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MetaCyc Protein: D1/D2 reaction center complex

Species: Synechococcus elongatus PCC 7942

Component of:
photosystem II monomer
photosystem II (extended summary available)

Subunit composition of D1/D2 reaction center complex = [PsbA1][PsbD1]
         D1 protein = PsbA1 (summary available)
         D2 protein = PsbD1 (extended summary available)

Summary:
The D1 and D2 proteins contain the cofactors that bring about charge separation, leading to the oxidation of water and reduction of the terminal electron/proton acceptor, a plastoquinone. Together, they bind six chlorophyll amolecules, two pheophytin molecules, two plastoquinones, one α-carotene on the D2 side of the RC, one α-carotene on the D1 side, and a non-haem iron [Loll05, Barber06].

Gene-Reaction Schematic: ?

Credits:
Created 19-May-2008 by Caspi R , SRI International


Subunit of: photosystem II monomer

Synonyms: PS II

Species: Synechococcus elongatus PCC 7942

Subunit composition of photosystem II monomer = [PsbX][PsbW][PsbV][PsbL][PsbK][PsbJ][PsbI][PsbU][PsbT][PsbO][PsbN][PsbM][PsbH][PsbC][PsbB][([PsbE]2)([PsbF]2)][(PsbA1)(PsbD1)]
         Photosystem II reaction center X protein = PsbX (summary available)
         Photosystem II reaction center psb28 protein = PsbW (summary available)
         cytochrome c550 = PsbV (summary available)
         Photosystem II reaction center protein L = PsbL (summary available)
         Photosystem II reaction center protein K = PsbK (summary available)
         Photosystem II reaction center protein J = PsbJ (summary available)
         Photosystem II reaction center protein I = PsbI (summary available)
         Photosystem II 12 kDa extrinsic protein = PsbU (summary available)
         Photosystem II reaction center protein T = PsbT (summary available)
         Photosystem II manganese-stabilizing polypeptide = PsbO (summary available)
         Protein psbN = PsbN
         Photosystem II reaction center protein M = PsbM (summary available)
         Photosystem II reaction center protein H = PsbH
         Photosystem II 44 kDa reaction center protein = PsbC (summary available)
         Photosystem II CP47 chlorophyll apoprotein = PsbB (summary available)
         cytochrome b559 = ([PsbE]2)([PsbF]2)
                 Cytochrome b559 α subunit dimer = (PsbE)2 (summary available)
                         Cytochrome b559 subunit α = PsbE
                 Cytochrome b559 subunit β = (PsbF)2 (summary available)
         D1/D2 reaction center complex = (PsbA1)(PsbD1) (summary available)
                 D1 protein = PsbA1 (summary available)
                 D2 protein = PsbD1 (extended summary available)

Component of: photosystem II (extended summary available)

Credits:
Created 19-May-2008 by Caspi R , SRI International


Subunit of: photosystem II

Synonyms: PS II

Species: Synechococcus elongatus PCC 7942

Subunit composition of photosystem II = [(PsbX)(PsbW)(PsbV)(PsbL)(PsbK)(PsbJ)(PsbI)(PsbU)(PsbT)(PsbO)(PsbN)(PsbM)(PsbH)(PsbC)(PsbB)([(PsbE)2][(PsbF)2])([PsbA1][PsbD1])]2
         photosystem II monomer = (PsbX)(PsbW)(PsbV)(PsbL)(PsbK)(PsbJ)(PsbI)(PsbU)(PsbT)(PsbO)(PsbN)(PsbM)(PsbH)(PsbC)(PsbB)([(PsbE)2][(PsbF)2])([PsbA1][PsbD1])
                 Photosystem II reaction center X protein = PsbX (summary available)
                 Photosystem II reaction center psb28 protein = PsbW (summary available)
                 cytochrome c550 = PsbV (summary available)
                 Photosystem II reaction center protein L = PsbL (summary available)
                 Photosystem II reaction center protein K = PsbK (summary available)
                 Photosystem II reaction center protein J = PsbJ (summary available)
                 Photosystem II reaction center protein I = PsbI (summary available)
                 Photosystem II 12 kDa extrinsic protein = PsbU (summary available)
                 Photosystem II reaction center protein T = PsbT (summary available)
                 Photosystem II manganese-stabilizing polypeptide = PsbO (summary available)
                 Protein psbN = PsbN
                 Photosystem II reaction center protein M = PsbM (summary available)
                 Photosystem II reaction center protein H = PsbH
                 Photosystem II 44 kDa reaction center protein = PsbC (summary available)
                 Photosystem II CP47 chlorophyll apoprotein = PsbB (summary available)
                 cytochrome b559 = ([PsbE]2)([PsbF]2)
                         Cytochrome b559 α subunit dimer = (PsbE)2 (summary available)
                                 Cytochrome b559 subunit α = PsbE
                         Cytochrome b559 subunit β = (PsbF)2 (summary available)
                 D1/D2 reaction center complex = (PsbA1)(PsbD1) (summary available)
                         D1 protein = PsbA1 (summary available)
                         D2 protein = PsbD1 (extended summary available)

Summary:
photosystem II (PSII) is a multiprotein complex contained within the thylakoid membranes of plants, algae and cyanobacteria, that utilizes solar energy for splitting water molecules. This reaction is the basis for oxygenic photosynthesis, transferring the electrons via an electron transfer chain that ends with a plastoquinone. This compound donates its electrons to the cytochrome b6f complex, which transfers them to plastocyanin, which in turn transfers the electrons to photosystem I, where they will eventually reduce NADP+ to NADPH. This provides reducing power for biosynthetic reactions and produces molecular oxygen as a by-product.

The heart of the PSII complex consists of the reaction center (RC) core, where light energy is converted into electrochemical potential energy and where the water-splitting reaction occurs. The RC core consists of two homologous proteins known as the D1 protein and D2 protein, which have five transmembrane α-helices each, and four chlorophylls that are associated with them, called P680. Closely associated with the D1/D2 reaction center complex are two chlorophyll-containing proteins, called Photosystem II 44 kDa reaction center protein and Photosystem II CP47 chlorophyll apoprotein. These too are structurally homologous, each having six transmembrane α-helices.

It had been shown that the PSII RC core complex of plants and cyanobacteria is dimeric, having a molecular mass of approx. 700 kDa.

There are many other intrinsic subunits in the complex, including a number of low-molecular mass proteins usually having a single transmembrane helix, which are rather featureless except for the Cytochrome b559 subunit α and PsbF proteins that provide histidine ligands for the high-potential haem of cytochrome b559.

Finally, the PSII RC core complex has several extrinsic proteins attached to its lumenal surface, forming a protein shield over the catalytic site of water splitting. The composition of these proteins is largely species-specific, although Photosystem II manganese-stabilizing polypeptide is found in all oxyphototrophs. In red algae and cyanobacteria, the Photosystem II 12 kDa extrinsic protein and cytochrome c550 proteins associate with Photosystem II manganese-stabilizing polypeptide.

The PSII RC core complex is attached to peripheral light-harvesting systems, which in cyanobacteria and red algae are composed of phycobilins, arranged within the extrinsically located phycobilisomes. There are usually 200-300 light harvesting pigment molecules serving one PSII RC [Barber06]. Most cyanobacteria have distinctly different pigment protein antenna systems, referred to as phycobilisomes [Hankamer01]. These hemidiscoidal structures consist of rod-like arrays of water soluble phycobiliproteins, bound to the stromal surface of the PSII core by linker proteins. The rods are composed of discs that are hexamers of αβ monomers. Each αβ monomer covalently binds two (allophycocyanin) or three (phycocyanin) or more (phycoerythrin) open chain tetrapyrroles.

Excitation of the RC, via light absorption by chlorophylls in the antenna, drives electron transfer from the cluster of four chlorophylls (P680) that are bound to the D1/D2 reaction center complex to a pheophytin acceptor, resulting in a P680* radical. This results in electron transfer activities both downstream and upstream of P680*.

The downstream flow starts with the transfer of the P680* electron to a firmly bound plastoquinone, called QA, and on to a second plastoquinone, named QB. When QB is fully reduced and protonated to plastoquinol (PQH2), it diffuses from the QB-binding site into the lipid matrix of the membrane, where it travels to deliver its electron to the plastoquinol- plastocyanin reductase (better known as the cytochrome b6f complex), which in turn transfers the electron to plastocyanin that travels to photosystem I, where it delivers the electron.

The upstream flow consists of several electron transfers, that can be summarized as the transfer of electrons from two water moleules to the oxidized form of P680. The P680 radical cation (P680*) has a very high redox potential, estimated to be approx. 1.3 V, which is required to drive the water-splitting reaction. The oxidized form of P680 "pulls" an electron from a tyrosine residue (TyrZ) of the D1 protein, one of the protein components of the reaction center. An electron is then transferred to the tyrosine from a manganese atom which is part of a catalytic site called the oxygen-evolving center (OEC). This site is composed of a cluster of four Mn and one Ca2+ ion. Each time P680 is excited by absorbing a photon, the electron transfer results in the removal of an electron from one of the Mn atoms. When four such cycles have occurred, the OEC is ready to receive the four electrons that are involved in the splitting of two water molecules to form dioxygen.

When the water molecules are split and their electrons are transferred to the OEC, two additional "by-products" are formed - molecular oxygen and protons. This accumulation of protons in the thylakoid lumen results in the build-up of a proton gradient across the memberane, which is utilized by ATP synthase to generate ATP.

Locations: thylakoid

Molecular Weight: 700.0 kD (experimental) [Barber06]

GO Terms:

Cellular Component: GO:0030075 - bacterial thylakoid

Credits:
Created 19-May-2008 by Caspi R , SRI International


Enzymatic reaction of: photosystem II

EC Number: 1.10.3.9

4 hν + 2 a plastoquinone + 2 H2O <=> 2 a plastoquinol + oxygen

The reaction direction shown, that is, A + B ↔ C + D versus C + D ↔ A + B, is in accordance with the Enzyme Commission system.

The reaction is physiologically favored in the direction shown.

In Pathways: ethylene biosynthesis V (engineered) , 1-butanol autotrophic biosynthesis , oxygenic photosynthesis , photosynthesis light reactions


Subunit of D1/D2 reaction center complex: D1 protein

Synonyms: photosystem II D1 protein

Gene: psbA1 Accession Number: SYNPCC7942_0424 (MetaCyc)

Locations: thylakoid membrane

Sequence Length: 360 AAs

Molecular Weight: 39.965 kD (from nucleotide sequence)

GO Terms:

Cellular Component: GO:0042651 - thylakoid membrane

Gene Class: UNCLASSIFIED

Unification Links: Protein Model Portal:P04996 , SMR:P04996 , String:1140.Synpcc7942_0424 , UniProt:P04996

Relationship Links: InterPro:IN-FAMILY:IPR000484 , InterPro:IN-FAMILY:IPR005867 , Pfam:IN-FAMILY:PF00124 , Prints:IN-FAMILY:PR00256 , Prosite:IN-FAMILY:PS00244

Summary:
Similar to higher plants, cyanobacteria utilize two photosystems to perform oxygenic photosynthesis [Oren77]. These are thylakoid membrane protein complexes composed of a number of polypeptide components [Golden86].

The heart of the PSII complex is the reaction center (RC) core, where light energy is converted into electrochemical potential energy and where the water-splitting reaction occurs. The RC core consists of two homologous proteins known as D1 and D2 that form a heterodimer, which have five transmembrane α-helices each [Rhee01]. These proteins bind several cofactors that facilitate primary charge separation, and are closely associated with two chlorophyll-containing proteins (CPs) called CP43 and CP47 [Barber06].

The product of the psbA gene is the D1 protein. The Synechococcus elongatus PCC 7942 genome contains three copies of this gene -psbA1, psbA2 and psbA3.


Subunit of D1/D2 reaction center complex: D2 protein

Synonyms: photosystem II D2 protein

Gene: psbD1 Accession Number: SYNPCC7942_0655 (MetaCyc)

Locations: thylakoid membrane

Sequence Length: 352 AAs

Molecular Weight: 39.446 kD (from nucleotide sequence)

GO Terms:

Cellular Component: GO:0042651 - thylakoid membrane

Gene Class: UNCLASSIFIED

Relationship Links: UniProt:PART-OF:P11005

Summary:
Similar to higher plants, cyanobacteria utilize two photosystems to perform oxygenic photosynthesis [Oren77]. These are thylakoid membrane protein complexes composed of a number of polypeptide components.

The heart of the PSII complex is the reaction center (RC) core, where light energy is converted into electrochemical potential energy and where the water-splitting reaction occurs. The RC core consists of two homologous proteins known as D1 and D2 that form a heterodimer and which have five transmembrane α-helices each [Rhee01]. These proteins bind several cofactors that facilitate primary charge separation, and are closely associated with two chlorophyll-containing proteins (CPs) called CP43 and CP47 [Barber06].

The product of the psbD gene is the D2 protein, also known as the QA protein. This protein is essential for oxygenic photosynthetic electron transport. The Synechococcus elongatus PCC 7942 genome contains two copies of this gene -psbD1 and psbD2. The two genes encode identical polypeptides [Golden88].


References

Barber06: Barber J (2006). "Photosystem II: an enzyme of global significance." Biochem Soc Trans 34(Pt 5);619-31. PMID: 17052167

Collins81: Collins MD, Jones D (1981). "Distribution of isoprenoid quinone structural types in bacteria and their taxonomic implication." Microbiol Rev 45(2);316-54. PMID: 7022156

Golden86: Golden SS, Brusslan J, Haselkorn R (1986). "Expression of a family of psbA genes encoding a photosystem II polypeptide in the cyanobacterium Anacystis nidulans R2." EMBO J 5(11);2789-98. PMID: 3098559

Golden88: Golden SS, Stearns GW (1988). "Nucleotide sequence and transcript analysis of three photosystem II genes from the cyanobacterium Synechococcus sp. PCC7942." Gene 67(1);85-96. PMID: 3138165

Hankamer01: Hankamer B, Morris E, Nield J, Carne A, Barber J (2001). "Subunit positioning and transmembrane helix organisation in the core dimer of photosystem II." FEBS Lett 504(3);142-51. PMID: 11532446

Loll05: Loll B, Kern J, Saenger W, Zouni A, Biesiadka J (2005). "Towards complete cofactor arrangement in the 3.0 A resolution structure of photosystem II." Nature 438(7070);1040-4. PMID: 16355230

Oren77: Oren A, Padan E, Avron M (1977). "Quantum yields for oxygenic and anoxygenic photosynthesis in the cyanobacterium Oscillatoria limnetica." Proc Natl Acad Sci U S A 74(5);2152-6. PMID: 16592398

Rhee01: Rhee KH (2001). "Photosystem II: the solid structural era." Annu Rev Biophys Biomol Struct 30;307-28. PMID: 11340062


Report Errors or Provide Feedback
Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
Page generated by SRI International Pathway Tools version 18.5 on Sun Nov 23, 2014, biocyc14.