Calcium modulates the photoassembly of photosystem II (Mn){sub 4}-clusters by preventing ligation of nonfunctional high-valency states of manganese
- Univ. of Kentucky, Lexington, KY (United States)
The requirement for Ca{sup 2+} in the Mn{sup 2+}-dependent photoactivation of oxygen evolution was re-evaluated using 17 kDa/24 kDa-less photosystem II (PSII) membranes depleted of (Mn){sub 4}-clusters by NH{sub 2}OH extraction. At optimum conditions (1 mM Mn{sup 2+}/10 {mu}M 2,6-dichlorphenolindophenol (DCIP)/20 mM Ca{sup 2+}), the light-induced increase of oxygen-evolution activity, the increase of membrane-bound Mn, and the B-band thermoluminescence emission intensity occurred in parallel. The extent of recovery of the oxygen-evolution activity was equivalent to 88% and 66% of the activity shown by parent NaCl-extracted PSII membranes and by PSII membranes, respectively. Analyses of the Ca{sup 2+} concentration dependence for the maximum recovery of oxygen evolution activity gave evidence for Ca{sup 2+}-binding site(s) having K{sub m} values of {approximately}38 and {approximately}1300 {mu}M. Illumination of membranes in the strict absence of Ca{sup 2+} resulted in large increases (up to 18 Mn/200 chlorophyll) of EDTA nonextractable, EPR silent, nonfunctional membrane-bound Mn{sup {ge}3+} and small increases of oxygen-evolution capability, dependent on pH and concentrations of Mn{sup 2+} and DCIP. In the strict absence of Ca{sup 2+}, significant recovery of oxygen-evolution activity was obtained under a limited set of conditions permitting photoligation of a limited abundance of the nonfunctional Mn{sup {ge}3+}. Simple addition of Ca{sup 2+} to membranes containing nonfunctional Mn{sup {ge}3+} followed by reillumination resulted in the conversion of Mn{sup {ge}3+} to (Mn){sub 4}-clusters. It is argued that Ca{sup 2+} promotes the conformational change involved in the conversion of the Mn{sup 2+} mononuclear intermediate to the Mn{sup 3+}-Mn{sup 2+} binuclear intermediate in the photoactivation mechanism, thereby permitting photoassembly of (Mn){sub 4}-clusters and preventing photo-inactivation by Mn{sup {ge}3+} ions. 68 refs., 11 figs., 1 tab.
- OSTI ID:
- 415156
- Journal Information:
- Biochemistry (Eaton), Vol. 34, Issue 41; Other Information: PBD: 17 Oct 1995
- Country of Publication:
- United States
- Language:
- English
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