Zinc complexes carbonic anhydrase

Studies on di- and trinuclear metal complexes bridged by carbonate or hydrogencarbonate are mainly noted from three areas (i) fixation and activation of C02 from the viewpoint of environmental protection 462 164 (ii) functional models for zinc-containing carbonic anhydrase (Volume 8) and (iii) the relationship between the bridging modes of carbonate and magnetic properties. 

The Bronsted relationship can be strictly accurate only over a certain range of acid and base strengths. When has diffusion-controlled values, which of course cannot be exceeded, the linear plot of log k/ y vs log must level off to a zero slope, that is a = 0. As well as being reported, although rarely, in simple metal complexes, the resultant curvature in the Bronsted plot is also shown by the zinc enzyme carbonic anhydrase (Chap. 8. Zn(II)). In... 

Subsequent to CO2 association in the hydrophobic pocket, the chemistry of turnover requires the intimate participation of zinc. The role of zinc is to promote a water molecule as a potent nucleophile, and this is a role which the zinc of carbonic anhydrase II shares with the metal ion of the zinc proteases (discussed in the next section). In fact, the zinc of carbonic anhydrase II promotes the ionization of its bound water so that the active enzyme is in the zinc-hydroxide form (Coleman, 1967 Lindskog and Coleman, 1973 Silverman and Lindskog, 1988). Studies of small-molecule complexes yield effective models of the carbonic anhydrase active site which are catalytically active in zinc-hydroxide forms (Woolley, 1975). In addition to its role in promoting a nucleophilic water molecule, the zinc of carbonic anhydrase II is a classical electrophilic catalyst that is, it stabilizes the developing negative charge of the transition state and product bicarbonate anion. This role does not require the inner-sphere interaction of zinc with the substrate C=0 in a precatalytic complex. 

The ability of zinc in carbonic anhydrase to become five-coordinate is also confirmed by the structural studies on enzyme-inhibitor complexes discussed in Section 62.1.4.2.1. There is much evidence for the coordination of anionic inhibitors to the metal, while the competitive inhibitor imidazole gives a five-coordinate centre. Sulfonamides are powerful inhibitors which bind directly to the zinc and also interact with the protein. The sulfonamide acetazolamide has significant affinity for the apoenzyme. It is probable " that the first interaction between the enzyme and aromatic sulfonamides is a hydrophobic interaction between the aromatic ring and residues in the active site cavity, followed by ionization of the SO2NH2 group prior to complex formation. Sulfonamides only bind to the zinc and cobalt enzymes, i.e. the two metals that give an active enzyme. 

The importance of coordination in the biochemistry of essential metallic elements may be illustrated by numerous examples of metal complexes of which the following are representative the iron complex hemoglobin and numerous enzymes containing the heme and related structures such as catalases, peroxidases and cytochromes and the iron-containing proteins ferritin, transferrin, and hemosiderin the zinc complexes zinc-insulin, carbonic anhydrase and the carboxypeptidases the cobalt complex vitamin B12 the copper complex, ceruloplasmin the molybdenum-containing enzymes, xanthine oxidase, and nitrate reductase DNA-metal ion complexes. 

Two model studies have been reported by Prince and Woolley which have a bearing on the role of the zinc in carbonic anhydrase. A macrocycle (4) has been synthesized which, when co-ordinated to zinc, produces a complex which also contains one water molecule in the inner co-ordination sphere this water molecule has a pXa of ca. 8—a. value considerably closer to that postulated for carbonic anhydrase (ca. 7) than is the value for hexa-aquozinc... 

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... 

The zinc acetate complex of tris(3-/-butyl-5-methylpyrazol-l-yl)borate was prepared as a structural model for carbonic anhydrase and comparison with the enzyme active site structures confirmed that the complexes are excellent structural models.239 A mononuclear zinc hydroxide complex can also be formed with the tris(pyrazolyl) borate ligand system as a structural model for carbonic anhydrase.240... 

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... 

There has been particular recent interest in zinc nitrate complexes as coordination models for bicarbonate binding in carbonic anhydrase. The mono- or bidentate coordination modes have been studied with tris-pyrazolyl borate complexes and can be rationalized in the context of the enzyme activity.433 Caution in this comparison is introduced by ab initio calculations on these model systems demonstrating both monodentate and bidentate coordination energy minima for nitrate binding to zinc 434... 

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

Synthesis of functional models of carbonic anhydrase has been attempted with the isolation of an initial mononuclear zinc hydroxide complex with the ligand hydrotris(3-t-butyl-5-methyl-pyrazolyl)borate. Vahrenkamp and co-workers demonstrate the functional as well as the structural analogy to the enzyme carbonic anhydrase. A reversible uptake of carbon dioxide was observed, although the unstable bicarbonate complex rapidly forms a dinuclear bridged complex. In addition, coordinated carbonate esters have been formed and hydrolyzed, and inhibition by small ions noted.462 A number of related complexes are discussed in the earlier Section 6.8.4. 

The engineering of zinc-binding sites in a-helical peptides, where metal binding stabilizes protein tertiary structure, has been reported by Handel and DeGrado (1990). In these experiments zinc-binding sites are incorporated into a dimeric helix-loop—helix peptide (H3 2) and a protein composed of four helices connected by three short loop sequences (H3 4). a model of one subunit of the H3 2 dimer is found in Fig. 47. In addition to metal complexation by two histidine residues at positions n and n+4 of one a helix, the metal is coordinated by a third histidine residue of an adjacent a helix. The composition of the zinc coordination polyhedron is like that of carbonic anhydrase (i.e., Hiss), and spectroscopic results suggest that all three histidine residues are involved in zinc complexation. This work sets an important foundation... 

Zn-organometallic compound and the complex was synthesized with the aim of modeling the active site of carbonic anhydrase during its first catalytic step, the acquisition of the NMR properties of nucleus at this Zn complex could be further exploited in studies where the zinc ion would be replaced by a Zn nucleus. 

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