pH-Dependent Regulation of Electron Flow in Photosystem II by a Histidine Residue at the Stromal Surface.

In photosystem II (PSII), the secondary plastoquinone electron acceptor Q B functions as a substrate that converts into plastoquinol upon its double reduction by electrons abstracted from water. It has been suggested that a histidine residue, D1-H252, which is located at the stromal surface near Q B , is involved in the pH-dependent regulation of electron flow and proton transfer to Q B . However, definitive evidence for the involvement of D1-H252 in the Q B reactions has not been obtained yet. Here, we studied the roles of D1-H252 in PSII using a cyanobacterial mutant, in which D1-H252 was replaced with Ala. Delayed luminescence (DL) measurement upon a single flash showed a faster Q B - decay at higher pH in the thylakoids from the wild-type strain due to the downshift of the redox potential of Q B [ E m (Q B - /Q B )]. This pH dependence of the Q B - decay was lost in the D1-H252A mutant. The experimental E m (Q B - /Q B ) changes were well reproduced by the density functional theory calculations for models with different protonation states of D1-H252 and with Ala replaced for H252. It was further shown that the period-four oscillation of the DL intensity by successive flashes was significantly diminished in the D1-H252A mutant, suggesting the inhibition of plastoquinone exchange at the Q B pocket in this mutant. It is thus concluded that D1-H252 is a key amino acid residue that regulates electron flow in PSII by sensing pH in the stroma and stabilizes the Q B binding site to facilitate the quinone exchange reaction.