Enzymes, active conformation flexibility

However, the nucleoside analogue 164 was found to be devoid of activity against HSV-1, HSV-2, VZV and the cytomegalovirus (CMV) in human fibroblast (MRC-5) cells. In this case the decreased conformational flexibility resulting from the introduction of the cyclopropyl group into 164 appeared to be unfavourable for interaction with the enzymes involved (vide supra. Sect. 2.9) [222]. Likewise, the cyclopropylpyrimidine 166c-f and 167, the cyclopropyl-purine nucleosides 168 showed no antiviral activity against HSV-1, HSV-2, HCMV and HIV-1 in cell culture, Eq. (66) [223]. 

The hydrophobic ionic liquid [BMIMJPFg has been consistently shown to provide the desired conformational flexibility to enzymes without drastically altering their catalytically active conformations. Therefore, this ionic liquid has been widely studied as a suitable medium for enzymatic transformations. 

It is worth noting that the enzyme can be withdrawn and recycled by using supercritical CO2. The success of the polymerizations carried out in organic solvents stems directly from the sustained activity of several lipases in organic solvents. In this respect, it must be noted that water has a manifold influence on the course of the polymerization. On the one hand, water can initiate the polymerization. On the other hand, a minimum amount of water has to be bound to the surface of the enzyme to maintain its conformational flexibility, which is essential for its catalytic activity [94]. Lipase-mediated polymerization cannot therefore be achieved in strictly anhydrous conditions. 

Early enzymatic theory emphasized the importance of high complementarity between an enzyme s active site and the substrate. A closer match was thought to be better. This idea was formally described in Fischer s lock and key model. The role of an enzyme (E), however, is not simply to bind the substrate (S) and form an enzyme-substrate complex (ES) but instead to catalyze the conversion of a substrate to a product (P) (Scheme 4.2). Haldane, and later Pauling, stated that an enzyme binds the transition state (TS ) of the reaction. Koshland expanded this theory in his induced fit hypothesis.5 Koshland focused on the conformational flexibility of enzymes. As the substrate interacts with the active site, the conformation of the enzyme changes (E — E ). In turn, the enzyme pushes the substrate toward its reactive transition state (E TS ). As the product forms, it quickly diffuses out of the active site, and the enzyme assumes its original conformation. 

For DMDBT oxidation, CPO physically immobilized on SBA-16 of 90 A had a higher thermostability than the free enzyme, retaining 50% of its activity at 45°C after 187 h while the free enzyme was half-inactivated after 68 h. This could be due to the restricted movement of the immobilized enzyme confined in the pores of this material. In contrast, the immobilization in material with a larger pore of 117 A did not improve the thermostability of the enzyme, probably due to the fact that larger pores did not prevent the increased conformational flexibility of the enzyme at this temperature. 

We may also speculate concerning a reason for the increase in the protein rigidity and correspondent decrease in sensitivity of the spin labels rotational diffusion to temperature increase above Tin = 312 K. Efficiency of the chemical processes requires optimum flexibility of the enzyme active site. If the low-temperature tendency toward an increase in the enzyme conformational flexibility in the active area would continue at high temperatures, such an optimization would be destroyed. Thus, the conformational transition may be necessary for maintaining a balance between activity and stability of the enzyme at high temperatures. 

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