Supported enzyme catalysis

Metal ions are clearly essential for the ribonucleoside triphosphate reductase isolated from the filamentous cyanophyte, Anabaena 7119, one of the many blue-green algae that depend on deoxyadenosylcobalamin for deoxyribonucleotide synthesis The purified enzyme possesses a molecular weight of 72,000 (estimated by gel filtration) with no subunit structure. It does not reduce ribonucleotides in the absence of divalent cations Ca" " is most effective but Mg" " and Mn" also support enzyme catalysis. Judging from their optimum concentration (5-10 mM) the metal ions are not only necessary to complex the substrate triphosphate but should have an effect on the enzyme protein itself. 

The entropic hypothesis seems at first sight to gain strong support from experiments with model compounds of the type listed in Table 9.1. These compounds show a huge rate acceleration when the number of degrees of freedom (i.e., rotation around different bonds) is restricted. Such model compounds have been used repeatedly in attempts to estimate entropic effects in enzyme catalysis. Unfortunately, the information from the available model compounds is not directly transferable to the relevant enzymatic reaction since the observed changes in rate constant reflect interrelated factors (e.g., strain and entropy), which cannot be separated in a unique way by simple experiments. Apparently, model compounds do provide very useful means for verification and calibration of reaction-potential surfaces... 

Tada, M. and Iwasawa, Y. (2009) Model Systems in Catalysis From Single Crystals and Size Selected Clusters to Supported Enzyme Mimics (ed. R.M. Rioux), Springer, in press. 

The idea that statistical fluctuations play a role in the mechanism of enzyme catalysis is intriguing. However, as far as I know, no experimental evidence exists to support such a hypothesis. 

The turnover frequency, N, (commonly called the turnover number) defined, as in enzyme catalysis, as molecules reacting per active site in unit time, can be a useful concept if employed with care. In view of the problems in measuring the number of active sites discussed in 1.2.4, it is important to specify exactly the means used to express Q in terms of active sites. A realistic measure of such sites may be the number of surface metal atoms on a supported catalyst but in other cases estimation on the basis of a BET surface area may be the only readily available method. Of course, turnover numbers (like rates) must be reported at specified conditions of temperature, initial concentration or initial partial pressures, and extent of reaction. 

The catalytic mechanism of GSHPx action is very complex and is even now only partially understood. The original 1975 proposal of Ganther[17] is shown in Fig. 4. This mechanism has however been challenged [18] and it is clear that the actual mechanism is very much more complex than that proposed by Ganther[17]. The specificity with respect to substrate is absolute with respect to GSH and even very similar analogues of GSH cannot support the enzymic activity. With respect to the substrate toward which the enzyme catalysis is... 

So far, only very little attention has been focussed on the use of zeolites in biocatalysis, i.e., as supports for the immobilization of enzymes. Lie and Molin [116] studied the influence of hydrophobicity (dealuminated mordenite) and hydrophilicity (zeolite NaY) of the support on the adsorption of lipase from Candida cylindracea. The adsorption was achieved by precipitation of the enzyme with acetone. Hydrolysis of triacylglycerols and esterification of fatty acids with glycerol were the reactions studied. It was observed that the nature of the zeolite support has a significant influence on enzyme catalysis. Hydrolysis was blocked on the hydrophobic mordenite, but the esterification reaction was mediated. This reaction was, on the other hand, almost completely suppressed on the hydrophilic faujasite. The adsorption of enzymes on supports was also intensively examined with alkaline phosphatase on bentolite-L clay. The pH of the solution turned out to be very important both for the immobilization and for the activity of the enzyme [117]. Acid phosphatase from potato was immobilized onto zeolite NaX [118]. Also in this study, adsorption conditions were important in causing even multilayer formation of the enzyme on the zeolite. The influence of the cations in the zeolite support was scrutinized as well, and zeolite NaX turned out to be a better adsorbent than LiX orKX. 

R.M. Rioux (ed.). Model Systems in Catalysis Single Crystals to Supported Enzyme Mimics, DOl 10.1007/978-0-387-98049-2 l, Springer Science+Business Media, LLC 2010... 

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