Functional and structural study on chelator-induced suppression of S2/S1 FTIR spectrum in photosynthetic oxygen-evolving complex.

Chelating agents have been shown to induce characteristic changes in the light-minus-dark Fourier transform infrared (FTIR) difference spectrum for the S(2)/S(1) difference in the oxygen-evolving complex (OEC). Addition of various ethylenediamine-N,N,N',N'-tetraacetic acid (EDTA)-type chelators, such as EDTA, O,O'-bis(2-aminoethyl)ethyleneglycol-N,N,N',N'-tetraacetic acid (EGTA), trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid (CyDTA), or N-(2-hydroxyethyl)ethylenediamine-N,N',N'-triacetic acid (HEDTA), to Ca(2+)-depleted PS II membranes resulted in the suppression of typical S(2)/S(1) vibrational features, including the symmetric (1365(+)/1404(-) cm(-1)) and the asymmetric (1587(+)/1566(-) cm(-1)) carboxylate stretching vibrations, as well as the amide I and II modes of the backbone polypeptides. In contrast, the addition of ethylenediamine-N,N'-diacetic acid (EDDA) showed less inhibitory effects. The effects of the chelators depended on the number of the carboxylate groups; chelators with more than two carboxymethyl groups were effective in altering the FTIR spectrum. The bridging structure that connects the two nitrogen atoms also influenced the inhibitory effects. However, the effects were not necessarily correlated with the stability constants of the chelators to Mn(2+). The vibrational modes that were suppressed by EDTA were almost completely restored by subsequent washing with Chelex-treated Ca(2+)-free buffer medium, indicating that the spectral changes are attributable to the reversible association of chelators with the Ca(2+)-depleted OEC. Nevertheless, prolonged incubation with chelators led to the impairment of the O(2)-evolving capability, with differences in the effectiveness, in the order that is consistent with that for the suppression effects on FTIR spectra. Chelators with carboxylate and/or carboxymethyl groups bound to a single nitrogen [nitrilotriacetic acid (NTA) and iminodiacetic acid (IDA)] or carbon (citric acid) were relatively ineffective for the suppression. A chelator that includes four phosphate groups, ethylenediamine-N,N,N',N'-tetrakis(methylenephosphonic) acid (EDTPO), also showed suppression effects on both the carboxylate and amide modes. Based on these findings, a possible mode of interaction between the chelators and the Mn cluster is discussed.