Mevalonate production from ethanol by direct conversion through acetyl-CoA using recombinant Pseudomonas putida, a novel biocatalyst for terpenoid production

BACKGROUND: Bioethanol is one of the most representative eco-friendly fuels developed to replace the non-renewable fossil fuels and is the most successful commercially available bio-conversion technology till date. With the availability of inexpensive carbon sources, such as cellulosic biomass, bioethanol production has become cheaper and easier to perform, which can facilitate the development of methods for converting ethanol into higher value-added biochemicals. In this study, a bioconversion process using Pseudomonas putida as a biocatalyst was established, wherein ethanol was converted to mevalonate. Since ethanol can be converted directly to acetyl-CoA, bypassing its conversion to pyruvate, there is a possibility that ethanol can be converted to mevalonate without producing pyruvate-derived by-products. Furthermore, P. putida seems to be highly resistant to the toxicity caused by terpenoids, and thus can be useful in conducting terpenoid production research. RESULTS: In this study, we first expressed the core genes responsible for mevalonate production (atoB, mvaS, and mvaE) in P. putida and mevalonate production was confirmed. Thereafter, through an improvement in genetic stability and ethanol metabolism manipulation, mevalonate production was enhanced up to 2.39-fold (1.70 g/L vs. 4.07 g/L) from 200 mM ethanol with an enhancement in reproducibility of mevalonate production. Following this, the metabolic characteristics related to ethanol catabolism and mevalonate production were revealed by manipulations to reduce fatty acid biosynthesis and optimize pH by batch fermentation. Finally, we reached a product yield of 0.41 g mevalonate/g ethanol in flask scale culture and 0.32 g mevalonate/g ethanol in batch fermentation. This is the highest experimental yield obtained from using carbon sources other than carbohydrates till date and it is expected that further improvements will be made through the development of fermentation methods. CONCLUSION: Pseudomonas putida was investigated as a biocatalyst that can efficiently convert ethanol to mevalonate, the major precursor for terpenoid production, and this research is expected to open new avenues for the production of terpenoids using microorganisms that have not yet reached the stage of mass production.