Improving isobutanol and 2-ketoisovalerate production by reducing by-product synthesis in Klebsiella pneumoniae.

Isobutanol is a promising next-generation biofuel with a high energy density and is considered a sustainable aviation fuel precursor. Klebsiella pneumoniae is an industrial workhorse microorganism used for the production of various chemicals. An endogenous isobutanol synthesis pathway, interconnected with the branched-chain amino acid and 2,3-butanediol biosynthesis pathways, has been identified in K. pneumoniae. However, isobutanol production by K. pneumoniae is inefficient due to the substantial formation of by-products. In this study, we enhanced isobutanol synthesis by systematically engineering K. pneumoniae to reduce by-product accumulation. The synthesis of 2-hydroxyisovalerate was reduced by knocking out panE. Formate synthesis was eliminated by disrupting pflB. The production of ethanol, acetate, and succinate was reduced by deleting aceE. Pyruvate accumulation was alleviated by overexpressing budB. In batch fermentation, the engineered strain produced 13.97 g/L of 2-ketoisovalerate, a key intermediate in the isobutanol pathway, with a conversion ratio of 0.43 mol/mol glucose. Fed-batch fermentation further increased the 2-ketoisovalerate titer to 26.63 g/L after 54 h of cultivation. The decarboxylation of 2-ketoisovalerate to isobutyraldehyde is catalysed by IpdC. Overexpression of the ipdC T290L variant shifted the major product from 2-ketoisovalerate to isobutanol, resulting in a batch fermentation titer of 5.32 g/L isobutanol and a conversion ratio of 0.28 mol/mol glucose. Fed-batch fermentation further increased the final isobutanol titer to 10.32 g/L after 52 h of cultivation. This study establishes an efficient strategy for high-level production of isobutanol or 2-ketoisovalerate in K. pneumoniae.