Carbonic anhydrase coordinated water

Several different carbonic anhydrases coordinate Zn + in their active sites using the amino acid side chains of His exclusively or of His and of Cys. Rationalize how the binding of water to the coordinated Zn " lowers the pK value of the water. 

X-ray diffraction studies on several forms of the enzyme have demonstrated that the active site is composed of a pseudo-tetrahedral zinc center coordinated to three histidine imidazole groups and either a water molecule [(His)3Zn-OH2]2+ (His = histidine), or a hydroxide anion [(His)3Zn-OH] +, depending upon pH (156,157). On the basis of mechanistic studies, a number of details of the catalytic cycle for carbonic anhydrase have been elucidated, as summarized in Scheme 22... 

The value of the tris(pyrazolyl)hydroborato complexes [TpRR ]ZnOH is that they are rare examples of monomeric four-coordinate zinc complexes with a terminal hydroxide funtionality. Indeed, [TpBut]ZnOH is the first structurally characterized monomeric terminal hydroxide complex of zinc (149). As such, the monomeric zinc hydroxide complexes [TpRR ]ZnOH may be expected to play valuable roles as both structural and functional models for the active site of carbonic anhydrase. Although a limitation of the [TpRR ]ZnOH system resides with their poor solubility in water, studies on these complexes in organic solvents... 

Water NMRD profiles acquired for other complexes and proteins always exhibit the same features of hexaaqua nickel(II). As an example, we report here the profile of the hexa-coordinate nickel(II)-substituted bovine carbonic anhydrase II 54,55) (Fig. 15). As in the aqua complex, (i) the low-field profile is flat, (ii) no dispersion appears, the cOg dispersion being quenched in S = 1 complexes with large static ZFS 56) (see Section I.A.5) and the... 

There are several types of -class CAs i.e., a-CA I-VII, reported in the literature, out of which the human carbonic anhydrase II (HCA II), the most extensively studied carbonic anhydrase, has an exceptionally high CO2 hydration rate and a wide tissue distribution 107). The HCA II comprises a single polypeptide chain with a molecular mass of 29.3 kDa and contains one catalytic zinc ion, coordinated to three histidine residues, His 94, His 96, and His 119. A tetrahedral coordination geometry around the metal center is completed with a water molecule, which forms a hydroxide ion with a pK value of 7.0 108). Quigley and co-workers 109,110) reported that the inhibition of the synthesis of HCO3 from CO2 and OH- reduces aqueous humor formation and lowers intra-ocular pressure, which is a major risk factor for primary open-angle glaucoma. 

XH NMR data of copper-carbonic anhydrase (CuCA) complexes in the presence of different anions indicated that water is present in the coordination sphere along with the anions (137). The three histidines, the anion, and the coordinated water molecule arrange themselves to maintain essentially a SQPY. His-94 would be in the apical position of the SQPY and two other histidine residues (His-96 and His-119) along with the anion and the coordinated water are positioned in the basal plane. Most likely the anion is present in the hydrophobic pocket or in the site and the coordinated water molecule is present in the C site or the hydrophilic binding site. 

The results of site-directed mutagenesis analysis of zinc ligands of higher plant p-carbonic anhydrase and of P. purpureum carbonic anhydrase have confirmed that zinc is essential for catalysis. X-ray fine structure data indicated that a water molecule is hydrogen bonded to the zinc-ligated Asp-151 and Asp-405. The water molecule is not directly coordinated to the zinc atom. A possible catalytic mechanism of C02 hydration cycle (211) has been proposed as given in Scheme 10. 

Roberts et al. reported a 27 kDa monomeric carbonic anhydrase, TWCAl, from the marine diatom Thalassiosira weissflogii (221). X-ray absorption spectroscopy indicated that the catalytic zinc is coordinated by three histidines and a water molecule, similar to the active sites of the a- and y-CAs (222). Also, the active site geometry is similar to that of a-CAs. Based on these results the catalytic mechanism is expected to be similar to that of the -class carbonic anhydrases. Tripp et al. (223) proposed that this TWCAl is the prototype of a fourth class carbonic anhydrase designated as 8-class CAs. In the... 

The iron(II)-iron(III) form of purple acid phosphatase (from porcine uteri) was kinetically studied by Aquino et al. (28). From the hydrolysis of a-naphthyl phosphate (with the maximum rate at pH 4.9) and phosphate binding studies, a mechanism was proposed as shown in Scheme 6. At lower pH (ca. 3), iron(III)-bound water is displaced for bridging phosphate dianion, but little or no hydrolysis occurs. At higher pH, the iron(III)-bound OH substitutes into the phosphorus coordination sphere with displacement of naphthoxide anion (i.e., phosphate hydrolysis). The competing affinity of a phosphomonoester anion and hydroxide to iron(III) in purple acid phosphatase reminds us of a similar competing anion affinity to zinc(II) ion in carbonic anhydrase (12a, 12b). 

Despite the fact that carbonic anhydrase was the first zinc metalloenzyme identified1233 and a good deal is known of its structure, there is still controversy about the nature of the various active-site species and the detailed mechanisms of their action. In particular, the identity of the group with a pXa of 7 that is involved in the mechanism, and the stereochemistry around the zinc ion during catalysis, are still in dispute. The various mechanisms proposed assume either ionization of a histidine imidazole group (bound or not to the zinc) and nucleophilic attack on C02 by the coordinated imidazolate anion,1273,1274 or ionization of the Znn-coordinated water and nucleophilic attack on C02 by OH. 1271 Many papers on this problem have appeared recently and the extensive literature is the subject of the several review articles referred to above. 

Water H R values have been measured for nickel(II)-substituted bovine carbonic anhydrase II [132,135] (Fig. 5.48), for which the following coordination polyhedron has been proposed [136,137] ... 

The study of zinc-aqua complexes as synthetic carbonic anhydrase models has shown a low coordination number and a hydrophobic environment to be prerequisites for a low pK value of the aqua ligand, which is essential for efficient enzyme function. The pK value in the case of 13 is expected to be greater than 10.7, which is the value determined for pentacoordinate [Zn(tren)(H20)](C104)2, in which the water ligand is more strongly bound and thus expected to be more acidic (cf. the discussion of bond lengths, above). Thus, 13 is not expected to show carbonic anhydrase-like reactivity. However, in related tetraazamacrocyclic systems it has been shown that upon... 

The active site of bovine carbonic anhydrase consists of a tetrahedrally coordinated zinc ion (1) [27] with a coordinated water molecule whose p(7.5) [28, 29] is con-... 

It is known that a vast variety of enzymes use metal ions in acid/base catalysts. In some cases the role of the metal is to activate water directly, e.g. Zn(OH)2 becomes Zn(OH ) in carbonic anhydrase, but in others it may be that the metal just forms a particularly constructive (useful) H-bond network, e.g. calcium in phospholipase A2 and in staph, nuclease. Substitution of one metal by other metals is now a critical test of the precision of the catalytic site and we know that nickel does not substitute for zinc in carbonic anhydrase, although it binds, and that Sr(II) has a different activity in lipases and nucleases from Ca(II). It is the water in the coordination sphere which is partly responsible for these changes. 

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