X-ray Absorption Spectroscopy of Calcium-Substituted Derivatives of the Oxygen-Evolving Complex of Phostosytem II

Abstract

X-ray absorption spectroscopy (XAS) has been used to characterize the structural consequences of Ca2+ replacement in the reaction center complex of the photosynthetic oxygen-evolving complex (OEC). EPR and activity measurements demonstrate that, in the absence of the 17 and 23 kDa extrinsic polypeptides, it is not necessary to use either low pH or Ca chelators to effect complete replacement of the active site Ca2+ by Sr2+, Dy3+, or La3+. The extended X-ray absorption fine structure (EXAFS) spectra for the OEC show evidence for a Mn···M interaction at ca. 3.3 Å that could arise either from Mn···Mn scattering within the Mn cluster or Mn···Ca scattering between the Mn cluster and the inorganic Ca2+ cofactor. There is no significant change in the either the amplitude or the phase of this feature when Ca2+ is replaced by Sr2+ or Dy3+, thus demonstrating that there is no EXAFS-detectable Mn···Ca contribution at ca. 3.3 Å in these samples. The only significant consequence of Ca2+ replacement is a small change in the ca. 2.7 Å Mn···Mn distance. The average Mn···Mn distance decreases 0.014 Å when Ca2+ is replaced by Sr2+ and increases 0.012 Å when Ca2+ is replaced by Dy3+. A structural model which can account both for the variation in Mn···Mn distance and for the known properties of Ca2+-substituted samples is one in which there is a hydrogen bond between a Ca2+-bound water and a Mn2(μ-O)2 unit. This scheme suggests that an important role for the Ca2+ may be to modulate the protonation state, and thus the redox potential, of the Mn cluster.

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

“…Peak III has been assigned to contributions from both Mn and Ca backscatterers at about 3.3 and 3.4 Å, respectively. 10,26,44 Studies from other groups prefer the assignment of peak III to backscattering from Mn or Ca at 3.3 Å, 45 or from Ca at 3.7 Å. 46 Presented in the lower part of Figure 3 are Fourier transforms from 0 to 10 Å.…”

Structural Effects of Calcium Depletion on the Manganese Cluster of Photosystem II: Determination by X-ray Absorption Spectroscopy

“…that interfere with other techniques. The different regions of the X-ray spectrum provide complementary information; X-ray absorption edges yield information about the oxidation states and site symmetry of the absorbing atom, and extended X-ray absorption fine structure (EXAFS) is sensitive to distances, numbers, and atomic number of atoms around the absorbing atom (for reviews of XAS in biological systems, see refs [27][28][29] 37 and Booth et al 38 ).…”

Reduction potentials of heterometallic manganese–oxido cubane complexes modulated by redox-inactive metals

“…The calcium center has been proposed to coordinate water or hydroxide ligands that participate in O-O bond formation (5)(6)(7). Other proposed functions include affecting proton-coupled electron transfer and the redox properties of the cluster, possibly via interactions with neighboring tyrosine residues (8)(9)(10)(11).…”