Ammonia binding to the oxygen-evolving complex of photosystem II identifies the solvent-exchangeable oxygen bridge (μ-oxo) of the manganese tetramer

Abstract

The assignment of the two substrate water sites of the tetramanganese penta-oxygen calcium (Mn 4 O 5 Ca) cluster of photosystem II is essential for the elucidation of the mechanism of biological O-O bond formation and the subsequent design of bio-inspired water-splitting catalysts. We recently demonstrated using pulsed EPR spectroscopy that one of the five oxygen bridges (μ-oxo) exchanges unusually rapidly with bulk water and is thus a likely candidate for one of the substrates. Ammonia, a water analog, was previously shown to bind to the Mn 4 O 5 Ca cluster, potentially displacing a water/substrate ligand [Britt RD, et al. (1989) J Am Chem Soc 111(10):3522-3532]. Here we show by a combination of EPR and time-resolved membrane inlet mass spectrometry that the binding of ammonia perturbs the exchangeable μ-oxo bridge without drastically altering the binding/exchange kinetics of the two substrates. In combination with broken-symmetry density functional theory, our results show that (i) the exchangable μ-oxo bridge is O5 {using the labeling of the current crystal structure [Umena Y, et al. (2011) Nature 473(7345):55-60]}; (ii) ammonia displaces a water ligand to the outer manganese (Mn A4 -W1); and (iii) as W1 is trans to O5, ammonia binding elongates the Mn A4 -O5 bond, leading to the perturbation of the μ-oxo bridge resonance and to a small change in the water exchange rates. These experimental results support O-O bond formation between O5 and possibly an oxyl radical as proposed by Siegbahn and exclude W1 as the second substrate water.PSII | OEC | water oxidizing complex | water-oxidation | Mn cluster I n oxygenic photosynthesis, light-driven water splitting is catalyzed by the oxygen-evolving complex (OEC) of the membrane bound, pigment-protein complex photosystem II (PSII). The OEC consists of an inorganic tetra-manganese penta-oxygen calcium (Mn 4 O 5 Ca) cluster (1-3) and the nearby redox-active tyrosine residue Y Z (D1-Tyr161) that couples electron transfer from the Mn 4 O 5 Ca cluster to P680, the photo-oxidant of PSII. The cluster resembles a "distorted chair", where the base is formed by an oxygen-bridged (μ-oxo) cuboidal Mn 3 O 4 Ca unit (1) (Fig. 1A). The fourth Mn (Mn A4 ) is located outside of the cuboidal unit and is linked via a μ-oxo-bridged ligation (O4) to one of its corners (Mn B3 ). A second linkage between the outer Mn and the cube is provided by a fifth oxygen O5. The Mn 4 O 5 Ca cluster is also held together by six carboxylate ligands and has only one directly coordinating nitrogen ligand, D1-His332 (Fig. 1B).The OEC cycles through a series of five intermediate states that are known as S states (4) (Fig. 1A): S 0 , S 1 (dark stable), S 2 , S 3 , and S 4 (not yet isolated), where the subscript refers to the number of oxidizing equivalents stored in the OEC through successive electron withdrawals by Y Z • . In the 1.9-Å resolution structure, the S state of the cluster was assigned to be S 1 (1). However, this is unlikely as all Mn-Mn, Mn-Ca, and Mn-O/N distances of the crystal structure are ∼...

Structural rearrangements preceding dioxygen formation by the water oxidation complex of photosystem II

“…Recent analysis indicates that both fast and slowly exchangeable substrate waters (W f and W s , respectively) are bound to Mn and that W s is either O5 or possibly, W2 (28,35,36,45,51,54). These results have been interpreted to favor a direct radical coupling mechanism.…”

“…The S 3 ground state (S + 3 Y Z ) seems to exist with four Mn ions in the IV oxidation state, each having octahedral local geometry, and the O5 oxo bonded to Mn4, which puts the overall arrangement of the Mn 4 CaO 5 as being in the "open cube" configuration, structurally analogous to the S 2 + low-spin conformational isomer (45). Presently, there are two classes of dioxygen formation mechanisms that are most actively discussed: models involving nucleophilic (28,35,45,(49)(50)(51)(52). Regarding the role of water movement in relation to the nucleophilic attack models, a deprotonation on W2 (-OH) preceding the oxidation of Mn4 or a deprotonation yielding the nucleophilic hydroxide may require rearrangements of titratable moieties, such as water, and may account for at least part of the polyphasic lag that could be altered by mutations.…”

Artificial photosynthesis: understanding water splitting in nature

“…Its binding mode was recently determined as a terminal ligand to the Mn4 ion, replacing the water ligand W1 [71,72]. Such a binding motif readily explains the observed hyperfine and quadrupole couplings; the asymmetric quadrupole coupling is a property of the H-bonding network of the W1/NH 3 site (Asp61) [57,72].…”

“…This result is readily understood as due to a trans effect: the W1 ligand is trans to the O5 bridge. Displacement of W1 by NH 3 elongates the O5-Mn4 distance perturbing the observed hyperfine coupling (figure 5c,d) [71].…”

“…Such a binding motif readily explains the observed hyperfine and quadrupole couplings; the asymmetric quadrupole coupling is a property of the H-bonding network of the W1/NH 3 site (Asp61) [57,72]. W1 displacement as opposed to other binding modes (m-oxo bridge displacement) is also preferred based on energetic considerations and an analysis of available vibrational data [71][72][73]. In samples where NH 3 is added, the hyperfine coupling of the exchangeable m-oxo bridge is strongly perturbed (ca 30%) [57,72].…”