Catalytic reactions of the proton and electron transfers occurring at the oxygen-evolving complex (OEC) of photosystem II during the S-2-S-3 transition were investigated by the quantum mechanics/molecular mechanics (QM/MM) methodology. Two favorable reaction pathways were elucidated. Both reactions start by moving the Ca-bound water (W3) to the vacant Mn(III) coordination at the left-opened (L) or right-opened (R) form. The former reaction pathway, in which W3 coordinates to the Mn4 at the S-2-L form, has lower activation barriers than the latter. Thus, easier proton transfers from W3 to the Tyrl 61 phenol anion can be performed. (C) 2015 Elsevier B.V. All rights reserved.