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Carbonic Anhydrase.

This page shows some interactive JSmol views of human carbonic anhydrase, a hydrolytic zinc metalloenzyme. Explore the structures for yourself:

The first figure, [CA 1], shows an overview of the metalloenzyme unit. Note that one side of the protein combrises a "wall" of β-sheet and the three His ligands are anchored there.
The [second figure [CA 2] shows the active site of the enzyme. The Zn²⁺ ion (green) is 4-coordinate and its coordination geometry is approximately tetrahedral. The ligands are 3 His residues and one H₂O/OH^-; the carbon dioxide is in the pocket but has not yet reacted. The coordinated H₂O/OH^- is hydrogen bonded to a Thr residue, which is itself hydrogen bonded to a Glu residue. This chain assists in deprotonating the water molecule.

Distances:

• H₂O/OH^- to Thr 2.63 Å
• Thr to Glu 2.52 Å
• Zn to O of O=C=O 3.18 Å
• H₂O/OH^- to C of CO₂ 2.77 Å

A later stage of the process is shown in figure [CA 3]. The OH^- has carried out a nucleophilic attack on the δ+ carbon atom of CO₂, resulting in formation of a bound HCO₃^- group (H atom not shown).

Finally, [CA 4] shows the van der Waals surface of the protein, the zinc and carbonate ions are shown in ball & stick style - note the restricted access to the active site. This channel provides the route for CO₂ to enter and HCO₃^- to leave.

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