The 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... 

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 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 ... 

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. 

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... 

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

Enzymes accelerate reactions by factors of as much as a million or more (Table 8.1). Indeed, most reactions in biological systems do not take place at perceptible rates in the absence of enzymes. Even a reaction as simple as the hydration of carbon dioxide is catalyzed by an enzyme—namely, carbonic anhydrase (Section 9.2). The transfer of CO2 from the tissues into the blood and then to the alveolar air would be less complete in the absence of this enzyme. In fact, carbonic anhydrase is one of the fastest enzymes known. Each enzyme molecule can hydrate 10 molecules of CO2 per second. 

In a few cases, theoretical calculations of transition state and intermediate energies and geometries provide confirmation of experimental studies of the mechanism of enzyme reactions and suggest directions for further study. One of these is the hydration of carbon dioxide catalyzed by carbonic anhydrase, a zinc enzyme. Below pH 7, the uncatalyzed reaction HC03 + H2O + CO2 is favored. Above pH 7, the reaction is... 

The most important t5q)es of homogeneous catalysis in water are performed by acids, bases and trace metals. A wide variety of mechanisms have been outlined for acid/base catalysis and are presented in kinetics texts (e.g. Moore and Pearson, 1981 Laidler, 1965). A number of bases have been observed to catalyze the hydration of carbon dioxide (Moore and Pearson, 1981 Dennard and Williams, 1966). Examples are listed in Table 9.7 for OH and the base Co(NH3)gOH2. The most dramatic effect is the catalysis of HS-oxidation by cobalt-4,4, 4",4"-tetrasulfophthalocyanine (Co-TSP ). At concentrations of 0.1 nM Co-TSP the reaction rate was catalyzed from a mean life of roughly 50 h to about 5 min. The investigators attributed the reason for historically inconsistent experimentally determined reaction rates for the H2S-O2 system by different researchers partly to contamination by metals. Clearly, catalysis by metal concentrations that are present in less than nanomolar concentrations is likely to be effective in aquatic systems. We shall see that similar arguments apply to catalysis by surfaces and enzymes. 

Figure 9.25 Mechanism of carbonic anhydrase. The zinc-bound hydroxide mechanism for the hydration of carbon dioxide reveals one aspect of metal ion catalysis. The reaction proceeds in four steps (1) water deprotonation (2) carbon dioxide binding (3) nucleophilic attack of hydroxide on carbon dioxide and (4) displacement of bicarbonate ion by water.

Figure 9,26 A synthetic analog mode system for carbonic anhydrase. (A) An organic compound, capable of binding zinc, was synthesized as a model for carbonic anhydrase. The zinc complex of this ligand accelerates the hydration of carbon dioxide more than 100-fold under appropriate conditions. (B) The structure of the presumed active complex showing zinc bound to the ligand and to one water molecule.

---