Zinc monohydroxide

A combination of steady-state and presteady sate kinetic and spectral and chemical modification studies led to a mechanistic scheme in which inhibition occurs through the binding of the zinc-monohydroxide species to the active EH species of the enzyme (Scheme 2) The pH independent constant for the inhibition by zinc is 0.71 J,M. The derived p/fa of 6 for the inhibition studies agrees with the corresponding value obtained in peptide hydrolysis experiments for the group, EH2, whose ionization leads to formation of the catalytically active form of the enzyme. 

On the contraiy, evidences were presented vdiich supports the formation of a cationic complex between acrylamide and zinc monohydroxide having the structure ... 

In water, the free zinc ion is thought to coordinate with six water molecules to form the octahedral aquo ion (Zn(H20)6)2+ in the absence of other complexing or adsorbing agents (Spear 1981). In freshwater, zinc exists almost exclusively as the aquo ion at pH >4 and <7 (Campbell and Stokes 1985). In freshwater at pH 6, the dominant forms of dissolved zinc are the free ion (98%) and zinc sulfate (2%) at pH 9 the dominant forms are the monohydroxide ion (78%), zinc carbonate (16%), and the free ion (6% USEPA 1987). In typical river waters, 90% of the zinc is present as aquo ion, and the remainder consists of ZnHC03+, ZnC03, and ZnS04 (Spear 1981). 

The rate of hydrolysis of bis(4-nitrophenyl) phosphate is approximately 10-fold higher for a binuclear Zn2(OH)2 complex of the L22 ligand (Fig. 52) versus a mononuclear zinc hydroxide complex of the L23 ligand at 308 K.105 In this system, a monohydroxide binuclear zinc complex (Zn2(p-OH)) does not promote phosphate diester hydrolysis. The 10-fold rate enhancement found for the reaction involving the binuclear Zn2(OH)2 complex was explained via cooperative interaction between the zinc centers. As shown in Fig. 54, the reaction is proposed to take place via attack of a terminal Zn-OH moiety on a phosphate diester substrate that is interacting with both zinc centers. This suggests a cooperative role for the two zinc centers in the phosphate diester hydrolysis reaction. 

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