Current Organic Chemistry - Volume 7, Issue 13, 2003

The Present issue of Current Organic Chemistry is dedicated to Enzymatic Catalysis. There is no doubt that enzymes are gaining an utmost important in organic chemistry thanks to their well-known selectivity (chemo-, regio- and enantio-) displayed under mild and environmental friendly reaction conditions. Moreover, the impressive development of molecular biology is making available an increasing number of enzymes with desired properties. We can reasonably affirm that biocatalysis has given its contribution to solve problems in any area of organic chemistry, showing that it is possible to find a biological equivalent for any chemical reaction (for instance, recent reports have described enzymes able to enzymes able to catalyze Diels-Alder reactions). The following collection of papers will offer a wide overview of the opportunities offered today by enzymatic catalysis. It is divided into two parts, the first devoted to the presentation of classes of enzymes not yet well-known to synthetic chemists and to new methodologies in biocataylsis. Martínkova and Mylerova contributed the first article on “Synthetic applications of nitrile-converting enzymes”, a subject that is relatively new for synthetic applications but that already sees several industrial applications on multi-tons scale. The second contribution by Poppe and Rétey is entitled “Properties and synthetic applications of ammonia-lyases”. It is a detailed report on the fundamental biochemistry and on the structure determination of enzymes that in vivo catalyze double bonds formation by ammonia elimination. They synthetic potentiality relays on the reverse reaction, that is on the stereoselective addition of ammonia to double bonds to give natural and unnatural amino acids. The third review by Burton concerns “Laccases and phenol oxidases in organic synthesis”. Using molecular oxygen as oxidant, these enzymes provide effective methods to achieve asymmetric oxy-functionalisation reactions and routes tocontrolled and predictable formation of oxy-and hydroxylated products and to polymers. At variance, the fourth review by DeMartin and coworkers, “Biocatalysis in reaction mixtures with undissolved solid substrates and products”, is related to the development of a reaction methodology in which substrates and / or products are mainly undissolved. Using this approach reaction yields can be improved and the necessity to use organic solvents to shift the thermodynamic equilibrium toward synthesis can be reduced by product precipitation, which makes the synthesis catalyzed by hydrolytic enzymes favourable even in water. The second part of this special issue is devoted to practical applications. Brenna describes in details the “Enzymes-mediated synthesis of chiral communication substances: fragrances for perfumery applications”. The use of enzyme-mediated reaction for the preparation of enantiomerically enriched chiral fragrances are exemplified by taking into consideration the main floral, musk and woody-ambery odorants employed in fine and functional perfumery. The microbial / enzymatic processes finalized to the synthesis of chiral intermediates for bioactive molecules (antihypertensive drugs, anticholesterol drugs, anticancer agents, antiviral agents, receptor agonists, and anti-Alzheimer's drugs), mainly taken from the research carried out at Brystol-Myers Squibb, have been described in the contribution by Patel: “Microbial / enzymatic synthesis of chiral intermediates for pharmaceuticals: case studies from BMS”. The contribution by Uyame Kobayashi (“Enzymatic Synthesis of Polyphenols”) exemplifies the non-conventional use of enzymes for polymer production. Specifically, oxidative enzymes have been reported to catalyze the formation of various polyphenols, included analogous of natural lacquers (artificial urushi). Finally, Galante and Formantici present a comprehensive overview on: “Enzyme Applications in Detergency and Manufactoring Industries”. This review is particularly impressive not only for the details and completeness of the description but because it draw the attention of those chemists who are mainly dedicated to academic research, to a practical aspect of enzyme utilization that is commonly overlooked and concerns the quality of our everyday life. We kindly thank all the authors for their excellent efforts and we welcome any ideas for new topics to be discussed in future COC issues.

The use of nitrile hydratases (RCN + H2O → RCONH2) and nitrilases (RCN + 2 H2O → RCOO- + NH4 +) in biocatalysis attracts profound attention from the beginning of 1980s since it provides a useful alternative to the conventional nitrile hydrolysis using strong acid or base catalysts. In the past 5 years the potential of this branch of biotransformations has been broadened significantly by both the isolation of new mesophilic and thermophilic nitrile-metabolizing microorganisms and the purification of new nitrile-converting enzymes, including thermostable ones. Currently, the scope of products afforded by nitrile-converting biocatalysts encompass hundreds of compounds such as aliphatic, alicyclic, aromatic and heterocyclic carboxylic acids and their amides. New nitrile hydratases show high enantioselectivity towards racemic substituted 2-arylpropionitriles, 2-arylbutyronitriles and prochiral 3-arylglutaronitriles. Synthesis of optically active α-hydroxy and α-fluoro amides and acids employs whole-cell biocatalysts or purified nitrilases. (E)-Selectivity for a,ß-unsaturated nitriles and cis- or trans-selectivity for alicyclic nitriles is also demonstrated with some nitrile-converting enzymes. Regioselectivity and chemoselectivity of the biocatalysts enable preparation of diverse functional group-containing products such as cyanocarboxylic acids, cyanoamides, hemiesters and their amides and carboxylic acids and amides with ether groups. Stabilization of the nitrile-converting biocatalysts is achieved by entrapment in hydrogels or by lyophilization. Organic-aqueous mixtures are suitable as reaction media for numerous nitrile-converting enzymes.

Ammonia-lyases catalyze a wide range of processes leading to α,β-unsaturated compounds by elimination of ammonia having few features in common. They utilize an extraordinary range of prosthetic groups; e.g. coenzyme B12, dipyrromethane cofactor, pyridoxal-phosphate, or the 3,5-dihydro-5-methylidene-4H-imidazol- 4-one (MIO) prosthetic groups. In this work, ammonia-lyases are reviewed with major emphasis on various aspects of biotransformations, covering topics from the fundamental biochemistry of ammonia-lyases to their structure determined by protein crystallography.

This is a review of recent progress in developing biocatalytic reactions involving two major groups of phenol oxidases, viz., laccases (benzenediol:oxygen oxidoreductase E.C. 1.10.13.2) and polyphenol oxidases (phenol: oxygen oxidoreductase, E.C. 1.14.18.1, also known as tyrosinase). Oxidases catalyse reactions involving direct activation of oxygen, and within this group, the multi-copper oxidases catalyse the four-electron reduction of oxygen to water with the concomitant one-electron oxidation of a substrate. Such reactions, applied in biocatalytic systems, provide effective methods to achieve oxyfunctionalisation, (particularly asymmetric oxyfunctionalisation reactions) and routes to controlled and predictable formation of oxy- and hydroxylated products, and polymers. The review includes discussion of the characteristics of these enzymes which are of relevance to their application as biocatalysts.

Enzymatic synthesis in reaction mixtures with mainly undissolved substrates and / or products is a synthetic strategy in which the compounds are present mostly as pure solids. It retains the main advantages of conventional enzymatic synthesis such as high regio- and stereoselectivity, absence of racemisation and reduced side-chain protection. The reaction yields are improved and the necessity to use organic solvents to shift the thermodynamic equilibrium toward synthesis is reduced by product precipitation, which makes the synthesis favourable even in water. The thermodynamics of these reaction systems have been investigated in the last few years, resulting in methods to predict the direction of a typical reaction a priori. Furthermore, studies on kinetics, enzyme concentration, pH / temperature effects, mixing and solvent selection have opened new perspectives for the understanding, modelling, optimisation and the possible large scale application of such a strategy. In this review we have tried to cover most of the literature published in the last five to ten years on biocatalysis in substrate suspensions, focusing especially on cases where the reaction products precipitate.

This work describes the use of enzyme-mediated reactions for the preparation of enantiomerically enriched chiral fragrances. The main floral, musk and woody-ambery odorants employed in fine and functional perfumery are taken into consideration.

There has been an increasing awareness of the enormous potential of microorganisms and enzymes for the transformation of synthetic chemicals with high chemo-, regio- and enatioselective manner. Chiral intermediates are in high demand from pharmaceutical industries for the preparation bulk drug substances. In this review article, microbial / enzymatic processes mainly taken from the research carried out at BMS have been described for the synthesis of chiral intermediates for antihypertensive drugs, anticholesterol drugs, anticancer agents, antiviral agents, β3-receptor receptor agonists, melatonin receptor agonists, and anti-Alzheimer's drugs.

In vitro synthesis of polyphenols using isolated enzymes as catalyst via non-biosynthetic pathways is reviewed. Various phenols have been subjected to an oxidative polymerization catalyzed by peroxidase, laccase, or bilirubin oxidase under mild reaction conditions. In most cases, polyphenols with a mixed structure of phenylene and oxyphenylene units have been formed. By utilizing specific enzymatic catalysis, regio- and chemoselective polymerizations have occurred, yielding functional and useful polymers, which are often difficult to synthesize by conventional methodologies. Enzymatic curing of urushiol analogues produces crosslinked polymeric films (“artificial urushi”) with high hardness and good elasticity.

Applications of enzymes in industrial and food processes have undergone remarkable developments in several areas in the last 10-20 years: detergent, textile, grain wet milling, food, monogastric animal feed, pulp & paper, leather, natural polymer modifications, organic chemical synthesis, diagnostics, etc. Recombinant DNA technology and protein engineering are currently the main technologies in the design and production of new industrial enzymes, because they allow to reach high production yields of purified products at competitive costs and to design new enzymes with novel properties well adapted to industrial conditions. Of the major classes of enzymes, about 80% of current industrial enzymes are hydrolases (e.g., carbohydrolases, esterases) and are extracellularly produced for ease of downstream recovery after fermentation. In recent years, a few oxidoreductases have been commercially introduced (e.g., catalase, peroxidase, laccase) and even one lyase (pectate lyase for raw cotton bioscouring). In this review, we give an overview of enzyme applications in: detergency, textile and leather, which are the three areas that represent the majority of industrial enzyme uses. In detergency, enzymes contribute to the highly efficient removal of stains made of proteins, starch and grease from garments and fabrics, thereby enhancing the action of surfactants and improving the performance of the washing process. In the textile industry, cellulases, amylases, proteases, catalases, pectin lyase, peroxidase and laccase, have all become enzymes commonly used in textile mills, dyehouses and industrial laundries. In tanneries, application of proteases at the bating step to soften the hides and prepare them for tanning has been a key step in leather making ever since ancient times. A vast range of proteases is now available for soaking, bating and for unhairing raw hides, and of lipases for degreasing.