Fig. 7. Comparative representations of the crystal structures of 3RGW (Re) and ; 4C3O (SeHyd-5), focusing on the key His229 and Glu residues (72 in ; 3RGW, 73 in ; 4C3O, carbon atoms black) in relation to the proximal and Ni–Fe clusters (Ni black, Fe magenta), the putative proton reservoir including two water molecules, and a water molecule (HOH3) that is hydrogen bonded to O of a cysteine bridge in the NiFe site and to mainchain O of Glu. In ; 3RGW the Glu side-chain is folded up with hydrogen bonds to O of Pro230 and HOH1, while in ; 4C3O it is folded down with different hydrogen bonds to O of Pro230 and HOH3.