Enzymatic catalysts in organic synthesis

A tutorial on asymmetric electrocatalysis

Electrochemistry has emerged as a powerful means to enable redox transformations in modern chemical synthesis. This tutorial review delves into the unique advantages of electrochemistry in the context of asymmetric catalysis. While electrochemistry has historically been used as a green and mild alternative for established enantioselective transformations, in recent years asymmetric electrocatalysis has been increasingly employed in the discovery of novel asymmetric methodologies based on reaction mechanisms unique to electrochemistry. This tutorial review first provides a brief tutorial introduction to electrosynthesis, then explores case studies on homogenous small molecule asymmetric electrocatalysis. Each case study serves to highlight a key advance in the field, starting with the historic electrification of known asymmetric transformations and culminating with modern methods relying on unique electrochemical mechanistic sequences. Finally, we highlight case studies in the emerging reasearch areas at the interface of asymmetric electrocatalysis with biocatalysis and heterogeneous catalysis.

Enzyme-catalyzed synthesis of bioactive heterocycles

Enzymes are proteins that functions as biological catalyst. It is now a known fact that enzyme can catalyze many synthetic operations better than the conventional reagents. Not only in the synthesis of natural products, they can also be applied for construction of varieties of unnatural compounds. In this chapter, Pariyar and Ghosh have discussed in brief synthesis of various biologically active heterocyclic compounds using different enzymes as catalysts. Among various enzymes, laccases, trypsin, α-amylase and Bakers’ yeast are few that are easily available and have been extensively explored for various synthetic strategies. This chapter will definitely serve as valuable source of information to the readers in the field of enzyme-catalyzed reactions.

CaLB Catalyzed Conversion of ε-Caprolactone in Aqueous Medium. Part 1: Immobilization of CaLB to Microgels

The enzymatic ring-opening polymerization of lactones is a method of increasing interest for the synthesis of biodegradable and biocompatible polymers. In the past it was shown that immobilization of Candida antarctica lipase B (CaLB) and the reaction medium play an important role in the polymerization ability especially of medium ring size lactones like ε-caprolactone (ε-CL). We investigated a route for the preparation of compartmentalized microgels based on poly(glycidol) in which CaLB was immobilized to increase its esterification ability. To find the ideal environment for CaLB, we investigated the acceptable water concentration and the accessibility for the monomer in model polymerizations in toluene and analyzed the obtained oligomers/polymers by NMR and SEC. We observed a sufficient accessibility for ε-CL to a toluene like hydrophobic phase imitating a hydrophobic microgel. Comparing free CaLB and Novozym® 435 we found that not the monomer concentration but rather the solubility of the enzyme, as well as the water concentration, strongly influences the equilibrium of esterification and hydrolysis. On the basis of these investigations, microgels of different polarity were prepared and successfully loaded with CaLB by physical entrapment. By comparison of immobilized and free CaLB, we demonstrated an effect of the hydrophobicity of the microenvironment of CaLB on its enzymatic activity.

Simple preparation of pacific cod trypsin for enzymatic Peptide synthesis

Trypsin from the pyloric caeca of Pacific cod (Gadus macrocephalus) was easily prepared by affinity chromatography on Benzamidine Sepharose 6B and gel filtration on Superdex 75. Pacific cod trypsin was composed of three isozymes, and their molecular masses were estimated 23,756.34 Da, 23,939.62 Da, and 24,114.81 Da by desorption/ionization time-of-flight mass spectroscopy (MALDI/TOF-MS) and their isoelectric points (pIs) were approximately 5.1, 6.0, and 6.2, respectively. The isolated Pacific cod trypsin showed high similarity to other frigid-zone fish trypsins. The kinetic behavior of tryptic hydrolysis toward N-p-tosyl-L-arginine methyl ester hydrochloride (TAME), N-benzoyl-L-arginine p-nitroanilide hydrochloride (BAPA), and p-amidinophenyl ester were also analyzed. In addition, the cod trypsin-catalyzed dipeptide synthesis was investigated using twelve series of "inverse subdtrates" that is p- and m-isomer of amidinophenyl, guanidinophenyl, (amidinomethyl)phenyl, (guanidinomethyl)phenyl, and four position isomers of guanidinonaphtyl esters derived from N-(tert-butoxycarbonyl)amino acid as acyl donor components. They were found to couple with an acyl acceptor such as L-alanine p-nitroanilide to produce dipeptide in the presence of the trypsin. All inverse substrates tested in this study undergo less enantioselective coupling reaction. The p-guanidinophenyl ester was most practical substrate in twelve series tested. The enzymatic hydrolysis of the resulting products was negligible.

Atlantic cod trypsin-catalyzed peptide synthesis with inverse substrates as acyl donor components

Atlantic cod trypsin-catalyzed peptide synthesis has been studied by using p-amidino- and p-guanidinophenyl esters of N-(tert-butyloxycarbonyl)amino acid as acyl donor components. The reaction temperature was optimized at 0 degrees C. The method was shown to be successful as effectively for synthesizing the peptide and useful for preparing dipeptide between D-amino acid with D-amino acid and beta-amino acid with beta-amino acid, respectively. The enzymatic hydrolysis of the resulting products was negligible.

Solid-phase peptide synthesis in water using microwave-assisted heating

An approach using water as a solvent (coupling and deprotection) was developed for the solid-phase synthesis of peptides using the most common Boc-amino acid derivatives. Key aspects of this methodology are the use of a PEG-based resin, EDC-HONB as a coupling method, and microwave irradiation as an energy source.

Papain kinetics in the presence of a water-miscible organic solvent

The effects of various concentrations of a water-miscible organic solvent [a 7:3 (v/v) mixture of N, N dimethylformamide and dimethylsulfoxide] on the kinetics of papain have been investigated. The parameters k(cat) and K(m) for the amidase and esterase activity of papain using N-alpha-benzoyl-DL-arginine-p-nitroanilide (BAPNA) and N-alpha-benzoyl-L-arginine ethyl ester (BAEE) as substrates were determined. For both types of activity, k(cat) initially increased (up to about 15% solvent), and then decreased with increasing concentrations of organic solvent. In contrast, K(m) increased sharply with the organic solvent concentration. Active site titration at 0 and 50% solvent indicated no change in the amount of active enzyme. Fluorometric measurements of the emission spectrum of papain did not indicate any major conformational changes with increasing concentrations of organic solvent.

Application of several types of substrates to ficin-catalyzed peptide synthesis

The capability of ficin, a cystine protease, to form peptide bonds was investigated using several types of N-Boc-amino acid phenyl and naphthyl esters as acyl donor components. Enzyme-catalyzed peptide synthesis was carried out under optimized reaction conditions of pH, acyl acceptor concentration and selection of the best yield organic solvent. It used a condensation of N-Boc-Ala-OpGu and Ala-pNA as a model reaction. The products were obtained in 72-96% yield using 10 different substrates, within a few minutes of reaction time.

An efficient asymmetric synthesis of (S)-atenolol: using hydrolytic kinetic resolution

Enantiomerically pure (S)-atenolol was prepared by using (R,R) salen Co(III) complex for the resolution of terminal epoxide. This process was carried out at room temperature in excellent enantio selectivity. The method can be applied for large-scale preparation of (S)-atenolol without any problem.

Microbial alkaline proteases: from a bioindustrial viewpoint

Alkaline proteases are of considerable interest in view of their activity and stability at alkaline pH. This review describes the proteases that can resist extreme alkaline environments produced by a wide range of alkalophilic microorganisms. Different isolation methods are discussed which enable the screening and selection of promising organisms for industrial production. Further, strain improvement using mutagenesis and/or recombinant DNA technology can be applied to augment the efficiency of the producer strain to a commercial status. The various nutritional and environmental parameters affecting the production of alkaline proteases are delineated. The purification and properties of these proteases is discussed, and the use of alkaline proteases in diverse industrial applications is highlighted.

Purification and characterization of an alkaline lipase from a newly isolated Pseudomonas mendocina PK-12CS and chemoselective hydrolysis of fatty acid ester

Lipase isolated from a soil isolate, Pseudomonas mendocina (PK-12CS) chemoselectively hydrolyzed the fatty ester group in presence of arbamate of compound 5-amino-2,4-dihydro-3H-1,2,4-triazole-3 ones, a class of compounds which are attractive starting materials for the synthesis of triazole annealed heterocycles. The enzymatic method provides an easy access to the synthesis of N-substituted glycine. Under optimized fermentation conditions the culture produced 3510 Lipolytic Units/mL of cell free fermentation broth in 20 h of fermentation. The purified lipase exhibited molecular mass of 80 kDa on SDS polyacrylamide gel electrophoresis. The enzyme was stable at room temperature for more than a month and expressed maximum activity at 37 degrees C and pH 8.

Thermostable NAD+-dependent (R)-specific secondary alcohol dehydrogenase from cholesterol-utilizingBurkholderia sp. AIU 652

An alcohol dehydrogenase produced by Burkholderia sp. AIU 652, which was isolated with a cholesterol medium, was purified to homogeneity and characterized. The enzyme had broad substrate specificity, and the best reaction was the reversible oxidation of 2-propanol to acetone and 2-butanol to 2-butanone. The K(m) values for secondary alcohols and ketones were much lower than those for primary alcohols or diols. In addition, the enzyme oxidized R-(-)-alcohols in preference to S-(+)-alcohols, and utilized NAD+, but not NADP+ as the cofactor. The molecular mass was 150 kDa with four identical subunits, and the activity was inhibited by o-phenanthroline, 8-hydroxyquinoline, and alpha,alpha'-dipyridyl. Thus, this enzyme was classified into a group of NAD+-dependent R-(-)-specific secondary alcohol dehydrogenases. However, this enzyme was better than the previously reported NAD+-dependent R-(-)-specific secondary alcohol dehydrogenases for chiral chemical synthesis in terms of substrate specificity, stereospecificity, and thermostability. This enzyme might be applicable as an effective biocatalyst for the production of chiral alcohols and related compounds.

Trypsin-catalysed synthesis of oligopeptide amides: comparison of catalytic efficiency among trypsins of different origin (bovine, Streptomyces griseus and chum salmon)

A procedure has been developed for the synthesis of oligopeptide amide using inverse substrates as acyl donors with amino acid amide instead of p-nitroanilide as acyl acceptor and trypsins of different origin (bovine, Streptomyces griseus and chum salmon trypsins) as the catalyst. The effectiveness of this procedure was demonstrated by the synthesis of a pentapeptide, Boc-[Leu5]-enkephalin amide, as a model compound. The method was the first enzymatic method shown to be successful at each successive coupling step for the synthesis of the oligopeptide. Bovine and chum salmon trypsins were superior to Streptomyces griseus trypsin as the catalyst.

Enzyme catalysis in reverse micelles

Water in oil microemulsions with reverse micelles provide an interesting alternative to normal organic solvents in enzyme catalysis with hydrophobic substrates. Reverse micelles are useful microreactors because they can host proteins like enzymes. Catalytic reactions with water insoluble substrates can occur at the large internal water-oil interface inside the microemulsion. The activity and stability of biomolecules can be controlled, mainly by the concentration of water in these media. With the exact knowledge of the phase behaviour and the corresponding activity of enzymes the application of these media can lead to favourable effects compared to aqueous systems, like hyperactivity or increased stability of the enzymes.

Microbial and reducing agents catalyze the rearrangement of taxanes

5alpha, 7beta, 9alpha, 10beta, 13alpha-Pentahydroxy-4(20),11(12)-taxadiene derivative 1 was converted to two unprecedented 1(15-->11)abeo-taxanes and a taxane derivative with a C10-C11 double bond by Absidia coerula ATCC 10738a. A similar compound was obtained from treatment with zinc of a triacetoxy-4(20),11(12)-taxadiene derivative.

Enzymatic coupling of specific peptides at nonspecific ligation sites: effect of Asp189Glu mutation in trypsin on substrate mimetic-mediated reactions

Two main drawbacks seriously restrict the synthetic value of proteases as reagents in peptide fragment coupling: (i) native proteolytic activity and, thus, risk of undesired peptide cleavage; (ii) limited enzyme specificities restricting the amino acid residues between which a peptide bond can be formed. While the latter can be overcome by the use of substrate mimetics achieving peptide bond formation at nonspecific ligation sites, the risk of proteolytic cleavage still remains and hinders the wide acceptance of this powerful strategy for peptide coupling. This paper reports on the effect of the trypsin point mutant Asp189Glu on substrate mimetic-mediated reactions. The effect of this mutation on the steady-state hydrolysis of substrate mimetics of the 4-guanidinophenyl ester type and on trypsin-specific Lys- and Arg-containing peptides was investigated. The results were confirmed by enzymatic coupling reactions using substrate mimetics as the acyl donor and specific amino acid-containing peptides as the acyl acceptor. The competition assay verifies the predicted shift in substrate preference from Lys and Arg to the substrate mimetics and, thus, from cleavage to synthesis of peptide bonds. The combination of results obtained qualifies the trypsin mutant D189E as the first substrate mimetic-specific peptide ligase.

Purification and characterisation of an ester hydrolase from a strain of Arthrobacter species: its application in asymmetrisation of 2-benzyl-1,3-propanediol acylates

An ester hydrolase (ABL) has been isolated from a strain of Arthrobacter species (RRLJ-1/95) maintained in the culture collection of this laboratory. The purified enzyme has a specific activity of 1700 U/mg protein and is found to be composed of a single subunit (Mr 32,000), exhibiting both lipase and esterase activities shown by hydrolysis of triglycerides and p-nitrophenyl acetate respectively. Potential application of the enzyme concerns the asymmetrisation of prochiral 2-benzyl-1,3-propanediol esters besides enantioselective hydrolysis of alkyl esters of unsubstituted and substituted 1-phenyl ethanols.

Protein engineering of subtilisin

The serine protease subtilisin is an important industrial enzyme as well as a model for understanding the enormous rate enhancements affected by enzymes. For these reasons along with the timely cloning of the gene, ease of expression and purification and availability of atomic resolution structures, subtilisin became a model system for protein engineering studies in the 1980s. Fifteen years later, mutations in well over 50% of the 275 amino acids of subtilisin have been reported in the scientific literature. Most subtilisin engineering has involved catalytic amino acids, substrate binding regions and stabilizing mutations. Stability has been the property of subtilisin which has been most amenable to enhancement, yet perhaps least understood. This review will give a brief overview of the subtilisin engineering field, critically review what has been learned about subtilisin stability from protein engineering experiments and conclude with some speculation about the prospects for future subtilisin engineering.