EXAFS, photosystem

The EXAFS of Mn in photosystem II looks very much like that of a mixed-valence di-/j-oxo-bridged Mn dimer model compound (33,35). In particular, the Mn-Mn distance of 2.7 A in photosystem II is characteristic of a di-/j-oxo-bridged structure. Recent EXAFS data have indicated that a second Mn-Mn distance of 3.3 A may also be present (37-38). Although a clear picture of all of the Mn-Mn distances is not yet available, the EXAFS results are consistent with a structure in which two di-/j-oxo bridged Mn dimers are present in close proximity. 

The Mn-Mn distance of 2.7 A determined by EXAFS is diagnostic of a di-/i-oxo-bridged Mn dimer in photosystem II (33,35). However, one could imagine a number of arrangements of the four Mn ions in photosystem II which contain a di-/i-oxo-bridged structure. Moreover, the observation of two different Mn-Mn distances by EXAFS, could only be accounted for by one of the following three arrangements of Mn a Mn trimer plus one isolated (more than 3.3 A away) Mn mononuclear center, two isolated inequivalent Mn dimers, or a Mn tetramer. 

If we return to the possible arrangements of Mn in photosystem II that are consistent with the EXAFS data, one can rule out the possibility that the Mn ions are arranged as two isolated inequivalent Mn dimers because this arrangement cannot account for the Sj -state g - 4.1 EPR signal. This leaves two possible arrangements of the Mn ions a Mn trimer plus a mononuclear Mn center, or a Mn tetramer. Consequently, one should look for an assignment for the 2-state multiline EPR signal from either a mixed-valence Mn trimer or tetramer rather than from a mixed-valence Mn dimer. 

I have presented an overview of the current state-of-the-art in studies of the Mn complex in photosystem II. There are many unresolved questions and a clear picture of the structure and function of Mn in photosynthetic water oxidation is still not available. One useful approach to help determine the structure of the Mn complex in photosystem II involves the synthesis and characterization of Mn model complexes for comparison with the properties of the Mn complex in photosystem II. Recently, several tetrameric high-valent Mn-oxo complexes have been reported (see the chapter in this volume by G. Christou). Further characterization of existing and new high-valent tetrameric Mn-oxo model complexes, especially EPR and EXAFS measurements, will no doubt help clarify the present uncertain picture of the structure of the Mn complex in photosystem II. 

Robblee, JH, Messinger, J, Cinco, RM, et al. The Mn cluster in the So state of the oxygen-evolving complex of photosystem II studied by EXAFS spectroscopy are there three Di-p-oxo-bridged Mn2 moieties in the tetranuclear Mn complex J Am Chem Soc. 2002 124(25) 7459-71. 

PSII = Photosystem II WOC = Water-oxidizing complex OEC = Oxygen-evolving complex (B)RC = (Bacterial) Reaction Center Chi = Chlorophyll Bchl = Bacteriochloro-phyll XRD = X-ray diffraction EPR = Electron paramagnetic resonance EXAFS = Extended X-ray absorption fine stmctnre ENDOR = Electron-nuclear double resonance ESEEM = Electron spin echo envelope modulation (Tyr = Yz) = DlTyrl61 ATP = Adenosine Triphosphate KIE = Kinetic isotope effect UV = Ultraviolet (FT-)IR = (Fourier Transform) InfraRed. 

AE McDermott, VK Yachandra, RD Guiles, K Sauer, MP Klein, KG Parrett and JH Golbeck (1989) EXAFS structural study ofFx, the low-potential FeS center in photosystem I. Biochemistry 28 8056-8059 V Petrouleas, JJ Brand, KG Parrett and JH Golbeck (1989) A M ssbuer a nalysis of the low-potential iron-sulfur center in photosystem I Spectroscopic evidence thatFx is a [4Fe-4S] cluster. Biochemistry 28 8980-8983 HV Scheller, I Svendsen and BL Mtller (1989) Subunit composition of photosystem I and identification of center X as a [4Fe-4S] iron-sulfur cluster. J Biol Chem 264 6929-6934... 

Cinco RM, Robblee JH, Rompel A et al. Sttondum EXAFS Reveals the Proximity of Calcium to the Manganese Cluster of Oxygen-Evolving Photosystem II. ] Phys Chem B 1998 102 8248-8256. 

An examination of a typical EXAFS spectrum from Mn in photosystem II shows beat patterns indicating that the modulations contain more than one damped sine wave. The Fourier transform of the Mn EXAFS data (Fig. 6) shows at least three clearly resolved features. Note that the peaks are not at the real distances of the shells from Mn but are shifted by about... 

Fig. 7. Fourier transform of the )t -weighted Mn EXAFS data from oriented photosystem II membranes in the 5i state from spinach. The membrane normal was oriented 15° (solid line) and 75° (dashed line) to the polarization direction of the X-rays. The orientation dependence of the EXAFS spectrum provides geometric information about the complex. [Adapted from V. K. Yachandra, V. J. DeRose, M. J. Latimer, I. Mukeiji, K. Sauer, and M. P. Klein, Science 260, 675 (1993).]...

Fig. 8. Structural model for the Mn complex in photosystem II derived from the EXAFS data. This is only one of many structures that are consistent with the EXAFS results. [From K. Sauer, V. K. Yachandra, R. D. Britt, and M. P. Klein, in Manganese Redox Enzymes (V. L. Pecoraro, ed.), p. 141. VCH, New York, 1992.]...

AN EXAFS STUDY OF THE MANGANESE O2 EVOLVING COMPLEX OF PHOTOSYSTEM 2... 

The isolation of the intact Photosystem I core protein has allowed us to determine the EXAFS and Mdssbauer spectrum of Fx in the absence of Fa and Fb. In addition, the isolation procedure is gentle enough to allow reconstitution of the Photosystem I complex from the P700 Fx-containing Photosystem I core protein and the isolated Fa/Fb polypeptide [9]. Here, we use the newly-developed Photosystem I core protein to show ... 

Fig. 1. Simulations of the k -weighted k-space Fe EXAFS data from the Photosystem I core protein containing Fx (solid line -experimental dotted line - simulation). After background removal and weighting by k, data from k=3 to k=12 A-i were Fourier filtered with window limits at R =0.5 and R =3.3 A. The similation shown was performed by the method of Teo and Lee using two shells. The parameters for simulation (a) minic a [4Fe-4S] center and employ 4 S atoms at 2.27 A with a Debye-Wdler disorder parameter of 0.075 A and 3 Fe neighbors at 2.7 A with a disorder parameter of 0.1 A. The parameters for simulation (b) mimic a [2Fe-2S] center and employ 4 S atoms at 2.26 A with a Debye-Wdler disorder parameter of 0.08 A and 1 Fe neighbor at 2.7 A with a disorder parameter of 0.07 A.

The major difficulty with assigning a cluster identity for Fx is that the Fa and Fb clusters, which constitute 8 out of the 12 iron atoms in the Photosystem I core complex, contribute most of the backscattering to the EXAFS spectrum. This problem can be circumvented by using the newly isolated Photosystem I core protein incorporating the components P700, Aq, (Ai by inference) and Fx, but devoid of Fa and Fb. In this preparation, the X-ray K-edge spectrum was found to be similar to that of four-coordinate [4Fe-4S] clusters and unlike six-coordinate iron complexes that are present in heat-denatured Photosystem I or oxidatively denatured ferredoxin. This indicates that the 4 iron atoms in the Photosystem I core protein are most certainly in the form of an intact iron-sulfur complex. The k-space spectrum of Fx (Fig. 1) can be simulated by assuming... 

An Exafs Study of the Manganese 02-Evolving Complex of Photosystem 2 793... 

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