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 Q(A)- emits the thermoluminescence A(T)-band [Ono, T., & Inoue, Y. (1991) FEBS Lett. 278,183-186]. By use of the A(T)-band as an index for histidine oxidation, and EPR signals II(f) and II(s) 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 A(T)-band emission with almost the same half-inhibition time of 1-2 s, while signal II(f) was well retained and signal II(s) 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 photooxidation was highly resistant. (iii) Weak-light photoinhibition simultaneously impaired the capabilities of photoactivation, A(T)-band emission, and signal II(f) with the same half-inhibition time of 1 min, leaving signal II(s) 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 A(T)-band emission. Based on these, a scheme of electron transfer on the donor side of PSII involving histidine oxidation via Z+ is proposed.