Zn Coordinative Bonds fold in tertiary structure polypeptide chain conecting residues His96,His94,His119 and water Wat-+263+ oxygen H₂O after dissociation of HCO₃^- in CA active site pocket 2VVB .
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.....-His96-His94-Zn-His119--Wat-+263+-.....

The active site of HCAII. The zinc ion is tetrahedrally coordinated by 3 histidines His-94, His-96, His-119 and catalytic water Wat-+263+. The deep water Wat-338sits in a hydrophobic pocket lined by Leu-198, Trp-209, Val-143, Val-121 at the bottom of the active site. Wat-318 is in a hydrophilic environment toward the mouth of the active site cone. The proton shuttle His-64, shown in both “in+H⁺” and “out (deprotonated)” positions, is linked via Wat-292, Wat-318 to the catalytic water Wat-+263+ after dissociation of HCO₃^- in CA active site pocket 2VVB. Hydrogen bonds are depicted as dotted lines, and waters are labeled with numbers only. Numbering is according to PDB code 2CBA.
Thr-199, a key residue of the second coordination sphere, is important for enzyme activity; together with Thr-200, it is involved in a finely tuned network of hydrogen bonds leading toward the solvent-exposed His-64, which is located at the entrance of the active-site channel. Thr-199 forms a hydrogen bond to the zinc-bound water/hydroxide Wat-+263+ after dissociation of HCO₃^- in CA active site pocket 2VVB, thereby orienting the 2 lone hydroxide electron pairs toward the 2 neighboring water molecules Wat-318 and Wat-338 that reside on potential substrate-binding sites. Although both positions are suitable for a nucleophilic attack of the zinc-bound hydroxide ion Zn²⁺-OH^-, their environments differ substantially.
Wat-318 is located in a hydrophilic environment on the way out of the active-site cone, whereas the “deep water” Wat-338 is located in a hydrophobic pocket that is lined by the following side chains: Leu-198, Trp-209, Val-143, Val-121 (Fig. 1). A wealth of indirect evidence indicates that the deep water Wat-338 position serves as the primary substrate-binding site (1–12), although the atomic details of enzyme and substrate–product interaction have remained elusive until now. The generally accepted catalytic mechanism of carbonic anhydrase is described by a 3-step kinetic scheme: (I) a ZnOH^- moiety catalyzes the interconversion of CO₂ to HCO₃^-, leaving a water molecule as the fourth zinc ligand (Eq. 1); (II) a proton is then transferred from the zinc-bound water to the imidazole ring of His-64 (Eq. 2); and (III) this proton then leaves His-64 for the surrounding water H₂O (Eq. 3).

His64-E-Zn²⁺-OH^-+CO₂+H₂O<=>His64-E-Zn²⁺-OHCO₂^-+H₂O(1a)
His64-E-Zn²⁺-OHCO₂^-+H₂O<=>His64-E-Zn²⁺-H₂O+HCO₃^- (1b)
His64-E-Zn²⁺-H₂O<=>H⁺-His64-E-Zn²⁺-OH^-(2);
H⁺-His64-E-Zn²⁺-OH^-+H₂O<=>His64-E-Zn²⁺-OH^-+H₃O⁺(3)
The pKa values for both the zinc-bound water and the proton shuttle (His-64) are close to 7.

off Water On Water Wat-338 is located in a hydrophobic pocket that is lined by the following side chains: Leu-198, Trp-209, Val-143, Val-121 (Fig. 1).
Wat-318 is in a hydrophilic environment Thr200, Thr199, L198, W263, W292 toward the mouth of the active site cone.
The proton shuttle His-64, shown in both “in+H⁺” and “out (deprotonated)” positions, is linked via Wat-292 and Wat-318 to the catalytic water Wat-+263+ after dissociation of HCO₃^- in CA active site pocket 2VVB.
Backbone traces conect amino acids alpha carbons CA with green color trace
.....-His96-His94-Zn-His119--Wat-+263+.....in tertiary structure of polypeptide
chain
Coordinative bonds -His96-His94-Zn--His119--Wat-+263+- conect in tertiary structure of polypeptide chain fragment
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