Wydrzynski (1977)

The structure of the Oxygen Evolving Complex (OEC) has been a work in progress for decades. Without crystallographic insight a variety of spectroscopic approaches have been employed to define the the ligands to the Mn ions, the number of Mn ions, the involvement of cofactors such as Ca, and the distances between the Mn ions. A review of this spectroscopy [1] can be found <pdf>. The figure on the left was possibly the first model of the OEC that used 4 Mn ions and was added at the insistence of a examiner to a thesis in 1977. It was subsequently published in [2a]. Since that time considerable effort has been directed at understanding the mechanism and structure of the catalyst. Molecular oxygen after all is the most vital of ingredients for life on earth and the molecular origins of this molecule have important consequences.

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• Over the past few years these structures have been improving and the haze has been clearing for this complex mechanism. The very first PSII structures came from a group in London [3] with ~8Å resolution and using 2-D x-ray crystallography and suggested locations of chlorophylls in the reaction centre and some general structural information.
• Following this in February 2001 the first data from a 3-D crystal was published from a group in Berlin at 3.8Å resolution (pdb 1FE1) [4]. This work providing a more detailed view of the chlorophylls and a density (blob) that contained the Mn ions. This blob clearly indicated the location of the catalytic site and revealed there were at least 4 metals present, but at the time there was discussion as to whether this was 3Mn+Ca or 4 Mn ions.
• Another group in Japan was working on the problem and in January 2003 published a 3.7Å structure (pdb 1IZL) [5]. In this work the Mn ions were also identified but seen to be translated relative to the initial Berlin work. Indeed, this variation in the Mn ions continues to dog the field.
• In Mar. 2004 a third group, the originators of the 2-D analysis returned to the scene and reported their 3.5Å structure (pdb 1S5L). This structure was the first to propose the complete structural assignment of all the amino acid side chains and identified all the chlorophylls. It was a hugely significant step forward [6].
• In Dec 2005 the Berlin group returned again and published a 3.0Å structure (pdb 2AXT) [7]. These last two models are illustrated below in animated <PyMOL> renditions of their structures and the overlay.
• In November 2006 X-ray absorption spectroscopy EXAFS [8] was used on the PSII crystals to map the Mn-Mn distances as a function of the angle of the crystal. This approach was the first attempt to use this approach to solve a metal structure in a crystal.