Carbon dioxide hydration catalysis

The Idnetic rate constants for CO2 hydration determined in the laboratory in sterile seawater (Table 4.6) are known sufficiently well that this value should create little uncertainty in the above calculation. However, in natural waters the reaction rates may be enzymatically catalyzed. Carbon dioxide hydration catalysis by carbonic anhydrase (CA) is the most powerful enzyme reaction known (see the discussion in Section 9.3). The catal5dic turnover number (the number of moles of substrate reacted, divided by the number of moles of enz5mie present) is 8 x 10 min for CA (Table 9.7), and marine diatoms are loiown to produce carbonic anhydrase (Morel et al, 1994). The calculations presented in Fig. 10.14 indicate that increasing the CO2 hydration rate constant by 10-fold should increase the gas exchange rate of CO2 in the ocean by 10%-50%. 

Carbon atom, 4. See also Atomic orbitals Carbon dioxide hydration, 197-199. See also Carbonic anhydrase Carbonic anhydrase, 197-199,200 Carbonium ion transition state, 154, 159 Carboxypeptidase A, 204-205 Catalysis, general acid, 153,164,169 in carboxypeptidase A, 204-205 free energy surfaces for, 160, 161 in lysozyme, 154... 

Table 9.7. I Homogeneous, surface and enzyme catalysis of carbon dioxide hydration, hydrogen sulfide oxidation and Mn oxidation... 

Catalysis of carbon dioxide hydration by carbonic anhydrase is the most rapid enz5fmatic reaction known (Table 9.7). It has been shown (Khalifah, 1971) that the Km for the human forms of the enzyme are between 4 and 9 mM, and the catal5dic turnover number, Vmax/ is 8 X 10 (Table 9.7). 

Pocket, Y. and D. W. Bjorkquist (1977) Stopped-flow studies of carbon dioxide hydration and bicarbonate dehydration in H2O and D2O. Acid-base and metal ion catalysis./. Am. Chem. Soc. 99, 6537-43. 

Essentially, the suggested mechanism includes metabolic production of carbon dioxide, hydration to carbonic acid, catalysis by carbonic anhydrase, dissociation of carbonic acid, and exchange of the hydrogen ions for sodium ions across the luminal border of the cell. 

Carbonic anhydrase is a zinc(II) metalloenzyme which catalyzes the hydration and dehydration of carbon dioxide, C02+H20 H+ + HC03. 25 As a result there has been considerable interest in the metal ion-promoted hydration of carbonyl substrates as potential model systems for the enzyme. For example, Pocker and Meany519 studied the reversible hydration of 2- and 4-pyridinecarbaldehyde by carbonic anhydrase, zinc(II), cobalt(II), H20 and OH. The catalytic efficiency of bovine carbonic anhydrase is ca. 108 times greater than that of water for hydration of both 2- and 4-pyridinecarbaldehydes. Zinc(II) and cobalt(II) are ca. 107 times more effective than water for the hydration of 2-pyridinecarbaldehyde, but are much less effective with 4-pyridinecarbaldehyde. Presumably in the case of 2-pyridinecarbaldehyde complexes of type (166) are formed in solution. Polarization of the carbonyl group by the metal ion assists nucleophilic attack by water or hydroxide ion. Further studies of this reaction have been made,520,521 but the mechanistic details of the catalysis are unclear. Metal-bound nucleophiles (M—OH or M—OH2) could, for example, be involved in the catalysis. 

Carbonic anhydrases catalyze the reaction of water with carbon dioxide to generate carbonic acid. The catalysis can be extremely fast molecules of some carbonic anhydrases hydrate carbon dioxide at rates as high as 1 million times per second. A tightly bound zinc ion is a crucial component of the active sites of these enzymes. Each zinc ion binds a water molecule and promotes its deprotonation to generate a hydroxide ion at neutral pH. This hydroxide attacks carbon dioxide to form bicarbonate ion, HCO3 ". Because of the physiological roles of carbon dioxide and bicarbonate ions, speed is of the essence for this enzyme. To overcome limitations imposed by the rate of proton transfer from the zinc-bound water molecule, the most active carbonic anhydrases have evolved a proton shuttle to transfer protons to a buffer. 

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. 

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 CO 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 CO per second. This cat-alyzed reaction is 10 times as fast as the uncata]yz.ed one. We will consider the mechanism of carbonic anhydrase catalysis in Chapter 9,... 

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.

A comparable degree of Interest has been shown In the transport of the other key physiological gas, carbon dioxide, as enhanced by reaction to form bicarbonate ion. The pioneering work of Longmuir et al. (6), Enns (7) and Ward and Robb (8) made it clear that fruitful theoretical analyses required consideration of the kinetics cf CO2 hydration and its catalysis by, in particular, the red blood cell enzyme carbonic anhydrase and arsenite ion. A review of this subject is forthcoming (9). 

The addition of water to carbon dioxide, CO2 + H2O OC(OH)2 H +HCOJ, is formally very similar to its addition to aldehydes and ketones, although here only 0.2% of the carbon dioxide is hydrated at equilibrium, and observations make use of the further equilibrium with and HCO3. A summary of work up to 1958 has been given by Edsall and Wyman,and several later kinetic studies have been made " the hydration process shows general catalysis by basic anions. It is particularly interesting that the enzyme carbonic anhydrase, which is active in maintaining the carbon dioxide-bicarbonate equilibrium in the body, is also an effective catalyst for the hydration of acetaldehyde and other carbonyl compounds. ... 

---