Water site-bound

Chemisorbed water molecules bound direcdy to surface Ti" " ions amounts to about 2—3 / nm for mtile HO2. Most if not all the Ti" " sites are occupied. 

Kimura and co-workers have synthesized a series of alkoxide complexes with the alcohol functionality as a pendent arm.447 674 737 A zinc complex of l-(4-bromophenacyl)-l, 4,7,10-tetraaza-cyclododecane was also synthesized by the same workers to mimic the active site of class II aldolases. The X-ray structure shows a six-coordinate zinc center with five donors from the ligand and a water molecule bound. The ketone is bound with a Zn—O distance of 2.159(3) A (Figure 12). Potentiometric titration indicated formation of a mixture of the hydroxide and the enolate. Enolate formation was also independently carried out by reaction with sodium methoxide, allowing full characterization.738... 

In Cu,Zn-SOD the copper atom is bound to three histidine groups and the zinc is bound to two histidines and an aspartate oxygen atom (Tainer et al., 1982, 1983) (Fig. 34). The Cu"-Zn distance is 6.3 A. The zinc-ligand geometry is tetrahedral, with a strong distortion toward a trigonal pyramid with aspartic acid at the apex. The coordination of the Cu(II) is tetrahedrally distorted square planar. The axial position of copper is more open on the solvent side than on the protein side probably, water is bound there. The Zn(II) is buried, while the Cu(II) site is solvent accessi-... 

Figure 3.4 (a) Catalytic efficiencies of subtilisin Carlsberg in THF with different water contents, (b) Polarity of subtilisin s active site (as indicated by the hyperfine splitting constant, A0, of the active-site bound spin label) with different water contents [54]. 

The active site contains two Zn2+ ions and one Mg2+ ion which are held by imidazole and carboxylate groups. The inorganic phosphate in an enzyme-product complex is bound to both zinc ions (Fig. 12-23). The Ser 102 side chain is above one Zn. In the enzyme-P intermediate it would be linked to the phospho group as an ester which would then be hydrolyzed, reversibly, by a water molecule bound to Zn.712 713a This water presumably dissociates to Zn+-OH and its bound hydroxyl ion carries out the displacement. This reaction may be preceded by a proton transfer to an oxygen atom of the phospho group.714... 

Figure 13-4 (A) Stereoscopic view of the active site of mitochondrial aconitase with a molecule of L-isocitrate placed by modeling next to the Fe4S4 cluster. This cluster still has a water molecule bound to one iron atom as in the free enzyme.83 Courtesy of C. D. Stout. (B) Interpretive diagram.

There is general agreement that carbonic anhydrase activity is linked to the zinc ion and its ligands. At the active site water is bound to the zinc but the state and involvement of the water in the actual catalysis is a matter of controversy. According to a widely accepted opinion the zinc-bound water is ionized and the activity is a function of pH1S6. ... 

The sheets formed by the apices of the tetrahedra are completed by hydroxyls and magnesium ions in octahedral coordination link the sheets. The one structural arrangement has nine octahedral sites and the other only eight. Both structures have channels on both sides and top and bottom of each ribbon which contains water molecules (zeolitic water). Additional water is bound to the edge of the ribbons and hydroxyls occur in the structure proper. 

The site bound ions accounts for its hydration state and are grouped either as outer-sphere or as inner-sphere [9, 11, 14]. In the later case, it is assumed that the water molecules in the hydration shell do not participate. The ions directly interact with the phosphate charges and anionic ligands [9, 11, 14], Since both outer- and inner-sphere interactions lead to formation of ion pairs, site bound ions are describable in terms of an association constant satisfying the law of mass action [9, 11, 14—16]. 

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