Mutation-induced shift of the photosystem II active site reveals insight into conserved water channels.

Photosystem II (PSII) is the water-plastoquinone photo-oxidoreductase central to oxygenic photosynthesis. PSII has been extensively studied for its ability to catalyze light-driven water oxidation at a MnCaO cluster called the oxygen-evolving complex (OEC). Despite these efforts, the complete reaction mechanism for water oxidation by PSII is still heavily debated.

Independent Mutation of Two Bridging Carboxylate Ligands Stabilizes Alternate Conformers of the Photosynthetic O-Evolving MnCaO Cluster in Photosystem II.

The O-evolving MnCaO cluster in photosystem II is ligated by six carboxylate residues. One of these is D170 of the D1 subunit. This carboxylate bridges between one Mn ion (Mn4) and the Ca ion.

Mutation of a metal ligand stabilizes the high-spin form of the S state in the O-producing MnCaO cluster of photosystem II.

The residue D1-D170 bridges Mn4 with the Ca ion in the O-evolving MnCaO cluster of Photosystem II. Recently, the D1-D170E mutation was shown to substantially alter the S-minus-S FTIR difference spectra [Debus RJ (2021) Biochemistry 60:3841-3855]. The mutation was proposed to alter the equilibrium between different Jahn-Teller conformers of the S state such that (i) a different S state conformer is stabilized in D1-D170E than in wild-type and (ii) the S to S transition in D1-D170E produces a high-spin form of the S state rather than the low-spin form that is produced in wild-type.

High-resolution cryo-electron microscopy structure of photosystem II from the mesophilic cyanobacterium, sp. PCC 6803.

Photosystem II (PSII) enables global-scale, light-driven water oxidation. Genetic manipulation of PSII from the mesophilic cyanobacterium sp. PCC 6803 has provided insights into the mechanism of water oxidation; however, the lack of a high-resolution structure of oxygen-evolving PSII from this organism has limited the interpretation of biophysical data to models based on structures of thermophilic cyanobacterial PSII.

Alteration of the O-Producing MnCa Cluster in Photosystem II by the Mutation of a Metal Ligand.

The O-evolving MnCa cluster in photosystem II (PSII) is arranged as a distorted MnCa cube that is linked to a fourth Mn ion (denoted as Mn4) by two oxo bridges. The Mn4 and Ca ions are bridged by residue D1-D170. This is also the only residue known to participate in the high-affinity Mn(II) site that participates in the light-driven assembly of the MnCa cluster.