Microbial degradation of model compounds of coal and production of metabolites with potential commercial value

Abstract

Due to the need of new strategies for improving the economical performance of coal technologies, efforts were undertaken to develop techniques of solubilizing coal by the action of microorganisms. Because of the poor information in the beginning of this research, the microbial metabolism of some monomeric structural elements of coal was investigated first. Dibenzofuran, for example, was chosen as it represents a model structure frequently found in many coals, i.e. the cyclic biarylether moiety. It was found to be degraded by many different organisms isolated from soil via a new degradative mechanism called "angular" dioxygenation. Fluorene, a model compound for dibenzo-cyclopentane structures in coal, surprisingly followed essentially the same metabolic steps. Additional compounds which were integrated in the research program like naphthalenes, biphenyls, biarylethers and carbazoles also exhibited an oxygenase-dependent mode of initial attack. Since all enzymes involved were not active outside the cells, there seems to be no way to employ them in biological depolymerisation of untreated coals. Current work therefore, by employing special selection substrates, concentrates on the detection of new enzyme systems which follow non-oxygenase dependent mechanisms. In a second line of research the enzyme systems mentioned above are used to synthesize new organic compounds from coal-derived substances. Due to the relaxed substrate specifity of the initial dioxygenases many structural analogues of dibenzofuran are metabolized. Several optically active compounds of the dihydrodiol-type were isolated and characterized by spectroscopic methods. We recently developed a preparative technique to produce these potentially valuable metabolites at a gram-scale. In addition, methods of genetic engineering are currently being adopted to create stable high expression organisms with improved productivity.