Blocking of Electron Donation by Mn(II) to Yz* Following Incubation of Mn-Depleted Photosystem II Membranes with Fe(II) in the Light

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The donation of electrons by Mn(II) and Fe(II) to Yz· through the high-affinity (HAz) site in Mn-depleted photosystem II (PSII) membranes has been studied by flash-probe fluorescence yield measurements. Mn(II) and Fe(II) donate electrons to Yz· with about the same efficiency, saturating this reaction at the same concentration (ca. 5 μM). However, following a short incubation of the membranes with 5 μM Fe(II), but not with Mn(II) in room light, added Mn(II) or Fe(II) can no longer be photooxidized by Yz· This blocking effect is caused by specifically bound, photooxidized Fe [≥Fe(III)] and is accompanied by a delay in the fluorescence yield decay kinetics attributed to the slowing down of the charge recombination rate between Qa- and Yz· Exogenously added Fe(III), on the other hand, does not donate electrons to Yz· does not block the donation of electrons by added Mn(II) and Fe(II), and does not change the kinetics of the decay of the fluorescence yield. These results demonstrate that the light-dependent oxidation of Fe(II) by Yz· creates an Fe species that binds at the HAz site and causes the blocking effect. The pH dependence of Mn(II) electron donation to Yz· via the HAz site and of the Fe-blocking effect is different. These results, together with sequence homologies between the C-terminal ends of the D1 and D2 polypeptides of the PSII reaction center and several diiron-oxo enzymes, suggest the involvement of two or perhaps more (to an upper limit of four to five) bound iron cations per reaction center of PSII in the blocking effect. Similarities in the interaction of Fe(II) and Mn(II) with the HAz Mn site of PSII during the initial steps of the photoactivation process are discussed. The Fe-blocking effect was also used to investigate the relationship between the HAz Mn site and the HA sites on PSII for diphenylcarbazide (DPC) and NH2OH oxidation. Blocking of the HAz site with specifically bound Fe leads to the total inhibition of electron donation to Yz· by DPC. Since DPC and Mn(II) donation to PSII is noncompetitive [Preston, C., and Seibert, M. (1991) Biochemistry 30, 9615-9624], the Fe bound to the HAz site can also block the DPC donation site. On the other hand, electron donation by NH2OH to PSII still occurs in Fe-blocked membranes. Since hydroxylamine does not reduce the Fe [≥Fe(III)] specifically bound to the HAz site, NH2OH must donate to Yz· through its own site or directly to P680+.

Original languageAmerican English
Pages (from-to)5854-5864
Number of pages11
Issue number18
StatePublished - 7 May 2002

NREL Publication Number

  • NREL/JA-590-32609


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