Oxygen-evolving complex oxidation states

Styring S, Rutherford AW. In the oxygen-evolving complex of photosystem II the So state is oxidized to the Si state by D+ (signal IIsiow). Biochemistry. 1987 26(9) 2401-5. 

Liang WC, Roelofs TA, Cinco RM, et al. Structural change of the Mn cluster during the S2 —> S3 state transition of the oxygen-evolving complex of photosystem II. Does it reflect the onset of water/substrate oxidation Determination by Mn X-ray absorption spectroscopy. J Am Chem Soc. 2000 122(14) 3 399 412. 

S2, S3, and S4. S is the most reduced state and S4 is the most oxidized state of the complex. It is now believed that the Mn cluster, a group of four manganese atoms bound to MSP near the PSII reaction center, is mostly responsible for these transitions. As the oxygen-evolving complex abstracts electrons and protons from HzO, it cycles through the five oxidation states. The electrons are transferred one at a time to a tyrosine residue on the Dj polypeptide and then to the P680 reaction center. The protons released in the process remain in the thylakoid lumen, where they contribute to the pH gradient that drives ATP synthesis. 

Other oxidases also derive function from bidirectional PCET pathways at the enzyme active site. The recent crystal structures of PSII [206, 207] support suggestions that as the oxygen evolving complex (OEC) steps through its various S-states [208, 209], substrate derived protons are shuttled to the lumen via a proton exit channel, the headwater of which appears to be the Dgi residue hydrogen-bonded to Mn-bound water [210]. The protons are liberated with the proton-coupled oxidation of the Mn-OH2 site. As shown by the structure reproduced in Fig. 17.23, Dgj is diametrically opposite to Y, which has long been known [148, 151, 152] to be the electron relay between the PS II reaction center and OEC. Notwithstanding,... 

T. thermophilus enzyme is isolated, a mixture of enzsme oxidation states including Mn", Mn"Mn", Mn ", and Mn" Mn" is observed 138, 139). The Mn"Mn " and Mn Mn" oxidation states are catal3d.ically inactive but can be reduced to active oxidation states, Mn"Mn or Mn "Mn ", by hydroxylamine 23). The Mn "Mn" oxidation state is quite intriguing because it is thought to contain a bis-(jU,2-0 ), core which previously has been implicated in the oxygen-evolving complex... 

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