result of advances in asymmetric synthesis and separation technologies, there were many cases in using the stereospecificity of enzyme for the production of single enantiomer to allow the study of its pharmacodynamics and pharmacokinetic properties. For example, in the case of total synthesis of D‐biotin, the novel enantioselective synthesis of the optically active (3aS,6aR)‐lactone (the key D‐biotin intermediate) was through kinetic resolution by inexpensive microbial lipase instead of pig liver esterase [26]. In Scheme 1.4, optically active (3aS,6aR)‐lactone 1 was enantioselectively produced with high enantiomeric excess.
Figure 1.2 Three‐point attachment rule shows that only one enantiomer of the asymmetric molecule can successfully bind with enzyme at the active site to produce the stereospecificity of enzyme.
(ee > 98%) and conversion ratio (≥ 40%) by dry microbial cells of Aspergillus oryzae WZ007 on racemic acid 1 (1,3‐dibenzyl‐5‐(hydeoxymethyl)‐2‐oxo‐4‐imidazolidinecarboxylic acid) that was obtained via chemical hydrolysis of racemic lactone 1.
Recently, an indirect strategy was used for the synthesis of (R)‐phenylephrine (an α1‐adrenergic receptor agonist) that is widely used in over‐the‐counter drugs to treat the common cold. An amino alcohol dehydrogenase gene isolated from Rhodococcus erythropolis BCRC 10909 was expressed in Escherichia coli NovaBlue, which is able to convert 1‐(3‐hydroxyphenyl)‐2‐(methylamino) ethanone (HPMAE) to (S)‐phenylephrine with more than 99% enantiomeric excess (ee) and 78% yield as shown in Scheme 1.5 [27]. The (S)‐phenylephrine was subsequently converted to (R)‐phenylephrine by Walden inversion reaction [28]. Since enantiopure vicinal diols are useful and valuable intermediate for pharmaceutical production, a simple and green method for preparing several enantiopure 1,2‐diols was developed via regio‐ and stereoselective concurrent oxidations of the racemates with microbial cell Sphingomonas sp. HXN‐200.
Figure 1.3 The stereospecificity of enzyme for two substrate enantiomers by two‐point binding.
Scheme 1.4 Enantioselective synthesis of (3aS,6aR)‐lactone.
As shown in Scheme 1.6, concurrent biooxidations of racemic 3‐O‐benylglycerol 1 with resting cells gave (S)‐1 in 99.2% enantiomeric excess (ee) and 32% yield. Similar biooxidations of racemic 1‐(4‐chlorophenyl)‐1,2‐ethanediol 2, 1‐(4‐methylp henyl)‐1,2‐ethanediol 3, and phenyl‐1,2‐ethanediol 4 gave (R)‐2 in 98.4% ee and 48% yield, (R)‐3 in 99.6% ee and 45% yield, and (R)‐4 in 98.7% ee and 36% yield, respectively [29].
Scheme 1.5 The enantioselective conversion of HPMAE to (S)‐phenylephrine.
Source: Lin et al. [27].
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. |