Enzymes chiral recognition

Bacteriological sulfur, 23 577-578 Bacteriophages, 3 135 12 474 in fermentation, 11 46 Bacteriorhodopsin, 20 826, 840 photochromic material, 6 603 Bacteriosins, 12 76. See also Bacteriocins Bacteriostatic water, 18 714 Bacterium lactis, 11 7 Baculovirus expression system, 5 346 Baddeleyite, 21 489 26 623-624 colorants for ceramics, 7 346t Badische Anilin und Soda Fabrik (BASF) terpenoid manufacture process, 24 481 Baeyer-Villiger oxidation reactions, 14 592 chiral recognition by enzymes, 3 675 microbial, 16 401 Baffled shellside flow, 13 262 Baffles, in stirred tank geometries,... 

Most of the work on chiral recognition in the ground state deals with salts having chiral, primary alkylammonium cations. Another approach is the chiral discrimination between two enantiomeric anions present as counterions in metal-cation complexes (Lehn et al., 1978). Discrimination between enantiomeric transition states will be dealt with in the next section together with non-chiral mimicry of enzymic catalysis. 

A different concept of chiral recognition was used by Lehn et al. (1978) for the differentiation between pairs of enantiomeric anions. Following the terminology used for metallo-enzymes, the chiral crown ether [309] acts as an apo-receptor, complexing a metal cation and thus becoming a chiral metal receptor that may discriminate between enantiomeric anions (cascade-type complexation). Extraction experiments with racemic mandelic acid dissolved in... 

Enzymes show a high degree of structural and chiral recognition of their substrates. 

Most of the enzymes show extremely strict chiral recognitions, and only one of the enantiomers can be the substrate of the enzyme. For example, chymotrypsin incorporates L-peptides only to the enzyme-substrate binding site to form enzyme-substrate complex, so it shows very high enantioselectivity (Figure 3 (a)). Oxidoreductases also form the enzyme-substrate complex of only one enantiomer, so enantioselectivities are high when isolated enzymes are used for reactions instead of whole cells containing both (R)- and (.S )-specific enzymes, which leads to overall low enantioselectivities. 

In the case of lipases and esterases, chiral recognitions are not so strict. Both enantiomers were incorporated to the enzyme to form the substrate-enzyme complex. However, the slow reacting enantiomer lacked the necessary hydrogen-bonding interaction, for example in the hydrolysis of menthol acetate, between the substrate menthol and the enzyme histidine group for the reaction to proceed further (Figure 3(b)).2 3 The explanation was also supported by the observation in the esterification reaction of 1-phenylethanol by lipases.4 Km values of the slow and fast reacting... 

Enomoto, N., Furukawa, S., Ogasawara, Y., Akano, H., Kawamura, Y., Yashima, E., and Okamoto, Y. (1996) Preparation of silica gel-bonded amylose through enzyme-catalyzed polymerization and chiral recognition ability of its phenylcarbamate derivative in HPLC, Anal. Chem. 68, 2798-2804. 

The first example of absolute chiral recognition of a prochiral centre by a small molecule (cf. enzyme reactions) is the reaction of A( — )436-oc[(2S,9S)-2,9-diamino-4,7-diazadecanecobalt(m)]dichloride with a,a-aminomethylmalonate, which produces A( — )436-P2[(2S,9S)-2,9-diamino-4,7-diazadecanecobalt(in)-(R)-aminomethyl-malonate]+,401 and a crystal structure determination of the product has been performed.402... 

Chiral recognition of epinephrine by an enzyme. Only the levorotatory enantiomer fits into the active site of OH rV0H... 

The racemase enzymes present an interesting situation in terms of chiral recognition. In terms of the classification discussed later, they lack both substrate and product selectivity. The simplest kinetic equations that could be written are as follows (D and L represent the two enantiomers of a substrate, E the enzyme) ... 

A completely different approach was pursued by Fuji and coworkers [14]. The enantiomeri-cally pure pyridine derivative 18 was prepared, the rationale being that the reduced steric hindrance in the proximity of the nitrogen nucleus should result in efficient catalytic turnover and that chiral recognition might be possible by remote asymmetric induction in a process similar to the induced fit mechanism in enzymes. When 18 was reacted with racemic 19b and isobutyric anhydride (l/ ,25)-19b was recovered with ee = 81% (71% conversion, 5 = 4.5,... 

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