Theoretical Study on the Role of Ca²⁺ at the S₂ State in Photosystem II

In photosynthesis, calcium is crucial for oxygen evolution. In the absence of Ca²⁺, the Kok cycle has been proven to stop at S₂ with Yz•. To explore the reason, photosystem II (PSII) S₂ models (in total 32452 atoms) with different metal ions (Ca²⁺, Sr²⁺, and K⁺) or without Ca²⁺ involved in the oxygen evolution complex (OEC) have been theoretically studied based on the previous dynamic study of PSII. It is found that the portion of the Mn1 d-orbital decreases in the highest occupied molecular orbitals for Ca²⁺-depleted PSII. This feature is unfavorable for the electron transfer from the OEC to the Yz•. Furthermore, the proton donor–acceptor distance was found elongated by the alternation of the binding water in the absence of Ca²⁺. The isolated vibrational modes of the key water molecules along the path and their high frequency of the OH stretching modes also suggested the difficulty of the proton transfer from the OEC toward the proton exit channel. This work provides one mechanistic explanation for the inactivity of Ca²⁺-depleted PSII