbC-CP43 Proteins
CP47 and CP43 proteins are important constituents of the RC core complex. Their absence can have a se-
rious impact on both PSII assembly and water oxidation function [27]. CP47 and CP43 proteins have
about 500 and 470 amino acid residues and their molecular masses are 56 and 50 kDa, respectively [6].
Their structures are similar in many ways. They both possess six transmembrane helices and large (200
and 150 amino acids, respectively) luminal loops between helices V and VI [26]. Both proteins contain a
considerable number of histidine residues, which bind chlorophyll and -carotenes. For example, CP47
PHOTOSYNTHETIC MEMBRANES IN HIGHER PLANTS 283