Whole-cell Biocatalysts from Organic Solvent-tolerant Bacteria
Thunyarat Pongtharangkul1 and Alisa S Vangnai2
1Department of Biotechnology, Faculty of Science, Mahidol University,
2Department of Biochemistry, Faculty of Science, Chulalongkorn University.
Abstract
Whole-cell biocatalyst is expected to be one of the most important tools in industrial synthesis of bulk chemicals as well as pharmaceutical and agrochemical intermediates and bioactive compounds. The use of whole cells not only provides a means for cost-effective production and regeneration of enzyme cofactors, such as NAD(P)H, but also allows the utilization of multicomponent and membrane-bound enzymes in biotransformation of chemicals. As several potential substrates and products are, however, sparingly water-soluble or water-immiscible, a second phase of organic solvent is required. Development of effective solvent-tolerant microbial platform is, therefore, a necessary step to make whole-cell biocatalyst applicable for production of waterimmiscible chemicals. Several examples of whole-cell biocatalysts based on organic solvent-tolerant bacteria are presented.
Keywords: biotransformation; organic-aqueous two-liquid phase system
Introduction
Chemical industry not only consumes a tremendous amount of energy and nonrenewable
resources, but also produces toxic wastes and pollutants. Therefore, attempts have been carried out in order to develop more environmentally friendly industrial chemical processes. The implementation of ‘green chemistry’ concept in which a chemical should be synthesized in a safe and energy-efficient
manner with the lowest impact on environment(1) emphasizes the role of biocatalysis processes. The fact that a high stereo- or enantiomerically pure product can be synthesized in a mild condition makes biocatalysts a promising alternative for production of several selective and complex chemicals(35).Comparing to an isolated enzyme system, whole-cell biocatalyst is more cost-effective in
terms of production, especially in systems that require regeneration of co-factors(16) and that
involve multi-component or membrane-bound enzymes. Various potential substrates and products, however, are water-immiscible and thus require the use of solvents for solubilization. Although the solvent phase in organic-aqueous two-phase system can be used as a means to overcome the toxicity of substrates and products as well as to recover products in situ, toxicity of the solvent phase itself has become a major drawback in the applications of whole-cell biocatalysts. Fortunately, recent discovery of solvent-tolerant bacteria has made whole-cell biocatalysis in an organic-aqueous two-phase system more promising.
Organic Solvent-tolerant Bacteria
Organic solvent-tolerant bacteria constitute a unique group of extremophilic microorganisms that can thrive in an environment with high concentrations of solvents(19). These bacteria have been considered
to be potentially useful in many applications of microbial transformation for environmental remediation as well as for production of valuable chemicals(32). Although most of the solvent-tolerant bacteria reported thus far are Gram-negative, a number of Grampositive bacteria, sometimes with even higher solvent tolerance level.