Oxygen-evolving complex OEC

As a sharp contrast to the large number of enzymes utilizing 02 as an oxidant, there is only a single enzyme capable of forming 02 from water. This is the OEC in PSII. A pentanuclear Mn4Ca complex bound to amino acid residues in the interior of PSII constitutes the heart of the OEC [14]. 

X-ray crystallography has provided some details of the location, structure, and protein environment of the Mn4Ca2+ cluster. However, because of the low resolution of the crystal structures reported to date, and the possibility of radiation damage at the catalytic centre, the precise position of each metal ion remains a matter of debate. To some extent these problems have been overcome by applying spectroscopic techniques [22], 

X-ray absorption spectroscopy (XAS), together with the compactness of the electron density assignable to the Mn complex and the relatively small number of potential amino acid ligands, suggest several p-oxo bridges between Mn ions [24], 

One of the ongoing mysteries of the OEC is how PSII could have evolved the ability to use water as an electron donor. It would seem that a number of changes, such as formation of a very high-potential oxidant in the charge-separation reaction and incorporation of a four-electron water-oxidation catalyst, would have had to occur simultaneously in order to convert an anaerobic photosynthetic reaction centre into PSII. 

Photosynthetic water oxidation in PSII involves the binding of two water molecules, the removal of four electrons and four protons from the Mn complex/sub-strate (water) entity, 0-0 bond formation, and dioxygen release. 

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The invention of aerobic photosynthesis, the light-driven oxidation of water to oxygen, stands as one of the pivotal evolutionary innovations in the history of life on Earth. The process is carried out only at the oxygen-evolving complex (OEC) of PSII in plants and algae, as well as in cyanobacteria. Despite the biological uniqueness of water oxidation to 02, several of the core proteins of PSII have homologues in the so-called type I and type II anaerobic photosynthetic reaction... 

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,... 

Figure 17.23 The 3.4 A resolution structure of the oxygen evolving complex (OEC) and the immediate peptide environment. The directions of proposed PT and ET pathways are indicated with arrows. Figure adapted from Ref [206].

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