Gwo-Jenn Shen

Enzymatic catalysis in organic synthesis.

Publisher Summary This chapter describes the fundamental concepts and the practical aspects regarding the design and development of enzymatic catalysts for synthetic organic transformations. The rate acceleration and specificity of enzymatic reactions that operate under mild conditions are the major advantages of enzymes used in organic synthesis. According to transition state theory and the thermodynamic cycle, in a given enzyme-catalyzed reaction the catalyst binds to the reaction transition state more strongly than to the ground state substrate by a factor approximately equal to the rate acceleration. All types of catalysis in enzymatic reactions, such as acid-base catalysis, nucleophilic–electrophilic catalysis, and catalysis by approximation, strain, and distortion, are just the contributing factors that lead to reducing the transition state energy. Enzyme-catalyzed organic reactions have been extended from the synthesis of chiral synthons and low molecular weight substances such as sugars and peptides to more complex molecules such as oligosaccharides, polypeptides, nucleotides, and their conjugates. All recombinant DNA work today requires several key enzymatic reactions to construct the gene for the expression of a desired protein. The recombinant DNA technology, however, has made possible the low-cost production of enzymes and the rational alteration of enzymatic properties. The area of enzymatic catalysis is further stimulated by the exciting new discovery of catalytically active antibodies. With the increasing environmental concerns and regulatory constraints faced in the chemical and pharmaceutical industries, enzyme-based organic synthesis becomes an attractive alternative that may offer clean and mild catalytic processes for the synthesis of single stereoisomers.

Overexpression, one-step purification and characterization of UDP-glucose dehydrogenase and UDP-N-acetylglucosamine pyrophosphorylase

Two enzymes of the Leloir pathway, UDP-GlcNAc pyrophosphorylase and UDP-Glc dehydrogenase, which are involved in the synthesis of activated sugar nucleotides have been cloned, overexpressed in Escherichia coli, and purified to homogeneity in only one step by chelation-affinity chromatography. The gene KfaC of E. coli K5 was thus demonstrated to encode UDP-Glc DH. Some properties of the cloned enzymes, such as stability, pH dependence, and substrate kinetics, were studied in order to facilitate the use of these enzymes in carbohydrate synthesis, especially in the synthesis of hyaluronic acid.

Cloning, overexpression and isolation of the type II FDP aldolase from E. coli for specificity study and synthetic application

A stable overexpression E. coli strain containing the plasmid pKEN 2 for the production of the Zn(2+)-dependent FDP aldolase from E. coli has been developed. Approximately 14,000 U of the enzyme (specific activity = 23.3 U/mg) can be obtained from 4-L of growth medium. The enzyme was isolated, purified to homogeneity and used for the studies of stability, substrate specificity and metal ion replacement and dissociation. Crystals of the enzyme have been obtained for structural analysis. This E. coli strain was deposited with the American Type Culture Collection (ATCC #77472).

A new NAD-dependent alcohol dehydrogenase with opposite facial selectivity useful for asymmetric reduction and cofactor regeneration

A new NAD-dependent alcohol dehydrogenase isolated from a Pseudomonas species catalysed the reduction of many acyclic ketones to optically active alcohols with very high enantioselectivity (90 to >98% enantiomeric excess); the stereochemical course of the reduction was determined to be the transfer of the pro-(R) hydrogen from NADH to the Si face of the carbonyl group, a process different from that for other known alcohol dehydrogenases.

Cloning and overexpression of rhamnose isomerase and fucose isomerase.

Rhamnose isomerase and fucose isomerase were overexpressed in E. coli, purified and characterized. The rhamnose isomerase gene was ligated to the restriction sites of PstI and Hind III of vector pTrcHis and the fucose isomerase gene was ligated to the EcoRI and PstI sites of vector pKK223-3 for overexpression of the enzymes in E. coli XL1-Blue MRF. Approximately 16,500 U of active fucose isomerase and 2400 of rhamnose isomerase can be obtained per liter of culture from these expression systems.

Enzymes for carbohydrate and peptide syntheses

New practical procedures have been developed for the enzyme-catalyzed synthesis of carbohydrates and peptides. Aldolases have been shown to be effective catalysts for the synthesis of uncommon sugars, particularly azasugars. Enzymatic methods for the large-scale synthesis of oligosaccharides have been developed with the use of glycosyltransferases coupled with the regeneration of sugar nucleotides. Engineered subtilisin variants that are stable and active in anhydrous dimethylformamide and in aqueous solution have been developed for peptide segment coupling.

A new alcohol dehydrogenase with unique stereospecificity from Pseudomonas sp.

Abstract A new nicotinamide cofactor-dependent alcohol dehydrogenase from Pseudomonas strain SBD6 (PADH) was isolated and purified 150-fold to homogeneity using a combination of salt precipitation, anion-exchange chromatography, gel filtration chromatography, and dye matrix chromatography. Approximately 10 mg of pure enzyme can be obtained from 10 g of wet cells. The enzyme has four subunits with a total molecular weight of 162,000. Incubation with the metal chelators 1,10-phenanthroline, 2-aminoethanethiol, hydroxyquinolinesulfonic acid, N-ethylmaleimide, and potassium cyanide result in complete loss of activity. The enzyme is very stable ( t 1 2 ∼ 7 days at pH 7 and 25°C in the absence of 2-propanol and ∼ 18 days in the presence of 10% 2-propanol, v/v) and possesses a broad substrate specificity with transfer of the pro-(R) hydride from NADH to the si face of carbonyl substrates to give (R)-alcohols in high enantiomeric excess, a stereochemical process different from that of other known alcohol dehydrogenases. Synthetic scale reductions are facilitated with 2-propanol as a hydride source for the regeneration of NADH. The kinetic mechanism is ordered bi-bi with the cofactor binding first. Based on NAD and 2-propanol, the kinetic parameters of the enzyme were determined to be Vmax = 29.9 Units mg−1 at 25°C and pH 8.5, KmNAD = 0.36 m m and Km2-propanol = 0.19 m m .

Cloning and Overexpression of a Tagged CMP-N-Acetylneuraminic Acid Synthetase from E. coli Using a Lambda Phage System and Application of the Enzyme to the Synthesis of CMP-N-Acetylneuraminic Acid

The gene coding from CMP-N-acetylneuraminic acid (CMP-NeuAc) synthetase (Ec 2.7.7.43) was amplified from total DNA of E. coli strain K-235 through a primer-directed polymerase chain reaction. The gene was fused with a modified ribosome binding site of the original CMP-NeuAc synthetase gene and a decapeptide tag sequence which served as a marker for screening of expressed proteins. The gene was cloned into lambda ZAP vector at EcoRI and XbaI sites and overexpressed in E. coli Sure at a level approximately 1000 times that of the wild type. The decapeptide-containing enzyme retained almost the same specificity as indicated by the Vmax and Km values using CTP and NeuAc as substrates. A preparative synthesis of CMP-NeuAc based on the recombinant enzyme was demonstrated.

Practical synthesis of carbohydrates based on aldolases and glycosyl transferases

With various recombinant DNA and protein engineering techniques now available, enzyme-bad technologies are emerging as practical methods for large-scale synthesis of chiral intermediates and bioactive molecules, especially carbohydrates, oligosaccharides, their conjugates and related substances. This paper describes recent developments in the synthesis of novel monosaccharides and aza sugars based on aldolases, and the synthesis of oligosaccharides and analogs based on glycosyltransferases coupled with in sins regeneration of sugar nucleotides. As many enzymes are available for the stereocontrolled synthesis of chiral synthons (l), attention has been extended to the development of more effective and stable enzymes for the synthesis of molecules with increasing complexity (2). One class of such complex molecules are carbohydrates and their conjugates, especially those that exist on cell surfaces (3). These molecules are involved in many types of recognition phenomena (3-6); however, most of their precise functions have not been clearly identified at the molecular level. Part of the reason is that these molecules have been difficult to isolate, characterize and synthesize. Enzyme-based technology seems to be well suited for the synthesis of glycoconjugates and related substances for the study of their functions as these molecules are multifunctional and highly soluble in polar solvents, and many enzymes are available for the transformation of these molecules (7). The following describes some new technologies developed for the synthesis of sugar- and peptide- related substances based on recombinant or engineered enzymes.

Construction of a Diverse Fab Expression Library from Autoimmunized Mice Based on an Improved Preparation of Cloning Arms from Bacteriophage Vectors: A New Library with Potential for Screening of Biocatalysts

This paper describes an improved procedure for the preparation of cloning arms from vectors Lc1 and Hc2 for the construction of cDNA libraries in phage δ for expression in E. coli of the combinatorial Fab fragments of the immunoglobulin repertoire from autoimmunized mice. It was found that annealing, dialysis and dephorphorylation after the restriction enzyme digestion of vectors were critical for the high efficient preparation of cloning arms. The construction of Fab cDNA libraries involves the use of primer-directed PCR amplification products of total RNA from autoimmunized mice. Fab production was confirmed by the immunoassay and sequence analysis. The partial sequenced genes for Fab fragments were different from each other in the FR4, CDR3 and FR3 regions, indicating a diverse nature of the library. This library may be used to screen for catalytic Fabs.