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Rapid Report

Evidence against Bicarbonate Bound in the O₂-Evolving Complex of Photosystem II^†

Supporting Info

Gözde Ulas^‡, Gerard Olack^§ and Gary W. Brudvig*^‡

Department of Chemistry and Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520-8107

Biochemistry, 2008, 47 (10), pp 3073–3075

DOI: 10.1021/bi8000424

Publication Date (Web): February 15, 2008

†

This work was supported by the National Institutes of Health (GM 32715).

‡

Department of Chemistry.

Department of Geology and Geophysics.

* To whom correspondence should be addressed. Phone: (203) 432-5202 . Fax: (203) 432-6144. E-mail: gary.brudvig@yale.edu.

Abstract

The oxidation of water to molecular oxygen by photosystem II (PSII) is inhibited in bicarbonate-depleted media. One contribution to the inhibition is the binding of bicarbonate to the non-heme iron, which is required for efficient electron transfer on the electron-acceptor side of PSII. There are also proposals that bicarbonate is required for formation of O₂ by the manganese-containing O₂-evolving complex (OEC). Previous work indicates that a bicarbonate ion does not bind reversibly close to the OEC, but it remains possible that bicarbonate is bound sufficiently tightly to the OEC that it cannot readily exchange with bicarbonate in solution. In this study, we have used NH₂OH to destroy the OEC, which would release any tightly bound bicarbonate ions from the active site, and mass spectrometry to detect any released bicarbonate as CO₂. The amount of CO₂ per PSII released by the NH₂OH treatment is observed to be comparable to the background level, although N₂O, a product of the reaction of NH₂OH with the OEC, is detected in good yield. These results strongly argue against tightly bound bicarbonate ions in the OEC.

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