WHOLE-CELL BIOCATALYSIS IN ORGANIC MEDIA

The use of water-immiscible organic solvents in whole-cell biocatalysi s has been exploited for biotransformations involving sparingly water- soluble or toxic compounds. These systems can overcome the problem of low productivity levels in conventional media due to poor substrate so lubility, integrate bioconversion and product recovery in a single rea ctor, and shift chemical equilibria enhancing yields and selectivities ; nevertheless, the selection of a solvent combining adequate physicoc hemical properties with biocompatibility is a difficult task. The cell membrane seems to be the primary target of solvent action and the mod ification of its characteristics the more relevant cellular adaptation mechanism to organic solvent-caused stress. Correlations between the cellular tonicity or the extractive capacities of different solvents a nd some of their physical properties have-been proposed in order to mi nimize preliminary, solvent-selection experimental work but also to he lp in the understanding of the molecular mechanisms of toxicity and ex traction. The use of whole cells in organic-media biocatalysis provide s a way to regenerate cofactors and carry out bioconversions or fermen tations requiring multi-step metabolic pathways; some processes alread y are commercially exploited;Immobilization can further protect cells from solvent toxicity, and has thus been effectively used in organic s olvent-based systems. Several examples of extractive fermentations and other whole-cell bioconversions in organic media are presented. (C) 1 998 Elsevier Science Inc.