Conformational Changes and H-Bond Rearrangements during Quinone Release in Photosystem II.

In photosystem II (PSII) and photosynthetic reaction centers from purple bacteria (PbRC), the electron released from the electronically excited chlorophyll is transferred to the terminal electron acceptor quinone, QB. QB accepts two electrons and two protons before leaving the protein. We investigated the molecular mechanism of quinone exchange in PSII, conducting molecular dynamics (MD) simulations and quantum mechanical/molecular mechanical (QM/MM) calculations. MD simulations suggest that the release of QB leads to the transformation of the short helix (D1-Phe260 to D1-Ser264), which is adjacent to the stromal helix de (D1-Asn247 to D1-Ile259), into a loop and to the formation of a water-intake channel. Water molecules enter the QB binding pocket via the channel and form an H-bond network. QM/MM calculations indicate that the H-bond network serves as a proton-transfer pathway for the reprotonation of D1-His215, the proton donor during QBH-/QBH2 conversion. Together with the absence of the corresponding short helix but the presence of Glu-L212 in PbRC, it seems likely that the two type-II reaction centers undergo quinone exchange via different mechanisms.