| Research Abstract |
Effects of photoinhibition on the redox properties of cytochrome b-559 were studied with NH_2OH-treated photosystem II membranes, which are depleted of the water-oxidizing complex. The membranes contained three redox forms (HP-, IP-and LP-forms) of cytochrome b-559, with Em valuse of +435, +237 and +45mV,respectively. A novel intermediate-potential form of cytochrome b-559 was generated during photoinhibition on the donor side of photosystem II : photoinhibitory illumination (7,000 muEm-^2s^1) for 1 min induced a 28% decrease in the level of the HP-form, with concomitant generation of an intermediate-potential (IP'-) form whose Em value was about+350mV.Prolonged illumination (10min) resulted in complete loss of the HP-form and an apparent increase in the level of the IP-form. After further photoinhibitory treatment (60min), complete loss of the IP'-form was observed and levels of the IP- and LP-forms each increased to about 50% of the total amount of cytochromeb-559. Kinetic analysis of these data led to the conclusion that the HP-form is converted to the LP-form via two intermediate-potential forms (IP' and IP), and that IP'-form appears only at the early phase of photoinhibition.
Kinetical analysis of the decays of the HP- and IP'-forms shows that the conversion of the HP- to the IP'-form occures in parallel with the photoinactivation of Tyr_Z, and that the disapperance of the IP'-form reflects the fate of Tyr_D. On these findings, we propose that the HP- and IP'-forms of cyotochrome b-559 donate electrons to Tyr_Z, respectively, to suppress their overoxidation, and so prevent the photoinhibition of the phtosystem II.
|
