Zinc-containing enzymes carbonic anhydrase

In mammals the bicarbonate ion, IICOT, is used as a buffer in blood and is catalysed by the zinc-containing enzyme carbonic anhydrase ... 

Zinc is the second most important of the essential trace elements for humans. It is a constituent of some enzymes, such as carbonic anhydrase. Zinc is sufficiently abundant that deficiencies of zinc are unknown. The highest levels of zinc are found in shellfish, which may contain 400 ppm. The level of zinc in cereal grains is 30 to 40 ppm. When acid foods such as fruit juices are stored in galvanized containers, sufficient zinc may be dissolved to cause zinc poisoning. The zinc in meat is tightly bound to the myofibrils and has been speculated to influence meat s water-binding capacity (Hamm 1972). 

Another way of bringing reactants into close proximity, which is encountered commonly in transition metal chemistry, is through metal ion complexation. The coordination of a reactant to a metal ion complex often activates its reactivity and can bring the reactant into close proximity with a second reactant or with a catalytic group. One example, shown in Fig. 6, is a zinc (11) complex of 1,5,9-triazacyclononane, as a model for the enzyme carbonic anhydrase, which contains a zinc (11) cofactor in its active site (4). In the aqua complex, the bound water molecule has a dramatically reduced pKa value of 7.3, which is similar to the pKa of the active site nucleophihc water. The corresponding cobalt (111) complex catalyzed ester hydrolysis at twice the rate because Co(lll) can coordinate both the hydroxide nucleophile and the ester carbonyl via a... 

Less than 10 years after the discovery of carbonic anhydrase in 1932, this enzyme was found to contain bound zinc, associated with catalytic activity. This discovery, remarkable at the time, made carbonic anhydrase the first known zinc-containing enzyme. At present, hundreds of enzymes are knovm to contain zinc. In fact, more than one-third of all enzymes either contain bound metal ions or require the addition of such ions for activity. The chemical reactivity of metal ions—associated with their positive charges, with their ability to form relatively strong yet kinetically labile bonds, and, in some cases, with their capacity to be stable in more than one oxidation state—explains why catalytic strategies that employ metal ions have been adopted throughout evolution. 

Carbonic anhydrase is an enzyme of central importance to the production of gastric acid. This enzyme, which contains zinc, accelerates the naturally occurring reversible reaction of CO2 with water. Before considering the mechanism of action of the enzyme, note the resonating structure of CO2. The carbon-to-oxygen bonds are polar the molecular structure can be represented by the resonance hybrids shown in Figure 2.54. Note that in two of the three forms shown the central carbon atom has a positive charge. 

Simulations of the native form of carboxypetidase A indicate that the zinc bound water forms a hydrogen bond with the carboxyl group of Glu270, positioning the water in a seemingly optimal orientation for nucleophilic attack on the substrate (Stote and Karplus 1995). A similar result, whereby the zinc plays a role in orienting residues important for the catalysis, was observed in the simulations of another zinc containing enzyme, carbonic anhydrase (Stote and Karplus 1995). The zinc also provides... 

The applications of these ligands have been limited to the work by Nakazawa et al.244 who found tris(pyrazolyl)methane titanium complexes to be high-activity catalysts for the polymerization of olefins, and the use of tris(pyrazolyl)methane zinc complexes to model zinc-containing enzymes, such as dihydrorootase and carbonic anhydrase.245 The structure of the free ligand HC(Me2pz)3 has also been reported.246... 

Triazinyl-containing sulfonamides 367 and 368, obtained from cyanuric chloride (2,4,6-trichloro-l,3,5-triazine) and amino-benzenesulfonamides 366, followed by treatment with water, MeNH2, or ROH, act as highly effective inhibitors of the zinc-containing enzyme, carbonic anhydrase (CA, EC 4.2.1.1) (Scheme 82) <2005BML3102>. 

Shown in Fig. 1 is the three-dimensional active-site structures of four representative zinc-containing enzymes carboxypeptidase and (3-lactamase belong to the hydrolase family, while carbonic anhydrase is one of... 

Fig. 1 Active site structures of four zinc-containing enzymes (a) carboxypeptidase A from bovine pancreas, (b) (3-lactamase from Bacteroides fragilis, (c) human carbonic anhydrase. and (d) horse liver alcohol dehydrogenase.

N.m.r. has been used to study the active site of other zinc-containing enzymes. In bovine carbonic anhydrase there is a single zinc co-ordination site available for Cl interaction, and the latter can be inhibited by CN and acetazolamide. C1 Resonance has also been used to investigate the environmental diflFerences of the zinc in carbonic anhydrase isozymes and the activation of pyruvate kinase with zinc, and to determine the pJ values in cobalt(u)-carbonic anhydrase. It is suggested that, although the TpK of aquozinc is about 9, the environment of the zinc ion in carbonic anhydrase B greatly increases the tendency of a zinc-bound water molecule... 

Recall from the "Chemistiy and Life" box in Section 2.7 that zinc is needed in our diets in trace amounts. Zinc is an essential part of several enzymes, the proteins that facilitate or regulate the speeds of key biological reactions. For example, one of the most important zinc-containing enzymes is carbonic anhydrase. This enzyme is found in red blood cells. Its job is to facilitate the reaction of carbon dioxide (CO2) with water to form the bicarbonate ion (HCOs ) ... 

Metallic elements are essential components of many important enzymes operating within our bodies. Carbonic anhydrase, which contains Zn, is responsible for rapidly interconverting dissolved CO2 and bicarbonate ion, HCO3. The zinc in carbonic anhydrase is ccx)idinated by three nitrogen-containing groups and a water molecule. The enzyme s action depends on the fact that the coordinated water molecule is more acidic than the bulk solvent molecules. Explain this fact in terms of Lewis add-base theory (Section 16.11). 

Carbonic Anhydrase (CA).—There has been continued interest in the mode of action of this widely found zinc-containing enzyme which catalyses the reversible interconversion of CO2 and HCOa". In a buffered solution not far from neutrality, the stoicheiometry of the reaction is... 

Prince and Woolley have reviewed the chemistry of the zinc-containing enzyme carbonic anhydrase with special reference to the part played by the metal. They point out that none of the results published to date shows beyond all doubt that the p Ta of ca. 7, which is a recurrent feature of carbonic anhydrase chemistry, is associated with the ionization of a zinc-bound water molecule (although it is difficult to offer an alternative explanation and the existence of this zinc aquo-complex seems to the authors to be a necessary condition for the activity of the enzyme). 

The ligand tris[2-(l-methylbenzimidazol-2-yl)ethyl] nitromethane (25) has been used in the formation of zinc complexes as models of the active site of carbonic anhydrase, and the formed complexes reveal affinity for the sulfonamide-containing enzyme inhibitor acetazolamide.248... 

Theoretical calculations have been carried out on a number of zinc-containing enzymatic systems. For example, calculations on the mechanism of the Cu/Zn enzyme show the importance of the full protein environment to get an accurate description of the copper redox process, i.e., including the electronic effects of the zinc ion.989 Transition structures at the active site of carbonic anhydrase have been the subject of ab initio calculations, in particular [ZnOHC02]+, [ZnHC03H20]+, and [Zn(NH3)3HC03]+.990... 

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