Hydration of carbon dioxide

Carbonic anhydrase is an enzyme that catalyzes the hydration of carbon dioxide to bicarbonate The uncatalyzed hydration of carbon dioxide is too slow to be effective m transporting carbon dioxide from the tissues to the lungs and so animals have devel oped catalysts to speed this process The activity of carbonic anhydrase is remarkable It has been estimated that one molecule of this enzyme can catalyze the hydration of 3 6 X 10 molecules of carbon dioxide per minute... 

Car bonic acid is formed when car bon dioxide reacts with water. Hydration of carbon dioxide is far- from complete, however. Almost all the carbon dioxide that is dissolved in water exists as carbon dioxide only 0.3% of it is converted to carbonic acid. Carbonic acid is a weak acid and ionizes to a small extent to bicarbonate ion. 

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... 

The system illustrated by (272) forms the basis of a model for the zinc-containing metalloenzyme, carbonic anhydrase (Tabushi Kuroda, 1984). It contains Zn(n) bound to imidazole groups at the end of a hydrophobic pocket, as well as basic (amine) groups which are favourably placed to interact with a substrate carbon dioxide molecule. These are both features for the natural enzyme whose function is to catalyze the reversible hydration of carbon dioxide. The synthetic system is able to mimic the action of the enzyme (although side reactions also occur). Nevertheless, the formation of bicarbonate is still many orders of magnitude slower than occurs for the enzyme. 

Carbonic anhydrase an enzyme found in cells and in mucus secretions responsible for the (reversible) hydration of carbon dioxide. 

The reaction catalysed by CA is the hydration of carbon dioxide thus ... 

The two ammonium ions produced from glutamine as illustrated in Figures 8.4 to 8.6 are secreted into the PCT lumen the by a Na+/H+ antiport (the NH4+ substitutes for H+). Subsequent metabolism of 2-oxoglutarate has the potential to generate two bicarbonate ions from the hydration of carbon dioxide by carbonic anhydrase ... 

The very small extent of hydration of carbon dioxide cannot be... 

Kang, S.-P. Lee, H. (2001). Enthalpies of dissociation of clathrate hydrates of carbon dioxide, nitrogen, (carbon dioxide + nitrogen), and (carbon dioxide + nitrogen + tetrahydrofuran). J. Chem. Thermodynamics, 33 (5), 513-521. 

Jones P., Haggett M.L., andEongridgeJ.E. (1964) The hydration of carbon dioxide./. Chem. Educ. 41, 610-612. 

Carbonic anhydrase influences the tubular reabsorption of sodium in proximal tubule where biocarbonate absorption occurs and in the distal tubule where sodium is exchanged for potassium or hydrogen ion and bicarbonate is formed as the accompanying anion. The hydration of carbon dioxide takes place under the influence of enzyme carbonic anhydrase which forms carbonic acid which dissociates and breaks into hydrogen and carbonate ions. 

The rate of reversible hydration of carbon dioxide has been studied extensively and a critical evaluation of different techniques used has been documented by Kern (21). In the absence of an externally added catalyst the hydration reaction can be described as follows ... 

M. Kem, The Hydration of Carbon Dioxide, J. Chem. Ed. 1960,37, 14. Great demonstrations with C02, including one with carbonic anhydrase, are described by J. A. Bell, Every Year Begins a Millennium, J. Chem. Ed. 2000, 77, 1098. 

One of the simplest biochemical addition reactions is the hydration of carbon dioxide to form carbonic acid, which is released from the zinc-containing carbonic anhydrase (left, Fig. 13-1) as HC03-. Aconitase (center, Fig. 13-4) is shown here removing a water molecule from isocitrate, an intermediate compound in the citric acid cycle. The H20 that is removed will become bonded to an iron atom of the Fe4S4 cluster at the active site as indicated by the black H20. An enolate anion derived from acetyl-CoA adds to the carbonyl group of oxaloacetate to form citrate in the active site of citrate synthase (right, Fig. 13-9) to initiate the citric acid cycle. 

This is of relevance to the mechanism of carbonic anhydrase. This enzyme, which catalyzes the hydration of C02, has at its active site a Zn2+ ion ligated to the imidazole rings of three of its histidines. The classic mechanism for the reaction is that the fourth ligand is a water molecule which ionizes with a pKa of 7.37 The reactive species is considered to be the zinc-bound hydroxyl. Chemical studies show that zinc-bound hydroxyls are no exception to the rule of high reactivity. The H20 in structure 2.31 ionizes with a pKa of 8.7 and catalyzes the hydration of carbon dioxide and acetaldehyde.38... 

Hydrates have also been applied to foodstuffs in Fennema s laboratory (Huang et al., 1965, 1966 van Hulle et al., 1966 van Hulle and Fennema, 1971, 1972 Scanlon and Fennema, 1972). A process for producing edible hydrates of carbon dioxide was patented by Baker (1993). 

Other natural hydrates have been suggested during the past few decades. Miller (1974) concluded that hydrates of carbon dioxide were on Mars, and Pang et al. (1983) indicated that the E rings of Saturn were hydrates. Delsemme and Miller (1970), Mendis (1974), and Makogon (1987) suggested that hydrates exist in comets in particular, carbon dioxide and water in comets were combined in the form of hydrates. 

Structure identification, quantifying relative cage occupancies. 1II NMR has been used for ethane, propane, and isobutane hydrates (Davidson et al., 1977 Garg et al., 1977), while 2H, 19F, 31P, and 77 Se NMR have been used for several si guests (Collins et al., 1990). 13C cross-polarization and magic angle spinning (MAS) NMR techniques have been applied to study hydrates of carbon dioxide, methane, and propane (Ripmeester and Ratcliffe, 1988, 1999 Wilson et al., 2002 Kini et al., 2004). 

FIGURE 6.24 Three-phase data for simple hydrates of carbon dioxide. 

A co-ordinated hydroxide ligand will still possess some of the nucleophilic properties of free hydroxide ion, and this observation proves to be the basis of a powerful catalytic method, and one which is at the basis of very many basic biological processes. In general, hydrolysis reactions proceed more rapidly if a water nucleophile is replaced by a charged hydroxide nucleophile. This is readily rationalised on the basis of the increased attraction of the charged ion for an electrophilic centre. However, in many cases the chemical properties of the substrate are not compatible with the properties of the strongly basic hydroxide ion. This is exactly the situation that biological systems find themselves in repeatedly. For example, the uncatalysed hydration of carbon dioxide is very slow at pH 7 (Fig. 5-61). 

Figure 5-61. The hydration of carbon dioxide is very slow. Biology has found very effective metal-mediated ways to speed up this process.

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