A possible evolutionary origin for the Mn4 cluster of the photosynthetic water oxidation complex from natural MnO2 precipitates in the early ocean
- *Melvin Calvin Laboratory, Physical Biosciences Division, Lawrence Berkeley National Laboratory, and †Department of Chemistry, University of California, Berkeley, CA 94720
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Communicated by Ignacio Tinoco, Jr., University of California, Berkeley, CA (received for review March 14, 2002)
Abstract
The photosynthetic water oxidation complex consists of a cluster of four Mn atoms bridged by O atoms, associated with Ca2+ and Cl−, and incorporated into protein. The structure is similar in higher plants and algae, as well as in cyanobacteria of more ancient lineage, dating back more than 2.5 billion years ago on Earth. It has been proposed that the proto-enzyme derived from a component of a natural early marine manganese precipitate that contained a CaMn4O9 cluster. A variety of MnO2 minerals are found in nature. Three major classes are spinels, sheet-like layered structures, and three-dimensional networks that contain parallel tunnels. These relatively open structures readily incorporate cations (Na+, Li+, Mg2+, Ca2+, Ba2+, H+, and even Mn2+) and water. The minerals have different ratios of Mn(III) and Mn(IV) octahedrally coordinated to oxygens. Using x-ray spectroscopy we compare the chemical structures of Mn in the minerals with what is known about the arrangement in the water oxidation complex to define the parameters of a structural model for the photosynthetic catalytic site. This comparison provides for the structural model a set of candidate Mn4 clusters—some previously proposed and considered and others entirely novel.
Footnotes
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↵ ‡ To whom reprint requests should be addressed. E-mail: khsauer{at}lbl.gov or vkyachandra{at}lbl.gov.
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↵ § Russell, M. J. & Hall, A. J., Sixth International Congress on Carbon Dioxide Utilization, Sept. 9–14, 2001, Breckenridge, CO, abstr., p. 49.
- Abbreviations:
- EXAFS,
- extended x-ray absorption fine structure;
- Ga,
- billion years ago;
- OEC,
- oxygen-evolving complex;
- PS II,
- photosystem II
- Copyright © 2002, The National Academy of Sciences
