Carbonic anhydrase intermediate

This suggestion is not necessarily intended to imply that the bicarbonate intermediate of the carbonic anhydrase cycle must exhibit unidentate coordination in the ground state. Rather, it is intended to imply that a unidentate species should be readily accessible. Moreover, it is not intended to suggest that this factor alone is responsible for influencing the activity of metal-substituted carbonic anhydrases,... 

The X-ray structure of the unsubstituted tris(pyrazolyl)borato zinc nitrate has been solved showing a unidentate coordination mode for nitrate, in contrast with the t-butyl substituted ligand, which shows anisobidentate nitrate coordination due to the steric effects.232 A partial explanation of the reduced activity of cadmium-substituted carbonic anhydrase is offered by Parkin on the basis of the comparison of nitrate coordination to cadmium and zinc trispyrazo-lylborate moieties. A contributing factor may be the bidentate coordination supported by the cadmium that does not allow the facile access to a unidentate bicarbonate intermediate, which could be highly important to carbonic anhydrase activity.233... 

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 interaction of carbon disulfide as a substrate in carbonic anhydrase model systems has been studied using density functional theory methods. A higher activation energy of CS2 compared to C02 in the reaction with [L3ZnOH]+ was due to the reduced electrophilicity of CS2. The reversibility of the reaction on the basis of these calculations is questionable with [L3ZnSC(0)SH]+ as intermediate.572... 

Figure 8.4 Nitrilase-catalyzed conversion of (5)-3-(thiophen-2-ylthio)butanenitri le to (S)-3,3,3-trifluoro-2-hydroxy-2-methyl-propionamide, an intermediate in the synthesis of carbonic anhydrase inhibitor Dorzolamine ...

The first step in the esterase activity of carbonic anhydrase (Fig. 3.15, e) is analogous to the first step in C02 hydration (Fig. 3.15,a). The tetrahedral intermediate so formed (Fig. 3.15,f) necessitates the participation of a proton donor for the departure of the leaving alcohol group (Fig. 3.15,f and g). It is possible that the Thr200 residue plays an important role in the esterase activity of carbonic anhydrase. Indeed, its replacement by other amino acids enhances the esterase activity but has no significant effect on the rate of C02 hydration [106],... 

In the other subdivision, water activation occurs in the first step of the enzymatic cycle. This activation is achieved by a carboxylate group in aspartic hydrolases (Fig. 3.10), Zn2+ and a carboxy group in metallopep-tidases (Fig. 3.12 ), a histidine side chain in calcium-dependent hydrolases (Fig. 3.14), or a Zn2+ in carbonic anhydrase (Fig. 3.15). The substrate, on the other hand, is polarized (activated) by a carboxy group in aspartic hydrolases or by a cation in metallopeptidases and calcium-dependent hydrolases. In this manner, the reactivity of both the water molecule and the substrate is enhanced and fine-tuned to drive formation of a tetrahedral intermediate that will break down to form the hydrolysis products. 

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). 

Scheme 4. Zinc proteases nucleophilic adduct is a reaction intermediate carbonic anhydrases nucleophilic adduct is the product.

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. 

Reduction of the sulfur-containing cyclic ketones A and B with (R,R)-39 gives the R alcohols C and D with 99% ee and 98% ee, respectively (Scheme 1.84) [321 ]. The chiral products are important intermediates in the synthesis of MK-0417, which is a carbonic anhydrase inhibitor [325],... 

Along the same lines, several groups of chemists at Merck successfully produced substantial quantities of various important pharmaceutical intermediates used in the preparation of drugs under development. Among these we can mention MK-0417 (7), a soluble carbonic anhydrase inhibitor useful to treat glaucoma,41 the LTD4 antagonist L-699392 (8),42 and the antiarrhythmic MK-499... 

Another yeast, Rhodotorula rubra MY 2169, has been shown to reduce a ketosulfone 8 to the corresponding tram-hydroxysulfone 9 (Scheme 19.8). This hydroxysulfone is an intermediate in the drug candidate L-685,393 (10), a carbonic anhydrase inhibitor.81 Results of this biotransformation yielded gram quantities of product with a de of >96%. Studies by Zeneca discuss additional screening experiments aimed at finding microorganisms to reduce a similar ketosulfone.82... 

In their role as enantioselective catalysts for the reduction of prochiral ketones, chiral oxazaborolidines have been used for the preparation of prostaglandins, PAF antagonists, a key intermediate of ginkgolide B, bilobalide, a key intermediate of forskolin, (/ )- and (S)-fluoxetine, (R)- and (S)-isopreterenol, vitamin D analogs, the carbonic anhydrase inhibitor the dopamine D1 agonist A-77636, ... 

Recall that, in aqueous solutions, CO2 exists as HCO3 with the aid of carbonic anhydrase (Section 9.2). The HCO3 is activated to carboxyphosphate. This activated CO2 is subsequently bonded to the N-1 atom of the biotin ring to form the carboxybiotin-enzyme intermediate (see Figure 16.27). The CO2 attached to the biotin is quite activated. The A G° for its cleavage... 

Figure 20.4). Ribulose 1,5-bisphosphate binds to Mg2+ through its keto group and an adjacent hydroxyl group. This complex is readily deprotonated to form an enediolate intermediate. This reactive species, analogous to the zinc-hydroxide species in carbonic anhydrase (Section 9.2.2). couples with CO2, forming the new carbon-carbon bond. The... 

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... 

Hydroboration of simple ketones eatalyzed by proline-derived oxazaborolidine 45 provides a practical method for the preparation of chiral secondary alcohols (Scheme 13). Under this protocol, a key intermediate in the synthesis of MK-0417 (44), a water-soluble carbonic anhydrase inhibitor, has been prepared... 

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