Metabolic engineering encompasses a number of processes that can be used to improve or change a given organism’s metabolic reactions. These methods are typically used to improve industrial biosynthetic pathways, allowing for increased yields and reduced costs.
There are six methods used to change an organism’s metabolism:
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Modifying a metabolic pathway to move flux towards a specific product. This could be targeting a specific limiting factor in the pathway, allowing it to be completed more quickly without intermediates accumulating in the cell.
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Increasing the precursor / cofactor supply (engineering central metabolism). If a cofactor (NAD, FAD, …) is required for a specific stage of the pathway, it may be the limiting factor. By engineering central metabolism to increase the concentration of that cofactor, the rate of the limiting step can be increased, allowing more flux through the pathway.
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Engineering transport systems. Accumulation of the product within the cell may prevent enzymes from working as efficiently as they should. Therefore, by upregulating or introducing efflux pumps, the microorganism will be able to secrete the product into the growth media. This also makes it easier to recover and extract the product at the end of the fermentation.
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Increasing tolerance to the product of interest. Some products may be toxic to the organism synthesising them, such as an antibiotic. By introducing resistance, higher levels of the product can be obtained, increasing the yield and reducing costs.
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Suppressing feedback inhibition pathways. If a product acts as a negative regulator of its synthetic pathway, it may be possible to engineer the regulatory step to allow more product to be obtained before shutting the pathway down. This is used in lysine biosynthesis from Corynebacterium glutamicum.
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Separating growth and product fermentation pathways. Growth pathways require high levels of energy, pulling flux away from the fermentation pathway and reducing the amount of product synthesised. If the growth pathways can be shut down after the microorganisms reach the correct phase in the fermentation vessel, more metabolic flux can be directed towards the product synthesis pathway, allowing for faster and higher yields. This could be done by using inducible promoters, allowing the inducer to be added at a specific point in the fermentation to ‘activate’ synthesis.
These methods can give small improvements to biosynthesis, leading to large improvements to the economic viability of the synthetic pathway.