View of hydrogen atoms in the X-ray structure. (a) Fixed H atoms: Trp 131 side-chain. (b) Mobile hydrogen atoms: Ile 177. (c) Water molecules: water 614 and 615 and their hydrogen bond network. Fobs-Fcalc omit map in blue. The contour level is 2.6σ in a), 2.7σ in b) and 2.5σ in c). Hydrogen atoms have been omitted in the structure factor calculation of the maps.