Ligands bidentate glutamate

Another contrast between the zinc proteases and the carbonic an-hydrases concerns the zinc coordination polyhedron. The carbonic an-hydrases ligate zinc via three histidine residues, whereas the zinc proteases ligate the metal ion through two histidine residues and a glutamate (bidentate in carboxypeptidase A, unidentate in thermolysin). Hence, the fourth ligand on each catalytic zinc ion, a solvent molecule, experiences enhanced electrostatic polarization in carbonic anhydrase II relative to carboxypeptidase A. Indeed, the zinc-bound solvent of carbonic anhydrase II is actually the hydroxide anion [via a proton transfer step mediated by His-64 (for a review see Silverman and Lindskog, 1988)]. 

Glu-252 (bidentate), and Asp-253. ASGPR has a Pro at the corresponding position of Glu-165 in MBL-A. The third calcium ion is unique to this protein. It is coordinated mainly by two glutamate residues, Glu-196 and Glu-227, both of which provide their two carboxyl oxygen atoms as bidentate ligands. The additional ligands come from the mainchain carbonyl of Val-190 and water. 

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