The Oxygen Evolving Complex OEC

The first EPR signal detected from the S2-state was reported by Dismukes and Siderer5 and showed a multiline signal (MLS) near g = 2. The MLS bears similarities to binuclear p-oxo bridged mixed valent MnmMnlv complexes that are coupled antiferromagnetically to an effective S = 1/2 state.5 

Due to this early work it became clear that the application of EPR techniques to the OEC would deliver very valuable information on the electronic and geometrical structure of this important enzyme. The work led to an enormous amount of publications in this field both on the native system and on related model compounds. These cannot all be discussed here in detail. In particular, the diverse field of synthetic Mn complexes serving as models for the OEC will not be reviewed the reader is referred to some review articles41 432 438 and a monograph edited by Pecoraro.439 

EPR signals detected from the S-states of the OEC are shown in Fig. 7 (see references 368,449). 

In the following we will briefly review the major results on the OEC that were derived from EPR and related techniques with special emphasis on the literature of the last 4 years. 

There is now fairly good evidence from EPR/ENDOR on the S2 state MLS that the observed signal and the 55Mn hyperfine structure results from a single Miu cluster with electron spin density distributed over all 4 Mn ions.462 465 In this work 55Mn ENDOR experiments, which deliver more precise hfc and nqc values463 and spectral simulations,462-466 have been of great importance. Independent support for the model came from simulations of the NH3-modified MLS that showed a spin redistribution in the Mm cluster rather than spin transfer to other atoms.462-463 

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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].

This paper will describe our use of polynuclear manganese complexes of the type believed to be present at the active site of the oxygen evolving complex (OEC) of photosystem II (PSII) as catalysts for hydrocarbon oxygenation. 

The correlation of the extent to which Tris inactivates photos)mthetic O2 evolution with the loss of 2/3 of the Mn bound to thylakoid membranes provided the first evidence that the oxygen-evolving complex (OEC) contained four bound Mn (1). Due to the linear dependence of activity and bound Mn, Tris was h) othesized to cause the concerted release of all four Mn from each reaction center. Later experiments which quantitated functional Mn after the removal of adventitious Mn by washing thylakoids with 50 mM CaCl2 confirmed the ratio of 4 Mn/OEC (2). However, in these experiments the amount of Mn released during inactivation by Tris or NH2OH was variable with the conditions of the extraction. This partial extraction of Mn was interpreted to result from a loss of one to three Mn/OEC in all of the reaction centers rather than the loss of 4 Mn/OEC in 1/4 to 3/4 of the reaction centers, respectively. Tris and NH2OH are now known to remove the extrinsic membrane proteins associated with the OEC (3,4). In the absence of these proteins, the mM concentrations of Ca " " used to remove adventitious Mn from thylakoids also activate the OEC and inhibit the loss of Mn from the enzyme (5-7). 

The fact that the oxygen evolving complex, OEC, did not have sufficient capacity to donate electrons to reduce Qa completely at this temperature. However, this does not seem to be likely because the light pulse was sufficiently long to reach maximal fluorescence. 

In green plants and cyanobacteria, there are two different PSs, PSI and PSII, coupled in series. PSI and PSII are together capable of oxidizing water to O2. This oxidation takes place in PSII. Typical for PSII is a manganese complex, called the oxygen-evolving complex (OEC), where water molecules enter and oxygen molecules exit. 

Nature represents the blueprint for storing sunlight in the form of chemical fuels (such as sugars) by CO2 conversion. The primary steps of natural photosynthesis involve the absorption of sunlight and its conversion into separated electron/hole pairs. The holes of this wireless current are then captured by the oxygen-evolving complex (OEC) to oxidize water to oxygen (reaction 1) that allows the electrons that are captured by the photo system to reduce NADP (nicotinamide adenine dinucleotide phosphate) to NADPH (the reduced form of NADP, (reaction 2) (Kanan et al., 2008) [4]. 

The electrons required to regenerate the chlorophyll molecules in PSll are then obtained from the oxidation of water by the Mn405Ca cluster in the oxygen-evolving complex (OEC). This catalyst is responsible for the activation of water... 

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