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Figure 3
Rotameric states of His64 and solvent positions at different internal CO2 pressures. (a) 15 atm CO2 hCA II, (b) 7.0 atm CO2 hCA II, (c) 2.5 atm CO2 hCA II, (d) 15 atm CO2 hCA II – 50s, (e) 15 atm CO2 hCA II – 1h. The electron density (2Fo − Fc) for W′I in (d) is contoured at 1.0σ. In all other cases, the electron density (2Fo − Fc) for His64 and the electron density (2Fo − Fc) for waters are contoured at 1.5 and 1.3σ, respectively. The intermediate waters WI and W′I are coloured light grey and the entrance-conduit water W′EC1 is coloured cyan for clarity. As the internal CO2 pressure decreases, W2′ gradually dissipates and the His64 side chain shifts from the `out' to the `in' position from (a) to (e). The intermediate water, WI, is clearly observed in (a) and the electron density gradually subsides (b, c) and finally disappears (d, e). In accordance to the decrease in WI, electron density for W1, which is not observable in (a), appears in (b) and subsequently increases gradually (c, d, e). When the models are refined with partial occupancy, the W1 occupancies are 0.8 in (b) and 0.9 in (c). Interestingly, the electron density for the newly observed intermediate water W′I increases gradually from (a) to (c), but decreases in (d) and disappears in (e). The measured distance between WI and W1 in (b) and (c) is 2.0 Å. The electron density for W3a is well isolated in all cases, but W3b shows an alternate position W3b′ in (a) which grows in (b) but subsequently disappears (c, d, e).

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