Zn Coordinative Bonds fold in tertiary structure polypeptide chain conecting residues His96,His94,His119 and acetazolamide AZM nitrogen AZM-N after dissociation of H⁺ in CA active site pocket 4G0C .
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.....-His96-His94-Zn-His119--Wat-+263+-.....


On Lined by the following side chains: Leu-198, Trp-209, Val-143, Val-121.
Water-1143 is in a hydrophilic environment Thr200, Thr199, L198, toward the mouth of the active site cone.
The proton shuttle His-64, shown in “in (protonated)” positions, is sighlencing proton shuttle to the catalytic proton dissociation in CA active site pocket 4G0C.
Coordinative bonds -His96-His94-Zn--His119--AZM- conect in tertiary structure of polypeptide chain.
Transfere Grafic File 4g0cZn.tgf Integrated Scientific Information System deposit DATA:2VVBZnPoc.tgf
The active site of HCAII. The zinc ion is tetrahedrally coordinated by 3 histidines His-94, His-96, His-119 and inhibitor AZM. Hydrophobic pocket lined by Leu-198, Trp-209, Val-143, Val-121 at the bottom of the active site. Water is in a hydrophilic environment toward the mouth of the active site cone. Hydrogen bonds are depicted as dotted lines, and waters are labeled with numbers only. Numbering is according to PDB code 4G0C. The pKa values for both the zinc-bound water and the proton shuttle (His-64) are close to 7.
The generally accepted catalytic mechanism of carbonic anhydrase is described by a 4-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)
Inhimition page 2 point:23: http://aris.gusc.lv/06Daugavpils/Research/CAS.pdf
H⁺-His64-E-Zn²⁺-OH^-+BNH₂<=>His64-E-Zn²⁺salt bridge^-BN^-H+H₂O
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