Enzyme-Based Organic Synthesis. Cheanyeh Cheng

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Scheme 1.6 Regio‐ and stereoselective concurrent oxidations of racemic vicinal diols to enantiopure 1,2‐diols.

1.5 Enzyme Classes and Nomenclature

      Since Duclaux proposed that all enzymes should give the suffix “ase” for an easy recognition [3], the suffix “ase” has been added to the name of many enzymes according to their substrate or to a word or phrase for describing the activity. For example, glucose oxidase catalyzes the oxidation of glucose to produce gluconolactone, and cellulase catalyzes the hydrolysis of cellulose to form glucose. However, enzymes such as pepsin and trypsin have names that do not relate with their substrates or functions. Because more and more enzymes are discovered accompanied with the progress of scientific researches, the name of new enzyme may have two or more names, or two different enzymes may be given the same name. To avoid the ambiguity for naming enzymes, a systematic method for naming and classifying enzymes should be used and agreed globally.

In 1960s, the Commission on Enzyme Nomenclature was formed by International Union of Biochemistry (IUB) to classify enzymes into six major classes according to the type of reaction catalyzed as indicated in Table 1.4 [2, 9, 30]. Each of the six major classes is further divided into subclasses and subgroups.

Table 1.4 Six major classes of enzyme.

      Source: Based on Armstrong [2]; Nelson and Cox [9]; Kula [30].

No.	Class	Catalytic function or reaction
1	Oxidoreductases	Transfer of electrons (hydride ions or H atoms), e.g.
2	Transferases	Group‐transfer reactions, e.g.
3	Hydrolases	Hydrolysis reactions (transfer of functional groups to water), e.g.
4	Lyases	Addition of groups to double bonds, or formation of double bonds by removal of groups, e.g.
5	Isomerases	Transfer of groups within molecules to yield isomeric forms, e.g.
6	Ligases	Formation of C–C, C–S, C–O, and C–N bonds by condensation reactions coupled to ATP cleavage, e.g.

      By international agreement, the catalytic reaction is assigned and identified by a group of four‐digit number according to the enzyme classification system. For example, the enzyme catalyzes the transfer of a phosphoryl group from ATP to D‐glucose is named as ATP:glucose phosphotransferase. The enzyme is classified as

The serial number of the last digit of an enzyme is identified by the first three entrees. Therefore, the Enzyme Commission number (E.C. number) of this enzyme is 2.7.11 denoted as E.C. 2.7.1.1. However, a trivial name, hexokinase, is more commonly used for this enzyme.

1.6 Enzyme and Green Chemistry

      Green chemistry, also known as sustainable chemistry, is an emerging field in chemistry and is highly advocated by global researchers recently. Green chemistry emphasizes the design of products and chemical processes that reduce or eliminate the use or production of hazardous substances [31]. Whereas sustainability has been defined as “meeting the needs of the present generation without compromising the ability of future generations to meet their needs.” Therefore, green chemistry can also be thought as a critical tool in attaining sustainability by developing new technologies in all kinds of applications such as food and drink, medicine, energy, biofuels, plastics, and nanotechnology. Also, the long‐term entanglement of the “three E’s” problems – Energy, Economy, and Environment – could be solved by applying the 12 principles of green chemistry to assure a sustainable society in the future.

The 12 principles of green chemistry listed in Table 1.5 [31, 32] were proposed by Paul T. Anastas and John C. Warner in 1998 that has become the index to implement the green/sustainable chemistry. How chemical synthesis catalyzed by enzyme is related with green chemistry? As mentioned before, the substrate specificity of enzyme greatly reduces the formation of byproducts, thus greatly increases the atom economy of the reaction. Enzymes are biodegradable natural material that does not threat human health and make environmental pollution. The regioselectivity, particularly, the stereoselectivity of enzymes can be used to design and produce safe drugs of no toxicity and without side‐effects by raising the enantiomeric excess (%ee) value or yield. The reaction conditions for enzyme‐catalyzed reaction are mild, usually at low temperature and under atmospheric pressure, and the solvent used is innocuous water that makes an energy efficient and clean synthetic process. The substrate for enzyme can be either renewable or nonrenewable source that gives the enzyme reaction wide and flexible applications. Various specificities of enzyme show great advantage in avoiding the use of blocking groups, protection/deprotection, or temporary modification of physical/chemical processes, thus reduce the synthetic steps for a final target product to a minimum that saves a lot of chemical reagents and generates minimal wastes. Immobilized enzyme‐catalyzed reaction is most feasible to couple an online analytical technique to perform real‐time monitoring and control of hazardous substances [33, 34].

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