EPR–ENDOR Characterization of (¹⁷O, ¹H, ²H) Water in Manganese Catalase and Its Relevance to the Oxygen-Evolving Complex of Photosystem II

Abstract

The synthesis of efficient water-oxidation catalysts demands insight into the only known, naturally occurring water-oxidation catalyst, the oxygen-evolving complex (OEC) of photosystem II (PSII). Understanding the water oxidation mechanism requires knowledge of where and when substrate water binds to the OEC. Mn catalase in its Mn(III)-Mn(IV) state is a protein model of the OEC’s S2 state. From 17O-labeled water exchanged into the di-μ-oxo di-Mn(III,IV) coordination sphere of Mn catalase, CW Q-band ENDOR spectroscopy revealed two distinctly different 17O signals incorporated in distinctly different time regimes. First, a signal appearing after two hours of 17O exchange was detected with a 13.0 MHz hyperfine coupling. From similarity in the time scale of isotope incorporation and in the 17O μ-oxo hyperfine coupling of the di-μ-oxo di-Mn(III,IV) bipyridine model (Usov, O. M.; Grigoryants, V. M.; Tagore, R.; Brudvig, G. W.; Scholes, C. P. J. Am. Chem. Soc. 2007, 129, 11886-11887), this signal was assigned to μ-oxo oxygen. EPR line broadening was obvious from this 17O μ-oxo species. Earlier exchange proceeded on the minute or faster time scale into a non-μ-oxo position, from which 17O ENDOR showed a smaller 3.8 MHz hyperfine coupling and possible quadrupole splittings, indicating a terminal water of Mn(III). Exchangeable proton/deuteron hyperfine couplings, consistent with terminal water ligation to Mn(III), also appeared. Q-band CW ENDOR from the S2 state of the OEC was obtained following multi-hour 17O exchange, which showed a 17O hyperfine signal with a 11 MHz hyperfine coupling, tentatively assigned as μ-oxo-17O by resemblance to the μ-oxo signals from Mn catalase and the di-μ-oxo di-Mn(III,IV) bipyridine model.

The biological water-oxidizing complex at the nano–bio interface

“…(vi) The assignments generally agree with comparisons with model systems [25,45,46]. (vii) The exchange rates are typically considered too fast for the existence of an m-oxo bridge, based on comparison with model complex data [45,46] and catalase data [25]. [47,48] and the displacement of NH 3 by W1 and O5 was assigned as the exchangeable bridge [49], the results showed that all 'waters', including the moxo bridge, were exchanged within 15 s. It can be concluded that all exchangeable sites could serve as a bound substrate in the S 1 state.…”

“…(v) The slow exchange rate is also perturbed by Ca/Sr exchange [44]. (vi) The assignments generally agree with comparisons with model systems [25,45,46]. (vii) The exchange rates are typically considered too fast for the existence of an m-oxo bridge, based on comparison with model complex data [45,46] and catalase data [25].…”

“…The characteristics and position of each metal ion in the Mn 4 CaO 5 cluster have been studied compared with artificial models [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] and using various methods [5][6][7][8][9][10][11][33][34][35]. However, the positions of the metal ions, together with the five oxygen atoms bridging the five metal ions, were clearly resolved by the high-resolution crystal structure analysis [10].…”

“…The Mn A4 (V)=O may equally be considered a Mn(IV)BO + or Mn(IV)-O species [23]. Adapted, with permission, from [25]; copyright American Chemical Society. forming a Mn-bound hydroperoxide [51,52] (see Figure 2A).…”

Manganese Oxidation State Assignment for Manganese Catalase

“…An experimental 17 O isotropic hfc of magnitude 1.5 or 3.8 MHz was recently measured for the water oxygen. 13,26 Spin projected 17 O isotropic hfcs for models A, A(OH), and B are shown in Table 3. A comparison of the calculated results for the three model systems with the experimental value strongly supports either model A or model A(OH).…”