Reactions Catalyzed by Carbons

Analysis of the pertinent literature shows that the most relevant conclusions regarding the role of surface chemistry in carbon catalysis have been derived following a common methodology whereby a series of catalysts are prepared from the same carbon material by suitable thermal or chemical treatments while keeping the textural properties essentially unchanged. It is then possible to correlate the catalytic properties with the surface chemistry of the carbons. This 

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Osteopetrosis (marble bone disease), characterized by increased bone density, is due to inability to resorb bone. One form occurs along with renal tubular acidosis and cerebral calcification. It is due to mutations in the gene (located on chromosome 8q22) encoding carbonic anhydrase II (CAII), one of four isozymes of carbonic anhydrase present in human tissues. The reaction catalyzed by carbonic anhydrase is shown below ... 

FIGURE 21. Mechanisms for the rearrangement of substrate radicals in the reactions catalyzed by carbon skeleton mutases. For 2-methyleneglutarate mutase and the acyl-CoA mutases both associative (upper pathway) and dissociative (lower pathway) mechanisms have been proposed (Halpem, 1985 Bucket Golding, 1996), whereas for glutamate only a dissociative mechanism appears feasible. 

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.)...

The msgor acids produced by the body include phosphoric acid, sulfuric acid, lactic acid, and the ketone bodies, acetoacetic acid and p-hydroxybutyric acid. C02 is also produced, which combines with H20 to form carbonic acid in a reaction catalyzed by carbonic anhydrase... 

The overall process by which parietal cells acidify the stomach lumen is Illustrated in Figure 7-28. In a reaction catalyzed by carbonic anhydrase the excess cytosolic OH combines with CO2 that diffuses in from the blood, forming HCO3 . Catalyzed by the basolateral anion antiporter, this bicarbonate ion is exported across the basolateral membrane (and ultimately into the blood) in exchange for a Cl ion. The Cl ions then exit through Cl channels in the apical membrane, entering the stomach lumen. To preserve electroneutrality, each Cl ion that moves into the stomach lumen across the apical membrane is accompanied by a ion that moves outward through a separate channel. In this way, the excess ions pumped Inward by the H /K ... 

FIGURE 28-2 NaHCO reabsorption in proximal tubule and mechanism of diuretic action of carbonic anhydrase (CA) inhibitors. A, antiporter S, symporter CH, ion channel. (The actual reaction catalyzed by carbonic anhydrase is OH + CO —> HCOg however, H2O —> OH + —> and HCO + > H2CO3, so the net reaction is H2O + CO2 —> H2CO2.)... 

Figure 8.9 Carbonic Anhydrase. (a) Reversible hydration reaction catalyzed by carbonic anhydrase (CA) (b) ribbon display structure of monomeric CA (Type I, human erythrocyte) (pdb 2cab) illustrating oc-helices (red) and (3-sheet (blue) plus a central zinc ion (Zn +) (yellow) rendered as van der Waals sphere for complete clarity.

Table 1 Kinetic constant for reactions catalyzed by carbonic anhydrase and peroxidases...

Although usually high, the proton-transfer steps may become the rate-limiting steps in catalysis. If the proton movement into and out of the active site is restricted, the state of protonation of the enzyme-substrate complex is not equilibrated rapidly with respect to the rate of reaction to give products or the rate of substrate dissociation (Section 14.6.2) that is why the enzymes have acid-base catalysis. On the other hand, when the chemical step is exceptionally fast, the proton-transfer reactions may become the rate-limiting steps in catalysis this case is verified in reaction catalyzed by carbonic anhydrase (Silverman Tu, 1975). 

In a brief overview of reactions catalyzed by both carbon materials and by carbon deposits, Trimm (1981) presented a list with reactions catalyzed by carbon materials ranging from hydrogenation through oxidation and reduction to polymerization and chlorination, a summarizing version of which is given in Table 8.7. 

Table 8.7. Some reactions catalyzed by carbon (Trimm, 1981 Rodrfguez-Reinoso, 1998).

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