Hydrated proton with active carbon, reaction

Reaction of the Hydrated Proton with Active Carbon... 

Carbonic anhydrases catalyze the reaction of water with carbon dioxide to generate carbonic acid. The catalysis can be extremely fast molecules of some carbonic anhydrases hydrate carbon dioxide at rates as high as 1 million times per second. A tightly bound zinc ion is a crucial component of the active sites of these enzymes. Each zinc ion binds a water molecule and promotes its deprotonation to generate a hydroxide ion at neutral pH. This hydroxide attacks carbon dioxide to form bicarbonate ion, HCO3 ". Because of the physiological roles of carbon dioxide and bicarbonate ions, speed is of the essence for this enzyme. To overcome limitations imposed by the rate of proton transfer from the zinc-bound water molecule, the most active carbonic anhydrases have evolved a proton shuttle to transfer protons to a buffer. 

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). 

Product analyses of hydrated DNA, irradiated at room temperature, yield substantial quantities of 5,6-dihydrothymine[18] as expected from ESR results which show large amounts of T(C6)H however, substantial quantities of 5,6-dihydrocytosine and its deamination product 5,6-dihydrouracil are also found [Swarts S, unpublished results]. These results suggest that at higher temperatures the activation barrier to protonation of one-electron reduced cytosine at a carbon site (reaction 3) is overcome, producing a reaction path which is competitive with reaction 2. 

As noted earlier, some carbonic anhydrases can hydrate carbon dioxide at rates as high as a million times a second (10 s ). The magnitude of this can be understood from the following observations. In the first step of a carbon dioxide hydration reaction, the zinc-bound water molecule must lose a proton to regenerate the active form of the enzyme (Figure 9.27). The rate of the reverse reaction, the protonation of the zinc-bound hydroxide ion. is limited by the rate of proton diffusion. Protons diffuse very rapidly with second-order rate constants near 10 M. Thus, the backward rale constant i must be less than 10 s F Because the equilibrium... 

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