Research of the past decade has demonstrated that the use of enzymes and whole-cell biocatalysts is environmentally friendly, economical and surprisingly, faces, few barriers when applied in organic syntheses. In nature, enzymes evolved to function within a living system and may not exhibit features desirable for large-scale in vitro syntheses. Thus, protein engineering has the potential to dramatically enhance enzyme performance in a wide variety of unusual - but technologically important - environments. Site-directed, cassette-mutagenesis and construction of chimeric enzymes commonly used in rational protein engineering are important strategies which reveal the structure-function relationship of a given enzyme. Notably, construction of hybrid enzymes has become increasingly important in the rational design of novel biocatalysts with desired properties and activities. However, ?irrational? protein design based on random mutagenesis technologies or the combination with directed evolution approaches has truly revolutionized the generation of functional biological molecules.