Introduction
The exploitation of new catalysts in an environmentally benign
manner has become crucially important in recent years [1].
Nature is an extraordinary chemist who evolves its catalysts over
millions of years [2]. Such catalysts are models of energy-efficient,
environmentally benign chemical agents, as virtually all do their
work under mild conditions and generate few waste products [3].
However, the development and use of these natural catalysts in
organic chemistry are very limited. To date, the most common
biocatalytic methods use either whole-cells or isolated enzymes for
a given chemical transformation. Each method has advantages
and challenges for both laboratory-scale and industrial-scale
chemistry [4]. The use of whole-cell systems benefits from the
ability to use low-cost and renewable feedstocks, and co-factor
addition and regeneration are not necessary. However, whole-cell
systems require expensive equipment and tedious work-up because
of large volumes, and have low productivity. More importantly,
the accumulation of products and by-products may be toxic to the
cell. Besides, the cell membrane may act as a mass transport
barrier between the substrates and the enzymes [5]. Compared to
whole cells, isolated enzymes offer several benefits, including
simpler reaction apparatus, higher productivity owing to higher
catalyst concentration, and simpler product purification [6]. Thus,
isolated enzymes are usually more efficient in bioconversion than
whole cells. However, enzyme purification involves expensive
strategies, usually employing costly and polluting chemicals, being
also laborious and time-consuming, which restricts the use of
isolated enzymes, making it difficult to scale-up [7]. Therefore, it is
obligatory to find easy methods to supply the gaps.