Effect of Osmolytes on Photoassembly of Functionally Active Mn 4 CaO 5 Cluster in Mn-Depleted Photosystem II Preparations Isolated from Spinach Leaves.

The effect of osmolytes (trehalose, sucrose, betaine, D-glucose and hydroxyectoine) on the photoassembly of the functionally active inorganic core of the water-oxidizing complex (Mn₄CaO₅ cluster) in Mn-depleted PSII preparations (apo-WOC-PSII) was investigated. It was revealed that the efficiency of the photoassembly of the Mn₄CaO₅ cluster was considerably (three times) increased in the presence of 1 M disaccharides (trehalose or sucrose) in contrast to other osmolytes. It was found that the osmolytes (especially trehalose or sucrose) improved the redox interaction of exogenous Mn²⁺ with apo-WOC-PSII, enhanced the protective effect of Mn²⁺ against the photoinhibition of apo-WOC-PSII, protected CaCl₂-treated PSII preparations against thermoinactivation, and stabilized the water-oxidizing complex and electron transport from Q_A to Q_B in native PSII preparations during heat treatment. It is suggested that the ability of osmolytes to enhance the efficiency of the photoassembly of a Mn₄CaO₅ cluster depends on their effect on the following key processes: the redox interaction of Mn²⁺ with apo-WOC-PSII; the stability of apo-WOC-PSII to photoinhibition during the photoactivation procedure; and the stabilization of both the newly assembled functionally active Mn₄CaO₅ cluster and the electron transfer from Q_A to Q_B. [ABSTRACT FROM AUTHOR]