Fig. 3. Substrate/product binding in apo- and Zn-CA II.

The intermediate water (W_I) is colored in steel blue for clarity. The electron density (2F_o–F_c, blue) is contoured at 2.2σ. a, b At 20 atm of CO₂ pressure, apo-CA II shows clear binding of CO₂ without the need of Zn²⁺ ion. c Zn-CA II shows similar binding of CO₂ as in apo-CA II while maintaining tetrahedral metal coordination. d Upon CO₂ binding (white) in Zn-CA II, W_Zn is located at the center of the hypothetical tetrahedral arrangement made up of Zn²⁺ ion, Thr199-Oγ1, position (1) (close to W1), and position (2) (close to the carbon atom in CO₂). In this configuration, a hybridized lone pair in W_Zn directly faces CO₂ molecule at a distance, appropriate for efficient nucleophilic attack. Distance between the position (2) and C atom of CO₂ is merely 0.36 Å.