Calcium EXAFS Establishes the Mn-Ca Cluster in the Oxygen-Evolving Complex of Photosystem II

Abstract

The proximity of Ca to the Mn cluster of the photosynthetic water-oxidation complex is demonstrated by X-ray absorption spectroscopy. We have collected EXAFS data at the Ca Kedge using active PS II membrane samples that contain approximately 2 Ca per 4 Mn. These samples are much less perturbed than previously investigated Sr-substituted samples, which were prepared subsequent to Ca depletion. The new Ca EXAFS clearly shows backscattering from Mn at 3.4 Å, a distance that agrees with that surmised from previously recorded Mn EXAFS. This result is also consistent with earlier related experiments at the Sr K-edge, using samples that contained functional Sr, that show Mn is ~ 3.5 Å distant from Sr. The totality of the evidence clearly advances the notion that the catalytic center of oxygen evolution is a Mn-Ca heteronuclear cluster. 4Calcium is essential as a cofactor for photosynthetic oxygen evolution in green plants and algae. Together with another cofactor, chloride, calcium is involved in a four-manganese cluster to catalyze the light-driven oxidation of water into dioxygen, protons and electrons for carbon fixation (1-4). The water-splitting reaction occurs within the oxygen-evolving complex (OEC 1 ) of photosystem II (PS II), located in the thylakoid membranes of green plants, cyanobacteria and algae. It is crucial to study the Ca cofactor as recent mechanisms of oxygen evolution (5-9) have proposed roles for Ca (and Cl) that require it to be located near the Mn cluster. Studies on photoactivation and assembly of the Mn cluster also implicate a requirement for nearby Ca in the light-induced assembly of the water-oxidizing complex (10-12).While the Mn cluster has been extensively researched, mainly by X-ray absorption spectroscopy (XAS) (13-16) and electron paramagnetic resonance (EPR) techniques (17), the calcium cofactor has been comparatively less studied (1,18,19). This is explained by the lack of spectroscopic handles for Ca compared to Mn. Previous work on the structure of the binding site of the Ca cofactor has mainly consisted of studies involving Ca depletion (20-23) or substituting other metals (24) including strontium, manganese or terbium for the cofactor (25-28). FTIR spectroscopy suggests a carboxylate bridge between Mn and Ca in PS II (29), but a later study disputes this finding (30). Structural information about the binding site of the Ca cofactor has been obtained recently through extended X-ray absorption fine structure (EXAFS) experiments on strontium-substituted PS II membranes (31-33) and through 87 Sr ESEEM (electron spin echo envelope modulation) spectroscopy (R. D. Britt, personal communication, 2002).Strontium is the only metal ion that replaces calcium and still maintains oxygen-evolving activity (up to 40% at saturating light levels) (34). Earlier efforts using Mn EXAFS on Srreactivated PS II (31) implicated a 3.5 Å distance between the Mn cluster and Sr. More recently, Sr EXAFS on similar samples confirms this finding of a 3.5 Å distance between Sr and Mn in the OEC (32)...

Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide

“…In the Mn 4 Ca complex of oxygenic photosynthesis, the 3.3-3.5 A vector detected by EXAFS spectroscopy relates to a Mn-Ca distance in a distorted Mn 3 Ca(m-O) 4 cube. 56,59,60 In the synthetic MnCat however, neither Ca ions nor any other bivalent cation is present. It is conceivable that, in the MnCat, Mn ions attached by monom-oxo bridges replace Ca with respect to its structural role in the photosynthetic Mn 4 Ca complex and in Mn 2 Ca-oxide particles.…”

Solar water-splitting into H2 and O2: design principles of photosystem II and hydrogenases

“…49 Nevertheless, the crystal structures provide an approximate overall shape of the cluster, and also affirm the binding place of Ca next to Mn as previously suggested by EXAFS spectroscopy. [50][51][52] A breakthrough was recently achieved by combining crystallography and EXAFS measurements. This approach allowed collecting polarized XANES and EXAFS spectra along the three crystallographic axes of PSII single crystals.…”

Functional models for the oxygen-evolving complex of photosystem II

“…Biochemical and biophysical experiments have long established that manganese is essential for photosynthetic water oxidation and EPR experiments have set the number of manganese atoms in the OEC at four [8,9,23]. In addition to the four manganese atoms, a calcium atom has been shown to be part of the OEC through EPR [10], EXAFS [24] and X-ray diffraction [1] experiments. Beyond the core structure of four manganese atoms and one calcium atom, a number of other components have been suggested for the OEC.…”