Phosphotriesterase, carboxylation

Figure 12-24 Hypothetical event in the action of a phosphotriesterase. A carbamylated lysine (lower center), as well as a water molecule, bridge the two Zn2+ ions, which are held by imidazole and aspartate carboxylate groups. The bound H20 can be deprotonated to give the HO complex shown. The substrate may displace the HO - ion from the right-hand zinc and thereby move close to the bound HO which attacks as indicated. Based on Cd2+-containing structure and discussion by Benning et al.722...

F long thin barrel structure group of a-helices in the carboxyl-terminal loop between /7-strand 1 and a-helix 1 require divalent transition metal ion phosphotriesterase, adenosine deaminase... 

The 2-hydroxypropyl 4-nitrophenyl phosphate (HPNP) transesterification reactivity (Fig. 40, bottom) of binuclear zinc complexes of the dianionic ICIMP and trianionic BCIMP ligands (Fig. 62) have been compared.245 Notably, both of these ligands contain imidazole and carboxylate donors akin to protein-derived residues that are found as ligands to the zinc centers in phosphate-ester-hydrolyzing enzymes such as phosphotriesterase. The initial rate for HPNP transesterification promoted by [(ICIMP)Zn2(Ph2Ac)]C104 in 50% acetonitrile-water solution (tris buffer) was determined to be five times faster than the reaction promoted by [(BCIMP)Zn2(Ph2Ac)]. 

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