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Figure 6
Proposed mechanism of water-network restoration. (a) In hCA II with fully bound CO2, WZn exists as OH− and nucleophilic attack occurs to form bicarbonate. In this state, only WI is present and not W1, suggesting that the water network for proton transfer is broken. (b) The sequential events for water-network restoration as the product bicarbonate leaves the active site. After the intermediate water WI fills the places for W1 and WDW, and these W1 and WDW waters or WI can move to the WZn position. WDW also fills the position for W′DW. Note that four waters (W1, WZn, WDW and W′DW) are eventually filled in from WI during this water-network restoration process. A newly found intermediate water W′I is located between WI and the outside bulk solvent, is stabilized by the entrance-conduit waters and seems to facilitate the fast charging process of WI. (c) Only after the water network is restored can proton transfer can occur from WZn to the outside through W1/W2/W2′/His64in/His64out. Now, with CO2 binding, hCA II is ready for the next catalytic turnover.

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