Carbon dioxide carbonic anhydrase

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). 

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... 

Carbon dioxide has an effect on Og binding by Hb that is similar to that of partly because it produces when it dissolves in the blood carbonic anhydrase ... 

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... 

Figure 6-9. The Bohr effect. Carbon dioxide generated in peripheral tissues combines with water to form carbonic acid, which dissociates into protons and bicarbonate ions. Deoxyhemoglobin acts as a buffer by binding protons and delivering them to the lungs. In the lungs, the uptake of oxygen by hemoglobin releases protons that combine with bicarbonate ion, forming carbonic acid, which when dehydrated by carbonic anhydrase becomes carbon dioxide, which then is exhaled.

Carbonic anhydrase inhibitors decrease aqueous humor production by inhibition of the carbonic anhydrase isoenzyme II located in the ciliary body. In the eye, carbonic anhydrase catalyzes the conversion of water and carbon dioxide to bicarbonate and hydrogen ion, which is a significant step in aqueous humor production. Carbonic anhydrase inhibitors are available in systemic and topical preparations.10,13,14... 

When carbonic acid levels become too high, the body releases an enzyme called carbonic anhydrase, which catalyzes the breakdown of carbonic acid into carbon dioxide and water ... 

Gutknecht, J. Bisson, M. A. Tosteson, F. C., Diffusion of carbon dioxide through lipid bilayer membranes. Effects of carbonic anhydrase, bicarbonate, and unstirred layers, J. Gen. Physiol. 69, 779-794 (1977). 

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... 

Synthesis of functional models of carbonic anhydrase has been attempted with the isolation of an initial mononuclear zinc hydroxide complex with the ligand hydrotris(3-t-butyl-5-methyl-pyrazolyl)borate. Vahrenkamp and co-workers demonstrate the functional as well as the structural analogy to the enzyme carbonic anhydrase. A reversible uptake of carbon dioxide was observed, although the unstable bicarbonate complex rapidly forms a dinuclear bridged complex. In addition, coordinated carbonate esters have been formed and hydrolyzed, and inhibition by small ions noted.462 A number of related complexes are discussed in the earlier Section 6.8.4. 

Different enzymes exhibit different specific activities and turnover numbers. The specific activity is a measure of enzyme purity and is defined as the number of enzyme units per milligram of protein. During the purification of an enzyme, the specific activity increases, and it reaches its maximum when the enzyme is in the pure state. The turnover number of an enzyme is the maximal number of moles of substrate hydrolyzed per mole of enzyme per unit time [63], For example, carbonic anhydrase, found in red blood cells, is a very active enzyme with a turnover number of 36 X 106/min per enzyme molecule. It catalyzes a very important reaction of reversible hydration of dissolved carbon dioxide in blood to form carbonic acid [57, p. 220],... 

The carbon dioxide produced during cellular metabolism diffuses out of the cells and into the plasma. It then continues to diffuse down its concentration gradient into the red blood cells. Within these cells, the enzyme carbonic anhydrase (CA) facilitates combination of carbon dioxide and water to form carbonic acid (H2C03). The carbonic acid then dissociates into hydrogen ion (H+) and bicarbonate ion (HC03). 

Stanton and Merz studied the reaction of carbon dioxide addition to zinc hydroxide, as a model for zinc metallo-enzyme human carbonic anhydrase IIJ 36. It was shown that the LDA calculations (DFT(SVWN)) were not reliable for locating transition state structures whereas the post-LDA ones (DFT(B88/P86)) led to the transition state structures and ener-... 

Although zinc, cadmium, and mercury are not members of the so-called main-group elements, their behavior is very similar because of their having complete d orbitals that are not normally used in bonding. By having the filled s orbital outside the closed d shell, they resemble the group IIA elements. Zinc is an essential trace element that plays a role in the function of carboxypeptidase A and carbonic anhydrase enzymes. The first of these enzymes is a catalyst for the hydrolysis of proteins, whereas the second is a catalyst for the equilibrium involving carbon dioxide and carbonate,... 

The system illustrated by (272) forms the basis of a model for the zinc-containing metalloenzyme, carbonic anhydrase (Tabushi Kuroda, 1984). It contains Zn(n) bound to imidazole groups at the end of a hydrophobic pocket, as well as basic (amine) groups which are favourably placed to interact with a substrate carbon dioxide molecule. These are both features for the natural enzyme whose function is to catalyze the reversible hydration of carbon dioxide. The synthetic system is able to mimic the action of the enzyme (although side reactions also occur). Nevertheless, the formation of bicarbonate is still many orders of magnitude slower than occurs for the enzyme. 

Carbonic anhydrase an enzyme found in cells and in mucus secretions responsible for the (reversible) hydration of carbon dioxide. 

Carbonic anhydrase 4.2.1.1 Carbon dioxide Bicarbonate pH electrode... 

The two ammonium ions produced from glutamine as illustrated in Figures 8.4 to 8.6 are secreted into the PCT lumen the by a Na+/H+ antiport (the NH4+ substitutes for H+). Subsequent metabolism of 2-oxoglutarate has the potential to generate two bicarbonate ions from the hydration of carbon dioxide by carbonic anhydrase ... 

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