A possible role of redox-active histidine in the photoligation of manganese into a photosynthetic O2-evolving enzyme.
Our previous experiments with a histidine modifier suggest that in Mn-depleted photosystem (PS) II a histidine residue is photooxidized and charge recombination between the oxidized histidine and QA-emits the thermoluminescence AT-band [Ono, T., & Inoue, Y. (1991) FEBS Lett. 278, 183-186]. By use of the AT-band as an index for histidine oxidation, and EPR signals IIf and IIs as indexes for tyrosine oxidation, we studied the role of this putative redox-active histidine in the photoactivation of the O2-evolving enzyme in NH2OH-treated PSII. The following results have been obtained. (i) Strong-light photoinhibition of NH2OH-treated PSII quickly impaired both capabilities of photoactivation and AT-band emission with almost the same half-inhibition time of 1-2 s, while signal IIf was well retained and signal IIs was not affected at all after complete loss of photoactivation capability. (ii) The capability of exogenous Mn2+ photooxidation was relatively sensitive to strong-light photoinhibition, but DPC was relatively sensitive to strong-light photoinhibition, but DPC photooxidation was highly resistant. (iii) Weak-light photoinhibition simultaneously impaired the capabilities of photoactivation, AT-band emission, and signal IIf with the same half-inhibition time of 1 min, leaving signal IIs unaffected. (iv) It was inferred that the putative redox-active histidine is essential for the photooxidation of coordinated Mn2+, the probable initial step of photoactivation, and its photodamage results in the loss of the capabilities of photoactivation and AT-band emission. Based on these, a scheme of electron transfer on the donor side of PSII involving histidine oxidation via Z+ is proposed.
Ono
T
T
Solar Energy Research Group, Institute of Physical and Chemical Research, Saitama, Japan.
Inoue
Y
Y
D015086:2,6-Dichloroindophenol / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D006639:Histidine / Q000378:metabolism*; D008345:Manganese / Q000378:metabolism*; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism*; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism; D045332:Photosystem II Protein Complex
D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D008345:Manganese; D006639:Histidine; D015086:2,6-Dichloroindophenol; D010088:Oxidoreductases; D010100:Oxygen