The role of Ca ²⁺ and protein scaffolding in the formation of nature’s water oxidizing complex

Abstract

Photosynthetic O2 evolution is catalyzed by the Mn4CaO5 cluster of the water oxidation complex of the photosystem II (PSII) complex. The photooxidative self-assembly of the Mn4CaO5 cluster, termed photoactivation, utilizes the same highly oxidizing species that drive the water oxidation in order to drive the incorporation of Mn2+ into the high-valence Mn4CaO5 cluster. This multistep process proceeds with low quantum efficiency, involves a molecular rearrangement between light-activated steps, and is prone to photoinactivation and misassembly. A sensitive polarographic technique was used to track the assembly process under flash illumination as a function of the constituent Mn2+ and Ca2+ ions in genetically engineered membranes of the cyanobacterium Synechocystis sp. PCC6803 to elucidate the action of Ca2+ and peripheral proteins. We show that the protein scaffolding organizing this process is allosterically modulated by the assembly protein Psb27, which together with Ca2+ stabilizes the intermediates of photoactivation, a feature especially evident at long intervals between photoactivating flashes. The results indicate three critical metal-binding sites: two Mn and one Ca, with occupation of the Ca site by Ca2+ critical for the suppression of photoinactivation. The long-observed competition between Mn2+ and Ca2+ occurs at the second Mn site, and its occupation by competing Ca2+ slows the rearrangement. The relatively low overall quantum efficiency of photoactivation is explained by the requirement of correct occupancy of these metal-binding sites coupled to a slow restructuring of the protein ligation environment, which are jointly necessary for the photooxidative trapping of the first stable assembly intermediate.

High-resolution model of Arabidopsis Photosystem II reveals the structural consequences of digitonin-extraction

“…when plants absorb more light than they can use for photosynthesis the D1 protein is replaced every 20–30 min, which means that the assembly state of PSII is in a constant switch between functional and repair/inactive state. To become functional, PSII needs to correctly assemble the site for water oxidation; a process driven by light and known as photoactivation 38 , 40 . At distinct steps of photoactivation, manganese ions (Mn 2+ ) interact with specific residues of the D1 protein, while being oxidized 40 .…”

High-resolution model of Arabidopsis Photosystem II reveals the consequences of digitonin-extraction

“…when plants absorb more light than they can use for photosynthesis the D1 protein is replaced every 20-30 minutes, which means that the assembly state of PSII is in a constant switch between functional and repair/inactive state. To become functional, PSII needs to correctly assemble the site for water oxidation; a process driven by light and known as photoactivation 37,39 . At distinct steps of photoactivation, manganese ions (Mn 2+ ) interact with specific residues of the D1 protein, while being oxidized 39 .…”

“…To become functional, PSII needs to correctly assemble the site for water oxidation; a process driven by light and known as photoactivation 37,39 . At distinct steps of photoactivation, manganese ions (Mn 2+ ) interact with specific residues of the D1 protein, while being oxidized 39 . One calcium ion (Ca 2+ ) and five oxygen atoms bridging the metal ions are indispensable to form the functional high-valence manganese cluster, Mn4CaO5.…”

“…When the full water oxidation complex is formed, Mn4CaO5 is coordinated to several D1 and CP43 amino acids 26 . Even though most of the photoactivation steps are still to be understood, there is evidence that manganese and calcium ions compete to bind at the high-affinity Mn-binding site (HAS) 39 -involving at least the D1 residue Asp170-in an early step of photoactivation.…”

“…The following steps are the oxidation of the first-bound Mn 2+ and the so-called 'dark rearrangement', proposed as a light-independent dynamic where the C-terminal of the D1 protein adopts a new structural conformation like the observed in the Mn4CaO5 binding pocket of mature PSII 39 . In an attempt to gain new information of this activation process, Gisriel and colleagues 40 isolated monomeric PSII from Synechocystis sp.…”