Movie frames for X-ray-induced ligand turnover in AcNiR, showing the conversion of nitrite to nitric oxide. The proximal and gatekeeper positions of Asp98 are modelled in the electron-density maps. (a) ds1 with dual-occupancy nitrite and a water molecule, (b, c) ds2, ds3 with dual-occupancy nitrite, (d) ds4 with single-occupancy nitrite in the side-on conformation, (e) ds11 with dual-occupancy nitrite and nitric oxide with water, (f–i) ds12–ds15 with dual-occupancy nitrite and nitric oxide, (j) ds16 with dual-occupancy nitrite and nitric oxide plus water, (k, l, m) ds17–ds24 with nitric oxide, (n) ds30 with nitric oxide and single-conformation Asp98 in the proximal position, (o) ds40 with bound water. Asp98, His100, His135, nitrite and nitric oxide are represented as sticks and water molecules and Cu atoms as spheres. 2Fo − Fc density is contoured in the range 0.53–0.36 e Å−3.