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  Reaction Steps:
  1. Start: Active Site
  2. CO₂ Binding
  3. Nucleophilic Attack
  4. HCO₃^- Dissociation
  5. H⁺ Dissociation/Shuttle
  6. His 64 Rotation/Flip
  7. Return to Start

  Reaction Sequence
    H-Bond Distances (Å)

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The numbered buttons show the sequence of steps in the hydration reaction:
  1. Start: Active Site The zinc is coordinated by the imidazole rings of three histidines (94, 96, and 119) and an OH^- ion. The geometry of the active site is tetrahedral.
  2. CO₂ Binding A water molecule (HOH338) is displaced by CO₂. The main chain NH of Thr 199 orients and polarizes the CO₂ molecule.
  3. Nucleophilic Attack The zinc-bound OH^- attacks the carbon of CO₂ to form HCO₃^-, labeled "BCT".
  4. HCO₃^- Dissociation A water molecule (HOH263) replaces the HCO₃^- product.
  5. H⁺ Dissociation/Shuttle The proton product dissociates from HOH263 and is transferred in three steps along a "wire" of H-bonded waters to His 64.
  6. His 64 Rotation/Flip The protonated His 64 side chain rotates from the "in" postion to the "out" position where it is exposed to bulk solvent on the enzyme exterior.
  7. Return to Start After releasing the H⁺ product, His 64 rotates back to the starting (in) postion. The enzyme is then ready for another cycle of catalysis. (Repeat the sequence at step 2.)
The rate limiting step in the reaction is H⁺ dissociation, i.e. steps 5-7 above.

"Reaction Sequence" This button shows all seven steps with a 1.0 sec delay between each.
  H-Bond Distances (Å) are between the waters in the proton dissociation pathway.

Carbonic Anhydrase Mechanism: Animation of the seven steps using chemical structures (in a pop-up window).