Cofactor Engineering Enhances the Physiological Function of an Industrial Strain

Microorganisms are able to produce a wide range of valuable chemicals and materials, and microbial fermentation is widely used as an alternative route for the production of chemicals in industry[1]. The key elements that determine the efficiency of a fermentation process are high titer, high yield, high productivity and process robustness[2]. These parameters are highly dependent on the host microorganism. In order to enhance the metabolic capabilities of the host microorganism, early research focused on screening appropriate microorganisms that naturally overproduce target products and improving their performance by random mutagenesis and by optimizing the fermentation processes. With the advent of metabolic engineering, many different genetic or metabolic engineering strategies have been adopted to improve the metabolic capabilities of the host strains, including relief of feedback inhibition, deletion of competing pathways, up-regulation of primary biosynthetic pathways, re-direction of central metabolism towards the target pathway, over-expression of export processes and insertion of new metabolic pathways. More recently, the emergence of systems biology integrated with metabolic engineering has provided a comprehensive understanding of microbial physiology, followed by a more global-wide identification of the target genes to be manipulated[3]. Those approaches have been proven to be powerful in developing microbial strains for the commercial production of organic acids[4], amino acids, biofuels and pharmaceuticals[5,6,7]. Nevertheless, problems such as the accumulation of toxic intermediates or metabolic stress resulting in decreased cellular fitness are still far from being solved. Over-expression, deletion or introduction of heterologous genes in target metabolic pathways does not always result in the desired phenotype. A good example is the attempts to increase the glycolytic flux, which cannot be increased by individual or combinational over-expression of genes encoding the key enzymes in either a eukaryotic or prokaryotic microorganism[8]. The essence of the problems listed above lies in the fact that, in addition to the modification of key genes by metabolic engineering, the researcher needs to study the effects of the internal environment (e.g. the intracellular energy charge and the interior redox potential and intracellular pH) on the phenotype, based on an accurate analysis of the metabolic network structure. If such an approach is adopted, manipulation of the form and level of intracellular cofactors can potentially be an efficient strategy for obtaining a desired phenotype.