CO2 hydrates

The concentration of tme carbonic acid (H2CO2) is negligible in comparison to dissolved carbon dioxide, eg, only 0.3% of the latter is hydrated to carbonic acid at 25°C. The ionization constant is a composite constant representing both the CO2 hydration reaction, iC, and ionization of tme H2CO2, ifj = ifjj QQ /(I + K). Temperature-dependent equations for and are (29)... 

Table 3.1 Incipient Equilibrium Data on CO2 Hydrate Formation in 20 (wt%) Aqueous Glycerol Solutions...

Fig. 4. CH4 - and CO2 hydrates stability curves showing C02 enhanced CH4 hydrate dissociation zone.

Fig. 3.15. Schematic mechanism of CO2 hydration (Steps a-d) and ester hydrolysis (Steps...

The crystal structure of the cobalt-substituted enzyme was obtained with bicarbonate bound to the metal (Iverson et al. 2000). The structure shows Asn 202 and Gln75 hydrogen bonded to the metal-bound bicarbonate, suggestzing potential roles for these residues in either transition-state stabilization or orientation and polarization of CO2 for attack from the zinc-hydroxyl (Fig. 11.5). The crystal structure also shows three discrete conformations for Glu 84, suggesting a role for this residue in the transfer of protons out of the active site indeed, kinetic analyses of Glu 84 variants combined with chemical rescue experiments establish this residue as critical for proton transfer (Tripp and Ferry 2000). The location of Glu 62 adjacent to Glu 84 suggests a potential role in proton transfer as well. Although kinetic analyses of site-specific variants establish an essential role for Glu 62 in the CO2 hydration steps (Eqs. 11.3 and 11.4), the results were inconclusive regarding an additional role in proton transfer (Eqs. 11.5 and 11.6). 

Human carbonic anhydrase II, found primarily in the erythrocyte, is the prototypical member of the family of carbonic anhydrases and has been extensively reviewed (Pocker and Sarkanen, 1978 Lindskog, 1983, 1986 Silverman and Lindskog, 1988). Within the erythrocyte carbonic anhydrase II hydrates CO2 to form bicarbonate ion plus a proton via tandem chemical processes (Silverman and Lindskog, 1988) (Scheme 2). Most of the carbon dioxide generated during the process of respiration requires this carbonic anhydrase Il-catalyzed event for transport out of the cell. The resultant protons of CO2 hydration are taken up by His-146)8, His-122a, and the amino terminus of the a subunits of the hemoglobin tetramer. As a reference. Scheme 3 outlines the interconversions... 

The catalysis of CO2 hydration by carbonic anhydrase II occurs via the two chemically independent steps outlined in Scheme 2 a general mechanistic profile is found in Fig. 23. The first step involves the association of substrate with enzyme and the chemical conversion of substrate into product. The second step is product dissociation and the regeneration of the catalytically active nucleophile zinc hydroxide (Coleman, 1967). Below, we address the structural aspects of zinc coordination in each of these steps. 

Fig. 23. A general mechanism of CO2 hydration as catalyzed by carbonic anhydrase II. Certain structural details (e.g., the function of pentacoordinate zinc or the degree of CO2—Zn interaction in enzyme-substrate association) remain to be elucidated.

There may be two proton transfers in the carbonic anhydrase II-catalyzed mechanism of CO2 hydration that are important in catalysis, and both of these transfers are affected by the active-site zinc ion. The first (intramolecular) proton transfer may actually be a tautomerization between the intermediate and product forms of the bicarbonate anion (Fig. 28). This is believed to be a necessary step in the carbonic anhydrase II mechanism, due to a consideration of the reverse reaction. The cou-lombic attraction between bicarbonate and zinc is optimal when both oxygens of the delocalized anion face zinc, that is, when the bicarbonate anion is oriented with syn stereochemistry toward zinc (this is analogous to a syn-oriented carboxylate-zinc interaction see Fig. 28a). This energetically favorable interaction probably dominates the initial recognition of bicarbonate, but the tautomerization of zinc-bound bicarbonate is subsequently required for turnover in the reverse reaction (Fig. 28b). 

Consider a CO2 droplet of radius 3 mm injected at 600 m seawater depth with temperature of 5.2°C (Zhang, 2005b). Under these conditions, density and viscosity of seawater are 1026 kg/m and 0.00161 Pa s, and density of liquid CO2 is 916kg/m, or 20.82 mol/L. Because of the formation of hydrate shell, the solubility of CO2 in seawater should be that of CO2 hydrate, which is 1.00 mol/L (CO2 liquid solubility is significantly greater), or Wq = 0.0429. Because solubility of CO2 is small, density of the interface water is similar to the bulk seawater. Hence, the... 

Feasibility of Large-Scale CO2 Ocean Sequestration. This project, operated by the Monterey Bay Aquarium Research Institute will usea Remotely Operated Vehicle (ROV) to deploy small quantities of liquid CO2 in the deep ocean. Below about 10,000 feet the density of liquid CO2 exceeds that of seawater, and the liquid CO2 is quickly converted into a solid hydrate by reacting with the surrounding water. Using a Raman spectrometer, scientists will assess the impact that the CO2 hydrate material has on the ocean floor and ecosystem. 

There are several types of -class CAs i.e., a-CA I-VII, reported in the literature, out of which the human carbonic anhydrase II (HCA II), the most extensively studied carbonic anhydrase, has an exceptionally high CO2 hydration rate and a wide tissue distribution 107). The HCA II comprises a single polypeptide chain with a molecular mass of 29.3 kDa and contains one catalytic zinc ion, coordinated to three histidine residues, His 94, His 96, and His 119. A tetrahedral coordination geometry around the metal center is completed with a water molecule, which forms a hydroxide ion with a pK value of 7.0 108). Quigley and co-workers 109,110) reported that the inhibition of the synthesis of HCO3 from CO2 and OH- reduces aqueous humor formation and lowers intra-ocular pressure, which is a major risk factor for primary open-angle glaucoma. 

For each turnover cycle of CO2 hydration, an H bond in H20 must be broken and in the dehydration step H2O must be formed. 

For isoenzymes I and II, the CO2 hydration rates are independent of buffer at high buffer concentrations, indicating thereby that a reaction step other than the buffer-dependent step becomes rate limiting. Studies of both hydration and dehydration reactions at high concentrations of buffers in H20 and DoO indicated that the kinetic parameter, kCSLt, for isoenzyme II has large isotope effect (k jkV) 3-4) (45b). This is consistent with involvement of H+ transfer in the rate-limiting step. The H+ transfer half-reaction is composed of at least two steps,... 

In general, anion inhibition is non-competitive below pH 7 and uncompetitive at pH 9 towards CO2 hydration, whereas it is competitive towards HCO3 dehydration at low and neutral pH (169,157c). Other inhibitors such as alkylcarbonates, acetates, alkoxides, alcohols (192a) and divalent metal ions like Cu(II), Hg(II) are also reported (160, 190,192b). 

Phenol binds to the hydrophobic pocket it is a competitive inhibitor for CO2 hydration and mixed non-competitive inhibitor of HCOg dehydration reaction (192a,193). In contrast, 2-nitrophenol is shown to be uncompetitive with respect to CO2 hydration at high pH. 1,2,4-Triazole is a non-competitive inhibitor towards both CO2 hydration and HCO3 dehydration (194). In contrast, tetrazole as an inhibitor is uncompetitive for C02 hydration and competitive for HCO3 dehydration at neutral and alkaline pH (194). Different inhibitors showing metal coordination with native enzyme and that of the Co-variant are presented in Table V. 

Uchida, T., Ebinuma, T., Kawabata, J., Narita, H., J. Cryst. Growth, 204, 348 (1999a). Uchida, T., Ebinuma, T., Mae, S., Formation Rate Measurements of CO2 Hydrate Film Formed at Liquid CO2 Water Interface, Greenhouse Gas Control Technologies, (Riemer, P., Eliasson, B., Wokaun, A., eds.) Elsevier, 1073 (1999b). 

Gas impurities may have caused the systematic deviation of the Berecz and Balla-Achs CH4 + CO2 hydrate data from those of Unmh and Katz (1949) and Adisasmito et al. (1991) and for those reasons the former data are excluded. 

Initial CO2 (bars) If alkalinity > 0.0 or CO2 hydrates are simulated, then specify the initial concentration of CO2 (g) in bars. 

Landolfi et al. (1997) reported a modified procedure for the measurement of carbonic anhydrase activity. The measure of carbonic anhydrase activity is based on the rate of CO2 hydration by the enzyme. Such transformation was monitored by a procedure which consists of the measure of time necessary for the pH of an appropriate buffer to decrease from 8 to 7.5 in the presence of a constant CO2 flow this time period is dose-dependently reduced by the addition of the enzyme and further modified in the presence of carbonic anhydrase inhibitory compounds. 

A structure-function study of a proton pathway in the y-class carbonic anhydrase from Methanosarcina thermophila was conducted in the work of Tripp and Ferry (2000). Four enzyme glutamate residues were characterized by site-directed mutagenesis. It was shown that Glu 84 and an active site residue, Glu 89, are important for CO2 hydration activity, while external loop residues, Glu 88 and Glu 89 are less important. Glu 84 can be substituted for other ionizable residues with similar pKa values and, therefore, participates in the enzyme catalysis not as a chemical reagent but as a proton shuttle. 

Carbonic anhydrases accelerate CO2 hydration dramatically. The most active enzymes, typified by human carbonic anhydrase II, hydrate CO2 at rates as high as k =10 s, or a million times a second. Fundamental physical processes such as diffusion and proton transfer ordinarily limit the rate of hydration, and so special strategies are required to attain such prodigious rates. 

CO2 hydration is associated with all fluxes between the atmosphere and the Earth surface, and the 0-C02 budget may be expressed, as done for as a mass balance with respect to the... 

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