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.