Faber - Biotransformations in Organic Chemistry: A Textbook
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This well-established textbook on biocatalysis provides a basis for undergraduate and graduate courses in modern organic chemistry, as well as a condensed introduction into this field. After a basic introduction into the use of biocatalysts--principles of stereoselective transformations, enzyme properties and kinetics--the different types of reactions are explained according to the reaction principle, such as hydrolysis, reduction, oxidation, C-C bond formation, etc. Special techniques, such as the use of enzymes in organic solvents, immobilization techniques, artificial enzymes and the design of cascade-reactions are treated in a separate section. A final chapter deals with the basic rules for the safe and practical handling of biocatalysts. The use of biocatalysts, employed either as isolated enzymes or whole microbial cells, offers a remarkable arsenal of highly selective transformations for state-of-the-art synthetic organic chemistry. Over the last two decades, this methodology has become an indispensable tool for asymmetric synthesis, not only at the academic level, but also on an industrial scale. In this 7th edition new topics have been introduced which include alcohol and amine oxidases, amine dehydrogenases, imine reductases, haloalkane dehalogenases, ATP-independent phosphorylation, Michael-additions and cascade reactions. This new edition also emphasizes the use of enzymes in industrial biotransformations with practical examples.
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John Whittall
Speedy Publishing
Francis Carey
Jean-Baptiste Sortais (editor)
Patel
Lutz F. Tietze
Clay S. Bennett
Felix A. Carroll
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Springer International Publishing AG 2018
Kurt Faber Biotransformations in Organic Chemistry1. Introduction and Background Information
Kurt Faber 1
(1)
Department of Chemistry, University of Graz, Graz, Austria
1.1 Introduction
Exponents of classical organic chemistry will probably hesitate to consider a biochemical solution for one of their synthetic problems due to the fact, that biological systems would have to be handled. Where the growth and maintenance of whole microorganisms is concerned, such hesitation is probably justified. In order to save endless frustrations, close collaboration with a microbiologist or a biochemist is highly recommended to set up fermentation systems [ In addition, modern methods of molecular biology became simple and reliable enough to be operated by any organic chemist with a minimal background in biosciences. Hence, the cloning and overexpression of a desired enzyme is nowadays feasible within a short time and at modest cost, in particular when it is derived from bacterial sources. Due to the enormous complexity of biochemical reactions compared to the repertoire of classical organic reactions, many methods described in this book have a strong empirical aspect. This black box approach may not entirely satisfy the scientific purists, but as organic chemists tend to be pragmatists, they accept that the understanding of a biochemical reaction mechanism is not a conditio sine qua non for the success of a biotransformation. After all, the exact structure of a Grignard-reagent is still unknown although its an indispensable reagent for organic synthesis. Consequently, a lack of detailed understanding of a biochemical reaction should never deter us from using it, if its usefulness has been established.
Worldwide, about 8590% of all chemical processes are performed catalytic [].
1.2 Common Prejudices Against Enzymes
If one uses enzymes for the transformation of nonnatural organic compounds, the following prejudices are frequently encountered: []
- Enzymes are sensitive .This is certainly true for most enzymes if one thinks of boiling them in water, but that also holds for most organic reagents, e.g., butyl lithium. When certain precautions are met, enzymes can be remarkably stable.].
- Enzymes are expensive .Some are, but others can be very cheap if they are produced on large scale []. In bulk, prices of enzymes range from ~100,000 $ per kg for a diagnostic enzyme to ~100 $ for crude preparations, which are adequate for most chemical reactions. Due to the rapid advances in molecular biology, costs for enzyme production are constantly dropping and proteins can be reused if they are immobilized.
- Enzymes are only active on their natural substrates .This statement is certainly true for enzymes derived from the primary metabolism, which provides energy for the maintenance of life. However, it is definitely false for proteins involved in secondary metabolism, which ensures detoxification of xenobiotics, defence against offenders and adaption to a constantly changing environment. Much of the early research on biotransformations was impeded by a tacitly accepted dogma of traditional biochemistry which stated that enzymes are natures own catalysts developed during evolution to enable metabolic pathways. This narrow definition implied that man-made organic compounds could not be regarded as substrates. Once this scholastic problem was surmounted [], it turned out that the fact that nature has developed its own peculiar catalysts over 3.9 109 years does not necessarily imply that they are designed to work only on their natural target molecules. Research during the past decades has shown that the substrate tolerance of many enzymes is much wider than previously believed and that numerous biocatalysts are capable of accepting nonnatural substrates of an unrelated structural type by often exhibiting the same high specificities as for the natural counterparts. It seems to be a general trend, that, the more complex the enzymes mechanism, the narrower the limit for the acceptability of foreign substrates. After all, there are many enzymes whose natural substrates if there are any are unknown.
- Enzymes work only in their natural environment .It is generally true that an enzyme displays its highest catalytic power in water, which in turn represents something of a nightmare for the organic chemist if it is the solvent of choice. However, biocatalysts can function in nonaqueous media, such as organic solvents, ionic liquids, and supercritical fluids, as long as certain guidelines are followed. Although the catalytic activity is usually lower in nonaqueous environments, many other advantages can be accrued by enabling reactions which are impossible in water (Sect. ].
1.3 Advantages and Disadvantages of Biocatalysts
1.3.1 Advantages of Biocatalysts
- Enzymes are very efficient catalysts .Typically the rates of enzyme-mediated processes are 1081010 times faster than those of the corresponding noncatalyzed reactions,).
- Enzymes are environmentally acceptable .Unlike many (metal-dependent) chemical catalysts, biocatalysts are environmentally benign reagents since they are completely biodegradable.
- Enzymes act under mild conditions .Enzymes act within a range of about pH 58 (typically around pH 7) and in a temperature range of 2040 C (preferably at around 30 C). This minimizes problems of undesired side-reactions such as decomposition, isomerization, racemization, and rearrangement, which often plague traditional methodology.
- Enzymes are compatible with each other .Since enzymes generally function under the same or similar conditions, several biocatalytic reactions can be performed in a cascade-like fashion in a single flask. Such systems are particularly advantageous if unstable intermediates are involved and furthermore, an unfavorable equilibrium can be shifted towards the desired product by linking consecutive enzymatic steps. Multienzyme cascades are often denoted as artificial metabolism (Sect. ].
- Enzymes are not restricted to their natural role .Many proteins exhibit a high substrate tolerance by accepting a large variety of man-made nonnatural substances. If advantageous for a process, the aqueous medium can often be replaced by an organic solvent (Sect. ).
- Enzymes can catalyze a broad spectrum of reactions .Like catalysts in general, enzymes can only accelerate reactions but have no impact on the position of the thermodynamic equilibrium of the reaction. Thus, in principle, enzyme-catalyzed reactions can be run in both directions. The catalytic flexibility of enzymes is generally denoted as catalytic promiscuity [], which is divided into substrate promiscuity (conversion of a nonnatural substrate), catalytic promiscuity (a nonnatural reaction is catalyzed), and condition promiscuity (catalysis occurring in a nonnatural environment).
Table 1.1
Catalytic efficiency of representative enzymes
Enzyme | Reaction catalyzed | TOF [s1] |
|---|---|---|
Carbonic anhydrase | Hydration of CO2 | 600,000 |
Acetylcholine esterase | Ester hydrolysis | 25,000 |
Penicillin acylase | Amide hydrolysis | 2000 |
Lactate dehydrogenase |
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