Anhydrase, carbonic

As a first comparison of a chemocatalytic reaction with an analogous enzyme-catalyzed reaction, we discuss the hydrolysis of CO2 by H2O to give HCO3 by the enzyme carbonic anhydrase. The reaction steps involved in the enzyme catalyzed mechanism will be compared with the chemocatalytic steps involved in the hydrolysis of acetonitrile by a Zn + containing zeolite as discussed on page 186 in Chapter 4. Similarly to the zeolite, the interior of the enzyme is hydrophobic except for the region close to the Zn + center. Its structure is shown in Fig. 7.6. 

The 0H bonded to Zn + reacts in a consecutive step with CO2 to form a bicarbonate ion via the sequence shown in Fig. 7.7. The bicabonate is released when another water molecule adsorbs and dissociate to form new OH sites. The proton initially attached to the His64 peptide is then released through the water network to charge compensate the bicarbonate molecule. This closes the catalytic cycle. The reactive center is regenerated in the last adsorption-induced desorption step. 

The similarities between the nitrile hydrolysis and CO2 hydrolysis system relate to the heterolytic H20-assisted splitting of H2O and the importance of the adsorption-induced desorption of the reactant molecule. The primary difference between the enzyme and the zeolite relates to the specific interactions with the imidizole group in the histidine framework, which can simultaniously interact with the reaction center to aid bond cleavage and bond formation reactions. 

The halide probe technique as proposed by Stengle and Baldeschwieler [502] was designed mainly to elucidate the properties of mercury complexes of macromolecules and in the following years several studies 

Carbonic anhydrase is a metalloenzyme containing one zinc atom per protein molecule of a molecular weight of about 30,000. The enzyme is of great physiological significance primarily since it catalyzes the reversible hydration of carbon dioxide to form bicarbonate. The enzyme has been studied in detail in solution by many methods and, furthermore, the three-dimensional crystal structure has been determined. An extensive review has been presented by Lindskog et at. [422], 35 

Cl NMR studies have been performed with human, horse and bovine carbonic anhydrases. Of these, the human and horse enzymes have been shown to exist in two principal forms differing in many properties and especially in the enzymatic activity. The forms of high specific activity are denoted C while the other major fractions of carbonic anhydrase are designed by the letter B. 

Ward s first study [426] was concerned with Cl NMR in solutions of bovine carbonic anhydrase. Comparison of results obtained with the 

From the equivalence points of the titration curves, as exemplified 

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Fig. 11.37 Free energy profile for the nucleophilic attack of water on CO2 (a) in aqueous solution and (b) in the enzyme carbonic anhydrase. (Graphs redrawn from Aqvist J, M Fothergill and A Warshel 1993. Computer Simulai of the COj/HCOf Interconversion Step in Human Carbonic Anhydrase I. Journal of the American Chemical Society 115 631-635.)...

Aqvist J, M FothergiU and A Warshel 1993. Computer Simulation of the CO2/HCO3 Interconversi Step in Human Carbonic Anhydrase I. Journal of the American Chemical Society 115 631-635. 

Z, ] McClarin, T Klein and R Langridge 1985. A Quantitative Structure-Activity Relationship and ecular Graphics Study of Carbonic Anhydrase Inhibitors. Molecular Pharmacology 27 493-498. 

Carbonic anhydrase red blood corpuscles carbonic acid CO, and H,0 6-8... 

Many important biochemical reactions involve Lewis acid Lewis base chemistry Carbon dioxide is rapidly converted to hydrogen carbonate ion m the presence of the enzyme carbonic anhydrase... 

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... 

In the chloride shift, Ck plays an important role in the transport of carbon dioxide (qv). In the plasma, CO2 is present as HCO, produced in the erythrocytes from CO2. The diffusion of HCO requires the counterdiffusion of another anion to maintain electrical neutraUty. This function is performed by Ck which readily diffuses into and out of the erythrocytes (see Fig. 5). The carbonic anhydrase-mediated Ck—HCO exchange is also important for cellular de novo fatty acid synthesis and myelination in the brain (62). 

Zinc. The 2—3 g of zinc in the human body are widely distributed in every tissue and tissue duid (90—92). About 90 wt % is in muscle and bone unusually high concentrations are in the choroid of the eye and in the prostate gland (93). Almost all of the zinc in the blood is associated with carbonic anhydrase in the erythrocytes (94). Zinc is concentrated in nucleic acids (90), and found in the nuclear, mitochondrial, and supernatant fractions of all cells. 

Metabolic Functions. Zinc is essential for the function of many enzymes, either in the active site, ie, as a nondialyzable component, of numerous metahoenzymes or as a dialyzable activator in various other enzyme systems (91,92). WeU-characterized zinc metahoenzymes are the carboxypeptidases A and B, thermolysin, neutral protease, leucine amino peptidase, carbonic anhydrase, alkaline phosphatase, aldolase (yeast), alcohol... 

Sulfonamides derived from sulfanilamide (p-arninoben2enesulfonainide) are commonly referred to as sulfa dmgs. Although several dmg classes are characterized by the presence of a sulfonamide function, eg, hypoglycemics, carbonic anhydrase inhibitors, saluretics, and tubular transport inhibitors, the antibacterial sulfonamides have become classified as the sulfa dmgs. Therapeutically active derivatives are usually substituted on the N nitrogen the position is generally unsubstituted. These features are illustrated by the stmctures of sulfanilamide (1) and sulfadiazine (2)... 

Thiadiazole-2-suifonamide, 5-acetamido-carbonic anhydrase inhibitor, 6, 576 crystal structure, 6, 548... 

FIGURE 1.19 Carbonic anhydrase, a representative enzyme, and the reaction that it catalyzes. Dissolved carbon dioxide is slowly hydrated by water to form bicarbonate ion and... 

At 20 C, the rate constant for this uncatalyzed reaction, uncat is 0.03/sec. In the presence of the enzyme carbonic anhydrase, the rate constant for this reaction, is 10 /sec. 

Thus carbonic anhydrase accelerates the rate of this reaction 3.3 X 10 times. Carbonic anhydrase is a 29-kD protein. 

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