CO2/HCO3−-binding site of native and V143I CA II. The intermediate waters (WI and WI′) are coloured steel blue for clarity. The electron density (2Fo − Fc) is contoured at 1.5σ where not indicated otherwise. (a)–(d) Native CA II structures. (e)–(h) V143I CA II structures. WI′ at 7, 13 and 15 atm is contoured at 1.25σ and WI at 0 atm is contoured at 1.0σ. Partial occupancies of HCO3− and WDW were determined in V143I-0atm and V143I-7atm, and partial occupancies of HCO3− and CO2 were determined at the higher CO2 pressures (see the supporting information). The inset (red box) in V143I-0atm shows the difference map (Fo − Fc contoured at 3.0σ; green) when the HCO3− molecule is not included in the structure refinement.