| Research Abstract |
Illumination of chloroplast thylakoids activates CF_0CF_1 to catalyze both ATP synthesis and ATP hydrolysis and changes the conformation of CF_0CF_1. I have previously shown that illumination of thylakoids changes the conformation of CF_0CF_1 to increase the reactivity of Lys-109 of the epsilon subunit of CF_1. By measuring the change in the epsilon-Lys-109 reactivity, I have demonstrated four distinct conformations of CF_0CF_1, E_L, E_M, E_<H^*>, and E_H, and clarified the light-induced activation process of CF_0CF_1.
After the onset of illumination, rapid formation of DELTApsi changes the conformation of CF_0CF_1 from E_L (E_L-ADP) to E_<H^*> (E_<H^*>-ADP) through E_M (E_M-ADP). The ADP binding site of E_L is half closed and E_L-bound ADP is unexchangable. E_M and E_<H^*> have high affinities for ADP. Therefore, E_M-bound and E_<H^*>-bound ADP do not release from E_M and E_<H^*>, respectively.
However, they exchange with the exogenously added ADP. Formation of DELTApH (or acidification of the lumen of thylakoids) changes the conformation of CF_0CF_1 form E_<H^*> (E_<H^*>-ADP) to E_H. E_H has a low affinity for ADP. Therefore, the release of bound ADP takes place with this change. In the postillumination dark, rapid decay of DELTApsi changes the conformation of CF_0CF_1 from E_H to E_M. With the decrease in DELTApH, CF_0CF_1 changes from E_M to E_L. E_M (E_M-ADP) and E_<H^*> (E_<H^*>-ADP) are ATP synthetically active. E_H is ATP synthetically less active. E_L is ATP synthetically inactive E_H, E_M, and E_L (ADP-free forms) are all ATP hydrolytically active.
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