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Probing reactive sites within the Photosystem II manganese cluster: Evidence for separate populations of manganese that differ in redox potential

Probing reactive sites within the Photosystem II manganese cluster: Evidence for separate... When the oxygen evolving complex of Photosystem II is depleted of essential cofactor atoms (Ca and Cl) by a high ionic strength treatment that extracts 23 and 17 kDa extrinsic polypeptides, Mn can be released by several large reductants (hydroquinone (HQ), ,,′,′-tetramethyl--phenylenediamine (TMPD), methyl derivatives of hydroxylamine). This reactivity can be slowed if the enzyme is reconstituted with Ca. For TMPD, data from EPR and X-ray absorption spectroscopy indicate that Ca reconstitution restricts initial Mn reduction to a site which contains a Mn atom. Dimethylhydroxylamine (DMHA), a much smaller Mn reductant, is unable to reduce the cluster under these same conditions, even though DMHA and TMPD can generate Mn from the Mn cluster in the absence of Ca. These reductants differ in redox potential by about 300 mV, and it is likely that the higher potential reductant, DMHA (+550 mV), is restricted to initially reacting with a Mn species that is screened by the Ca atom and is incapable, in the presence of Ca, of reducing lower potential atoms of the cluster that do react with TMPD. Reduction of the reactive Mn species by DMHA in the absence of Ca, however, subsequently allows reduction of the remaining 3 Mn. Such an arrangement of metals and potentials in the cluster is in accord with models for the site of water oxidation and for the structure of the Ca–Mn cluster. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Physical Chemistry Chemical Physics Royal Society of Chemistry

Probing reactive sites within the Photosystem II manganese cluster: Evidence for separate populations of manganese that differ in redox potential

Kuntzleman, Thomas; McCarrick, Robert; Penner-Hahn, James; Yocum, Charles
Royal Society of Chemistry — Oct 12, 2004
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Abstract

When the oxygen evolving complex of Photosystem II is depleted of essential cofactor atoms (Ca and Cl) by a high ionic strength treatment that extracts 23 and 17 kDa extrinsic polypeptides, Mn can be released by several large reductants (hydroquinone (HQ), ,,′,′-tetramethyl--phenylenediamine (TMPD), methyl derivatives of hydroxylamine). This reactivity can be slowed if the enzyme is reconstituted with Ca. For TMPD, data from EPR and X-ray absorption spectroscopy indicate that Ca reconstitution restricts initial Mn reduction to a site which contains a Mn atom. Dimethylhydroxylamine (DMHA), a much smaller Mn reductant, is unable to reduce the cluster under these same conditions, even though DMHA and TMPD can generate Mn from the Mn cluster in the absence of Ca. These reductants differ in redox potential by about 300 mV, and it is likely that the higher potential reductant, DMHA (+550 mV), is restricted to initially reacting with a Mn species that is screened by the Ca atom and is incapable, in the presence of Ca, of reducing lower potential atoms of the cluster that do react with TMPD. Reduction of the reactive Mn species by DMHA in the absence of Ca, however, subsequently allows reduction of the remaining 3 Mn. Such an arrangement of metals and potentials in the cluster is in accord with models for the site of water oxidation and for the structure of the Ca–Mn cluster.

Journal

Physical Chemistry Chemical Physics – Royal Society of Chemistry

Published: Oct 12, 2004

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