Chymotrypsin, optical activity

Wong and co-workers have prepared various quaternary cx-nitro-cx-methyl carboxylic acid esters by the palladium-catalyzed allylic alkylation of a-nitropropionate ester (Eq. 5.59). The products can be kinetically resolved by using cx-chymotrypsin and are converted into optical active a-methyl cx-amino acids. Such amino acids are important due to the unique biological activity of these nonproteinogenic a-amino acids.82... 

Life as we know it would be impossible without the astonishing characteristics of enzymic catalysis. This catalysis is not only highly efficient, so that reactions may proceed at low temperature and at neutral pH with the speed required by living cells, but it exhibits also a remarkable specificity. Let us cite two typical examples First, the enzyme urease catalyzes the hydrolysis of urea but of no other compound (1). Second, the catalytic action is frequently restricted to one of the antipodes of optically active substrates. Thus, chymotrypsin will catalyze the hydrolysis of acylated L-tyrosinamides, but will not catalyze the reaction of the corresponding derivatives of D-tyrosine (2). 

When racemic 173 (R = Me) was hydrolyzed in the presence of chymo-trypsin, the resulting optically active acid 173 (R = H) exhibited an ORD absorption spectrum characteristic of L-phenylalanine the starting ester possessed an axially oriented carbomethoxy group.338-362 Atropisomerism and conformational asymmetry of a precisely definable nature in a substrate are therefore recognized by chymotrypsin. X-ray diffraction studies confirmed that the chymotrypsin-active isomer has an axial ester moiety in the solid state, and that the ester mutarotates in solution to a CD-inactive isomer, whose ester group is in the equatorial position.363... 

Peptide-based polymers 62, containing imidazole, carboxyl, and hydroxymethyl functionalities, have been prepared from optically active 50d and tested as mimics of enzymes, such as chymotrypsin, which have the same functionalities (Scheme 41) [70]. These polymers exhibit markedly higher activities than the corresponding low molecular weight compounds in the hydrolysis of nitrophenyl and dinitrophenyl esters. Increased activities were... 

Figure 25.3 shows the relationship of active site of serine hydrolases. The serine hydrolases include serine proteases, lipases, and PHB depolymerases. A common feature of the serine proteases is the presence of a specific amino acid sequence -Gly-Xl-Ser-X2-Gly-. The catalytic mechanism of these enzymes is very similar and the catalytic center consists of a triad of serine, histidine, and aspartate residues [54]. The serine from this sequence attacks the ester bond nucleophilically [55]. Lipases and PHB depolymerases also have a common amino acid sequence around the active site, -Gly-Xl-Ser-X2-Gly-. These serine hydrolases may share a similar mechanism of substrate hydrolysis [21, 56]. In terms of origin of enzymes, it would be wise to consider that the enzyme had wide substrate specificity initially, and then it started to evolve gradually for each specific substrate. In the case of polyester hydrolysis, lipases showed the widest substrate specificity among serine hydrolases for hydrolysis of various polyesters ranging from a-ester bonds to (o-ester bonds. PHB depolymerases would become more specific for microbial PHB that has / -ester bonds, though it could also hydrolyze other polyesters that have -ester and y-ester bonds. Serine proteases such as proteinase K, subtilisin, a-chymotrypsin, elastase, and trypsin hydrolyze only optically active PLLA with a-ester bonds and various proteins with a-amido bonds. 

This biphenyl model compound was shown to possess the so-called primary optical specificity of a-chymotrypsin. That is, only the enantiomer related to L-phenylalanine was hydrolyzed by the enzyme. Generally speaking, there are three more levels of specific substrate recognition in enzyme catalysis. Let us consider a peptide bond in a polypeptide chain. The lateral side chain R2 is responsible for the normal specificity of the enzyme. For a-chymotrypsin, R2 is an aromatic side chain and the hydrophobic cavity a aromatic hole in the active center is there to accommodate the amino acid to be recognized by the enzyme. This is referred to as the primary structural specificity. 

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