CO2 hydration catalyzed by

Although the magnitudes of /c at and /c cat differed significantly, the two pH-dependent rate curves (Figures la and lb) are symmetrical (with scales adjusted) with an inflection point at pH ca. 7.4, a value close to the p a value of 7.3. Thus, the macrocyclic complex (33) was the first to mimic the pH-dependent behavior of reversible CO2 hydration catalyzed by CA. This fact implies that in CA, too, the CO2 hydration/HCOs dehydration should be essentially controlled by the zinc(II)-OH /zinc(II)-OH2 equilibrium at the active center. 

CO2 hydration catalyzed by 1 was followed by production at 25°C. which was detected by using a pH-indicator in buffer solution (pH 6-10). The kinetics demonstrated the catalytic nature of the zinc(II) complex 1 at various pHs. A plot of the initial rates against total zinc(II) complex concentrations (=[l]totai) indicated that the CO2 hydration rate varied linearly with [1]total and [CO2] to give an observed second-order rate constant ( at)obs- Shown in Fig. la is a plot of (kcat)obs data as a function of pH. The sigmoidal curve is characteristic of a... 

Activation parameters calculated for CO2 hydration catalyzed by BCA, by the solvent, and by OH , are listed in Table II. Although temperature effects were examined in the plateau region of the plot kcat/ against pH for CO2 hydration, similar measurements of temperature coefficients were found to be impractical for HCO dehydration. Since pK values at different temperatures are not yet known for BCA, tha activation parameters for HCO dehydration are not available at this time. 

An example is the hydration of CO2, as catalyzed by carbonic anhydrasek The catalytic reaction requires proton transfer from the zinc-bound water at the active site to solution to regenerate Zn-OH in each catalytic cycle. The most efficient isozyme forms use His-64 as a nearby proton shuttle group other forms contain residues that are less effective in proton transfer and limit overall catalytic efficiency. 

Honda M, Kuno S, Begum N, Fujimoto K-I, Suzuki K, Nakagawa Y, Tomishige K (2010) Catalytic synthesis of dialkyl carbonate from low pressure CO2 and alcohols combined with acetonitrile hydration catalyzed by Ce02. App Catal A Gen 384(1-2) 165-170... 

Like butadiene, allene undergoes dimerization and addition of nucleophiles to give 1-substituted 3-methyl-2-methylene-3-butenyl compounds. Dimerization-hydration of allene is catalyzed by Pd(0) in the presence of CO2 to give 3-methyl-2-methylene-3-buten-l-ol (1). An addition reaction with. MleOH proceeds without CO2 to give 2-methyl-4-methoxy-3-inethylene-1-butene (2)[1]. Similarly, piperidine reacts with allene to give the dimeric amine 3, and the reaction of malonate affords 4 in good yields. Pd(0) coordinated by maleic anhydride (MA) IS used as a catalyst[2]. 

Hemoglobin carbamates account for about 15% of the CO2 in venous blood. Much of the remaining COj is carried as bicarbonate, which is formed in erythrocytes by the hydration of COj to carbonic acid (H2CO3), a process catalyzed by carbonic anhydrase. At the pH of venous blood, HjCOj dissociates into bicarbonate and a proton. 

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... 

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). 

To illustrate this a model transesterification reaction catalyzed by subtilisin Carls-berg suspended in carbon dioxide, propane, and mixtures of these solvents under pressure has been studied (Decarvalho et al., 1996). To account for solvent effects due to differences in water partitioning between the enzyme and the bulk solvents. Water sorption isotherms were measured for the enzyme in each solvent. Catalytic activity as a function of enzyme hydration was measured, and bell-shaped curves with maxima at the same enzyme hydration (12%) in all the solvents were obtained. The activity maxima were different in all media, being much higher in propane than in either CO2 or the mixtures with 50 and 10% CO2. Considerations based on the solvation ability of the solvents did not offer an explanation for the differences in catalytic activity observed. The results suggest that CO2 has a direct adverse effect on the catalytic activity of subtilisin. 

Figure 10-14 Ion and fluid movement in the nonpigmented ciliary epithelium. Na+ enters the nonpigmented ciliary epithelium from the stromal side either by diffusion or by NaVH+ exchange. Na+, the main cation involved in aqueous formation, is transported extraceUularly into the lateral intercellular channel by a Na+-K+-adenosine triphosphatase-dependent transport system. HC03 forms from the hydration of CO2, a reaction catalyzed by carbonic anhydrase. HC03", the major anion involved in aqueous formation, balances a portion of the Na+ being transported into the lateral intercellular channel. Cl" enters the intercellular space by a mechanism that is not understood. This movement of ions into the lateral intercellular space creates a hypertonic fluid, and water enters by osmosis. Because of the restriction on the stromal side of the channel, the newly formed fluid moves toward the posterior chamber. A rapid diffusional exchange of CO2 allows for its movement into the posterior chamber. (Adapted from Cole DF. Secretion of aqueous humor. Exp Eye Res 1977 25(suppl) l6l-176.)...

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... 

Catalysis Hydration and dehydration reactions are catalyzed by various bases, by certain metal chelates, and by the enzyme carbonic anhydrase. Natural waters may contain some natural catalysts. Reaction 81 is essentially a catalysis of reaction 79. The conversion of CO2 into H2CO3 or HCO by both mechanisms is... 

Many addition and elimination reactions, e.g., the hydration of aldehydes and ketones, and reactions catalyzed by lyases such as fumarate hydratase are strictly reversible. However, biosynthetic sequences are often nearly irreversible because of the elimination of inorganic phosphate or pyrophosphate ions. Both of fhese ions occur in low concentrations within cells so that the reverse reaction does not tend to take place. In decarboxylative eliminations, carbon dioxide is produced and reversal becomes unlikely because of fhe high sfabilify of CO2. Further irreversibility is introduced when the major product is an aromatic ring, as in the formation of phenylpyruvate. 

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%. 

All reactions in biochemical systems are carried out by enzymes. This is true even for such a simple reaction as the hydration of CO2, which is catalyzed by the enzyme carbonic anhydrase ... 

Reclamation of bicarbonate. The filtered Na is reabsorbed by the proximal tubular cell in exchange for H". The filtered HCOj is converted to H2O and CO2 catalyzed by the luminal carbonic anhydrase IV (CAIV). CO2 diffuses in the tubular cell where it is hydrated to H2CO3 by carbonic anhydrase II and dissociated to H+ and HCOJ. Three molecules of HCO3 and one of Na" are transported to the peritubular capillary by the basolateral cotransporter. 

This reaction is catalyzed by carbonic anhydrase, an enzyme particularly abundant in erythrocytes. Carbon dioxide is not very soluble in aqueous solution, and bubbles of CO2 would form in the tissues and blood if it were not converted to bicarbonate. As you can see from the equation, the hydration of CO2 results in an increase in the concentration (a decrease in pH) in the tissues. The binding of oxygen by hemoglobin is profoundly influenced by pH and CO2 concentration, so the interconversion of CO2 and bicarbonate is of great importance to the regulation of oxygen binding and release in the blood. 

Propionyl-CoA is first carboxylated to form the d stereoisomer of methylmalonyl-CoA (Fig. 17-11) by propionyl-CoA carboxylase, which contains the cofactor biotin. In this enzymatic reaction, as in the pyruvate carboxylase reaction (see Fig. 16-16), CO2 (or its hydrated ion, HCO3) is activated by attachment to biotin before its transfer to the substrate, in this case the propionate moiety. Formation of the carboxybiotin intermediate requires energy, which is provided by the cleavage of ATP to ADP and P,. The D-methylmalonyl-CoA thus formed is enzymatically epimerized to its l stereoisomer by methylmalonyl-CoA epimerase (Fig. 17-11). The L-methylmalonyl-CoA then undergoes an intramolecular rearrangement to form succinyl-CoA, which can enter the citric acid cycle. This rearrangement is catalyzed by methylmalonyl-CoA mutase, which requires as its coenzyme 5 -deoxyadenosyl-cobalamin, or coenzyme B12, which is derived from vitamin B12 (cobalamin). Box 17-2 describes the role of coenzyme B12 in this remarkable exchange reaction. 

Figure 1 The rate-pH profile for (a) a CO2 hydration (b) HCO3 dehydration, catalyzed by...

The reaction of CO2 with a metal hydride produces formate complexes M-0C(0)H, not formyl derivatives M-C(0)0H, and the insertion into M-C bonds gives the appropriate carboxylate compounds M-0C(0)R. In a similar fashion, the reactions with M-OH and M-OR (R = alkyl, aryl) generate the corresponding bicarbonate M-0C(0)0H and carbonate M-0C(0)0R species, respectively. The reaction of CO2 with a zinc hydroxide moiety is particularly important in biological systems, namely, for the reversible hydration of CO2 to HCOs catalyzed by Zn(ll) in carbonic anyhdrases. Moreover, it has been postulated that the insertion of CO2 into M-O bonds is essential in the co-polymerization of CO2 and epoxides and in the preparation of cyclic carbonates and polycarbo-In a similar vein, the insertion of CO2 into the M-N bond of both main group and transition metal... 

However, the 12-membered macrocyclic triamine ([12]aneN3) zinc(II) complex 1 has. for the first time, provided a chemical mechanism that shows the role of zinc(II) ion in the reversible CO2 hydration and HC03 dehydration catalyzed by The fast kinetics of the... 

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