Different localization of two Ca2+ in spinach oxygen-evolving photosystem II membranes. Evidence for involvement of only one Ca2+ in oxygen evoluation
1993
Han, K.C. | Katoh, S.
Localization of the two Ca2+ bound to oxygen-evolving photosystem II (PSII) membranes from spinach was investigated by fractionating the membranes into the PSII reaction center core complexes and peripheral antenna Chl a/b-proteins after solubilization with n-heptylthioglucoside. The core complex fraction contained one Ca2+ per PSII, while another Ca2+ was found in the solubilized major light-harvesting Chl a/b-proteins (LHCII). LHCII isolated with Triton X-100 or dodecylmaltoside also contained Ca2+ in an amount corresponding to one per PSII. The Ca2+ bound to LHCII could not be removed by treatment with Chelex 100, which effectively sequestered extraneous Ca2+ bound to LHCII, or by preparation of LHCII in the presence of 40 mM citrate. Localization of the two Ca2+ in different functional domain of PSII membranes conclusively indicates that the number of the bound Ca2+ that can function in oxygen evolution is one per PSII. The results also suggest that one Ca2+ has a structural role in the peripheral antenna assembly.
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