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Ether bonds are found in a wide variety of natural products--mainly secondary metabolites--including lipids, oxiranes, terpenoids, flavonoids, polyketides, and carbohydrate derivatives, to name some representative examples. To furnish such a biodiversity of structures, a large number of different enzymes are involved in several different biosynthetic pathways. Depending on the compound and on the (micro) environment in which the reaction is performed, ethers are produced by very different (enzymatic) reactions, thus providing an impressive display of how Nature has combined evolution and thermodynamics to be able to produce a vast number of compounds. In addition, many of these compounds possess different biological activities of pharmacological interest. Moreover, some of these ethers (i.e., epoxides) have high chemical reactivity, and can be useful starting materials for further synthetic processes. This review aims to provide an overview of the different strategies that are found in Nature for the formation of these "bioethers". Both fundamental and practical insights of the biosynthetic processes will be discussed.

Pig liver esterase (PLE, EC 3.1.1.1) has been employed extensively for research purposes during the last three decades, especially in kinetic resolutions, in desymmetrizations of prochiral substrates, and in the synthesis of nucleosides. Its practical use, however, has been traditionally hampered for several reasons. The existence of several isoenzymes with different (enantio)selectivities has caused problems in reproducibility when different PLEs have been used for a certain reaction. In addition, being an animalderived enzyme, its use in several fields, such as pharmaceuticals, is excluded, as the enzyme could act as a source of viral transmission. To overcome these drawbacks -and thus make this powerful enzyme useful for organic chemists -many efforts have been devoted to cloning and over-expressing PLE in some heterologous hosts, thus assuring the recombinant production of (pure) PLE. After solving some technical problems, this has recently been achieved, when successful cloning of isoenzyme g from PLE (g-rPLE) in E. coli at high productivities was reported. This important achievement reestablishes the potential use of this enzyme as a biocatalyst in organic (asymmetric) synthesis. Furthermore, it also opens the possibility of developing new recombinant PLEs -through biological strategiesleading to new PLEs with better (novel) applications than those reported for wild-type PLEs.

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