Enzymes comparison with other catalysts

The most important property of a catalyst for application in a process is not its activity but rather its selectivity, followed by its stability, which is just activity integrated over time. In comparison with other catalysts, enzymes often feature superior selectivity, especially regio- and enantioselectivity. Enzymes are destined for selective synthesis of molecules with several similar functional groups or chiral centers. A growing emphasis is laid on the synthesis of enantiomerically pure compounds (EPCs). The interest in EPCs shown by all areas of the life science industries (e.g., pharmaceuticals, food and agriculture), stems from the challenge to develop structurally optimized inhibitors, almost always containing chiral centers. 

A discussion of catalysis would not be complete without a comparison of the catalysts we normally encounter in the laboratory or in industry with those that occur naturally (i.e., enzymes). Enzymes are proteins that are either soluble in the aqueous medium of the cell or attached to a cellular membrane. Soluble enzymes resemble homogeneous transition metal catalysts in ways other than their solubility characteristics. Enzymes have at least one region that serves as... 

A catalyzed reaction follows a path of relatively low free energy of activation, in the sense of Eyring s theory, as compared with the same reaction proceeding without a catalyst. Where a comparison is possible, it would seem that inorganic catalysts lower the energy (heat content change) of activation (3), and the same holds for enzymes (131). The effect of catalysts on the entropy of activation, so far as the present author is aware, is, in general, less marked. In some way the molecules adsorbed on the metal, metal oxide, or other catalysts, or held to a specific protein, are converted into a more labile form, i.e., the potential hill for one specific mode of reaction (out of perhaps a number of such) is considerably lowered. 

Complexes with modified cyclodextrins were initially used as models of hydrolytic enzymes [20,181,197,198]. Then, Breslow and Kato studied such complexes in biomimetic oxidation and epoxidation. Reetz and some other researchers investigated them in hydroformylation. We examined cyclodextrin-based catalysts in hydroxylation and Wacker oxidation. Note that some of these studies demonstrated a substantial increase in the activity of the catalysts in comparison with their analogues prepared as ordinary mixtures, which was due to the cooperative substrate binding. 

If metallocenes, especially zirconocenes but also titanocenes, hafnocenes and other transition metal compounds (Figure 2) are treated with MAO, then catalysts are acquired that allow the polymerization of up to 100 tons of ethene per g of zirconium [151-153]. At such high activities the catalyst can remain in the product. The insertion time (for the insertion of one molecule of ethene into the growing chain) amounts to some 10 s only (Table 6). A comparison with enzymes is not far-fetched. 

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... 

Hence the dimension ("the order") of the reaction is different, even in the simplest case, and hence a comparison of the two rate constants has little meaning. Comparisons of rates are meaningful only if the catalysts follow the same mechanism and if the product formation can be expressed by the same rate equation. In this instance we can talk about rate enhancements of catalysts relative to another. If an uncatalysed reaction and a catalysed one occur simultaneously in a system we may determine what part of the product is made via the catalytic route and what part isn t. In enzyme catalysis and enzyme mimics one often compares the k, of the uncatalysed reaction with k2 of the catalysed reaction if the mechanisms of the two reactions are the same this may be a useful comparison. A practical yardstick of catalyst performance in industry is the space-time-yield mentioned above, that is to say the yield of kg of product per reactor volume per unit of time (e.g. kg product/m3.h), assuming that other factors such as catalyst costs, including recycling, and work-up costs remain the same. 

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