Alteration of the O₂-Producing Mn₄Ca Cluster in Photosystem II by the Mutation of a Metal Ligand

Abstract

The O 2 -evolving Mn 4 Ca cluster in photosystem II (PSII) is arranged as a distorted Mn 3 Ca 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 Mn 4 Ca cluster. In this study, we use Fourier transform infrared difference spectroscopy to characterize the impact of the D1-D170E mutation. On the basis of analyses of carboxylate and carbonyl stretching modes and the O−H stretching modes of hydrogen-bonded water molecules, we show that this mutation alters the extensive network of hydrogen bonds that surrounds the Mn 4 Ca cluster in the same manner as that of many other mutations. It also alters the equilibrium between conformers of the Mn 4 Ca cluster in the dark-stable S 1 state so that a high-spin form of the S 2 state is produced during the S 1 -to-S 2 transition instead of the low-spin form that gives rise to the S 2 state multiline electron paramagnetic resonance signal. The mutation may also change the coordination mode of the carboxylate group at position 170 to unidentate ligation of Mn4. This is the first mutation of a metal ligand in PSII that substantially impacts the spectroscopic signatures of the Mn 4 Ca cluster without substantially eliminating O 2 evolution. The results have significant implications for our understanding of the roles of alternate active/inactive conformers of the Mn 4 Ca cluster in the mechanism of O 2 formation.

Rapid-Scan Fourier Transform Infrared Monitoring of the Photoactivation Process in Cyanobacterial Photosystem II

“…The Mn 2+ location between the D1-D170 and D1-E189 side chains is consistent with the estimation of the distance between the Mn 2+ ion and Y D • in apo-PSII using pulsed electron–electron double resonance (PELDOR) . The involvement of D1-D170 in the high-affinity site is also consistent with the previous studies using D1-D170 mutants, especially D1-D170S, although it was recently pointed out that caution is necessary in the interpretation of the previous results from D1-D170 mutants as well as mutants at other ligand sites because post-translational amino-acid conversion could take place to generate original Asp/Glu residues when amino acids have relatively long side chains. , Mutations need to be examined directly by mass spectrometry or by amino acid sensitive spectroscopy such as FTIR. ,,, …”

“…26,27 Mutations need to be examined directly by mass spectrometry or by amino acid sensitive spectroscopy such as FTIR. 26,27,39,40 Mechanism of the Dark Rearrangement Process. The dark rearrangement process during the B → C step (Figure 1c) has been thought to be a protein conformational change and/ or the relocation of a photo-oxidized Mn 3+ ion.…”

Probing the proton release by Photosystem II in the S1 to S2 high-spin transition

“…Similarly, a mutation of a Mn4CaO5 ligand that stabilizes the S2 HS state, as the D170E mutation, also modifies the H-bond network, e.g. [87].…”

Probing the proton release by Photosystem II in the S1 to S2 high-spin transition

“…Similarly, a mutation of a Mn 4 CaO 5 ligand that stabilizes the S 2 HS state, as the D170E mutation, also modifies the H-bond network, e.g. [87].…”