Optimized structures of ‘second coordination sphere’ in the S3 state; (a) W1&W2-OH− fixing the α-carbon and its two nearest atoms; (b) W1&W2-OH− fixing the α-carbon atoms of Asp61, Lys317, His337 and Arg357; (c) W2&inserted water-OH− fixing the α-carbon and its two nearest atoms; (d) W2&inserted water-OH− fixing the α-carbon atoms of Asp61, Ly317, His337 and Arg357. A red broken line is used if Mn—O bond distance is between 2.0 and 2.5 Å or Ca—O bond distance is between 2.8 and 3.0 Å. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)