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Cytochrome b559, alpha (gene psbE) and beta (gene psbF)subunits
Structure of Photosystem II from Thermosynechococcus elongatus.
Identifiers
Symbol	Cytochrom_B559
Pfam	PF00283
InterPro	IPR013081
PROSITE	PDOC00464
OPM superfamily	2
OPM protein	2axt
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Lumenal portion of Cytochrome b559, alpha (gene psbE) subunit
Structure of oxygen-evolving photosystem II from Thermosynechococcus vulcanus.
Identifiers
Symbol	Cytochrom_B559a
Pfam	PF00284
InterPro	IPR013082
PROSITE	PDOC00464
OPM superfamily	2
OPM protein	2axt
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Cytochrome b559 is an important component of Photosystem II (PSII) is a multisubunit protein-pigment complex containing polypeptides both intrinsic and extrinsic to the photosynthetic membrane. Within the core of the complex, the chlorophyll and beta-carotene pigments are mainly bound to the antenna proteins CP43 (PsbC) and CP47 (PsbB), which pass the excitation energy on to chlorophylls in the reaction centre proteins D1 (Qb, PsbA) and D2 (Qa, PsbD) that bind all the redox-active cofactors involved in the energy conversion process. The PSII oxygen-evolving complex (OEC) provides electrons to re-reduce the PSII reaction center, and oxidizes 2 water molecules to recover its reduced initial state. It consists of OEE1 (PsbO), OEE2 (PsbP) and OEE3 (PsbQ). The remaining subunits in PSII are of low molecular weight (less than 10 kDa), and are involved in PSII assembly, stabilisation, dimerization, and photoprotection.

Cytochrome b559, which forms part of the reaction centre core of PSII, is a heterodimer composed of one alpha subunit (PsbE), one beta (PsbF) subunit, and a heme cofactor. Two histidine residues from each subunit coordinate the heme. Although cytochrome b559 is a redox-active protein, it is unlikely to be involved in the primary electron transport in PSII due to its very slow photo-oxidation and photo-reduction kinetics. Instead, cytochrome b559 could participate in a secondary electron transport pathway that helps protect PSII from photo-damage. Cytochrome b559 is essential for PSII assembly.

This domain occurs in both the alpha and beta subunits of cytochrome B559. In the alpha subunit, it occurs together with a lumenal domain (InterPro: IPR013082), while in the beta subunit it occurs on its own.

Cytochrome b559 can exist in three forms, each with a characteristic redox potential. These forms are very low potential (VLP), ≤ zero mV; low potential (LP) at 60 mV; and high potential (HP) at 370 mV. There is also an intermediate potential (IP) form that has a redox potential at pH 6.5-7.0 that ranges from 170 to 240 mV. In oxygen-evolving reaction centers, more than half of the cyt b559 is in the HP form. In manganese-depleted non-oxygen evolving photosystem II reaction centers, cyt b559 is usually in the LP form.

References

1. Loll B, Kern J, Saenger W, Zouni A, Biesiadka J (December 2005). "Towards complete cofactor arrangement in the 3.0 A resolution structure of photosystem II". Nature. 438 (7070): 1040–4. Bibcode:2005Natur.438.1040L. doi:10.1038/nature04224. PMID 16355230. S2CID 4394735.
2. Kamiya N, Shen JR (January 2003). "Crystal structure of oxygen-evolving photosystem II from Thermosynechococcus vulcanus at 3.7-A resolution". Proc. Natl. Acad. Sci. U.S.A. 100 (1): 98–103. Bibcode:2003PNAS..100...98K. doi:10.1073/pnas.0135651100. PMC 140893. PMID 12518057.
3. Kamiya N, Shen JR (2003). "Crystal structure of oxygen-evolving photosystem II from Thermosynechococcus vulcanus at 3.7-A resolution". Proc. Natl. Acad. Sci. U.S.A. 100 (1): 98–103. Bibcode:2003PNAS..100...98K. doi:10.1073/pnas.0135651100. PMC 140893. PMID 12518057.
4. Blankenship RE, Raymond J (2004). "The evolutionary development of the protein complement of photosystem 2". Biochimica et Biophysica Acta (BBA) - Bioenergetics. 1655 (1–3): 133–139. doi:10.1016/j.bbabio.2003.10.015. PMID 15100025.
5. Schroder WP, Shi LX (2004). "The low molecular mass subunits of the photosynthetic supracomplex, photosystem II". Biochimica et Biophysica Acta (BBA) - Bioenergetics. 1608 (2–3): 75–96. doi:10.1016/j.bbabio.2003.12.004. PMID 14871485.
6. Burda K, Kruk J, Borgstadt R, Stanek J, StrzaBka K, Schmid GH, Kruse O (2003). "Mössbauer studies of the non-heme iron and cytochrome b559 in a Chlamydomonas reinhardtii PSI- mutant and their interactions with alpha-tocopherol quinone". FEBS Lett. 535 (1–3): 159–165. doi:10.1016/S0014-5793(02)03895-4. PMID 12560096. S2CID 1636274.
7. Mizusawa N, Yamashita T, Miyao M (1999). "Restoration of the high-potential form of cytochrome b559 of photosystem II occurs via a two-step mechanism under illumination in the presence of manganese ions". Biochimica et Biophysica Acta (BBA) - Bioenergetics. 1410 (3): 273–286. doi:10.1016/S0005-2728(99)00005-5. PMID 10082793.

• Photosynthesis
• Protein families