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Figure 1
Surface rendition of hCA II depicted using the ultrahigh-resolution (0.9 Å) crystal structure of 7.0 atm CO2 hCA II. The hydrophobic and hydrophilic regions are coloured green and yellow, respectively. (a) The overall hCA II shows the CO2-binding active site that is located at a depth of 15 Å from the surface and is open to the outside bulk solvent through an entrance (the entrance diameter is ∼7–10 Å and it is referred to as the `entrance conduit' in this study). The substrate and product (CO2/bicarbonate) of hCA II as well as water molecules can pass in and out through this open entrance conduit. (b) A closer view of the active site (Zn, CO2 binding site and His64) is shown through the entrance conduit with a 2Fo − Fc map (in blue) contoured at 2.5σ (for His64) and 5.0σ (others). The isolated electron density of the C atom of CO2 is clearly visible in this ultrahigh-resolution structure. The protein surface around His64 is removed and the proton-transfer water network (WI/W2/W3a/W3b) is not shown for clarity. The proton transfer during the catalytic activity is thought to occur via His64 through the proton wire rather than through the open entrance conduit.

IUCrJ
Volume 5| Part 1| January 2018| Pages 93-102
ISSN: 2052-2525
https://doi.org/10.1107/S2052252517017626