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. In this study, we employed EPR spectroscopy to test if a high-spin form of the S state is formed preferentially in D1-D170E PSII. Our data show that illumination of dark-adapted D1-D170E PSII core complexes does indeed produce a high-spin form of the S state rather than the low-spin multiline form that is produced in wild-type. This observation provides further experimental support for a change in the equilibrium between S state conformers in both the S and S states in a site-directed mutant that retains substantial O evolving activity.