Enzymes stereoselective

C2 conformation, unlike Neu5Ac occurring in the 2CS conformation), because the conformation of the aldol condensation products is due to the stereochemistry of the substrate on C-3. But in addition to that, the 3-position on the substrate is critical for the enzyme stereoselectivity. Indeed we first observed that condensation of D-arabinose with pyruvate in tenfold excess led to two diastereomers, 4-epi-KDO and KDO in the 56 44 ratio (Scheme 4) [35],... 

By reversing the ratio of acceptor/donor, namely by using D-arabinose in a 25-fold excess, the percentage of KDO, the product of the inverted enzyme stereoselectivity, could be increased up to 83% [48]. The reason for that is still unclear, but this interesting result was exploited in the preparation of KDO 2, which was achieved using the enzyme membrane reactor technique [48]. Compound 2 could be separated from its epimer according to the procedure previously described [16]. 

U. Kragl, A. Godde, C. Wandrey, N. Lubin, and C. Auge, New synthetic applications of sialic acid aldolase, a useful catalyst for KDO synthesis. Relation between substrate conformation and enzyme stereoselectivity, J. Chem. Soc. Perkin Trans. 7 119 (1994). 

Of particular interest is the study of the biological mechanisms associated with enzyme stereoselectivity and enantioselectivity. For example, MD simulations have been successful in explaining the different affinities of trypsin and acetylcholinesterase to the diastereomers of soman inhibitors [154] and the ability of subtilisin Carlsberg and a-chymotrypsin to discriminate between R-and S- configurations of chiral aldehyde inhibitors [155, 156]. 

It is generally held that an aqueous environment is desirable, although not a prerequisite, for the optimal activity of enzymes. It is assumed that in most cases, placing an enzyme in an organic medium will cause deleterious effects to the tertiary structure of the protein. However, if this was not the case, there would be potential advantages for enzyme catalysis in an organic medium. It would clearly facilitate conversion of water-insoluble substrates and aid in product recovery. The thermodynamic equilibria of certain reactions are unfavorable in aqueous systems. Enzyme stereoselectivity has been shown to improve in some nonaqueous enzyme transformations (67,68). 

In contrast to the various fermentation and bioconversion approaches that have used enzyme stereoselectivity to syntlresize L-phenylalanine as a single isomer, additional methods have been developed that rely on the same stereoselectivity to resolve racemic mixtures of chemically synthesized amino acids. One such general approach, which has been successfully commercialized for L-phenylalanine production, relies on cleavage of the L-isomer of a D,L-a-amino acid amide mixture by an enantiospecific amidase enzyme. The general procedure operated... 

Reetz, M.T., Prasad, S., Carballeira, J.D., Gumulya, Y. and Bocola, M. (2010) Iterative saturation mutagenesis accelerates laboratory evolution of enzyme stereoselectivity rigorous comparison with traditional methods. /. Am. Chem. Soc., 132,9144r-9152. 

In all cases, the immobilized and the native enzyme stereoselectively esterify the 5-(+) enantiomer. The enantiomeric excess obtained with the native enzyme increases with the amount of water as described by Hoegberg et al. [125], whereas the immobilized derivatives are not affected by the additional water, except in the case of the derivatives on SiOa. Finally, we can observe in Table 10 that at the same reaction time (192 h) and at the same amount of water (1 ml buffer), only the immobilized lipase on Si02 displayed the same enzymatic activity and enantioselectivity as the native form. 

The solvating power of an SCF resembles that of a liquid. It varies dramatically with variations of pressure and temperature near the critical point. This property is utilized for various separation processes [2,3]. Dielectric constant, solubility parameter, diffusivity, and viscosity can be adjusted simply by changing the reactor pressure or temperature [4]. This will also change enzyme stereoselectivity [5-7]. Without changing solvent and using pressure as the sole adjustable parameter, both enzyme activity and stereoselectivity can be predictably tailored in supercritical environments [8]. 

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