Production of Propionate by a Sequential Fermentation-Biotransformation Process via L-Threonine

Bio-based propionate is widely welcome in the food additive industry. The current anaerobic process by Propionibacteria endures low titers and a long fermentation time. In this study, a new route for propionate production from L-threonine was designed. 2-Ketobutyrate, deaminated from L-threonine, is cleaved into propionaldehyde and CO2 and then be oxidized into propionic acid, which is neutralized by ammonia released from the first deamination step. This CoA-independent pathway with only CO2 as a byproduct boosts propionate production from L-threonine with high productivity and purity. The key enzyme for 2-ketobutyrate decarboxylation was selected, and its expression was optimized. The engineered Pseudomonas putida strain, harboring 2-ketoisovalerate decarboxylase from Lactococcus lactis could produce 580 mM (43 g/L) pure propionic acid from 600 mM L-threonine in 24 h in the batch biotransformation process. Furthermore, a high titer of 62 g/L propionic acid with a productivity of 1.07 g/L/h and a molar yield of >0.98 was achieved in the fed-batch pattern. Finally, an efficient sequential fermentation-biotransformation process was demonstrated to produce propionate directly from the fermentation broth containing L-threonine, which further reduces the costs since no L-threonine purification step is required.

Automated summary

This study presents a novel approach to efficiently and purely produce propionic acid from L-threonine. By optimizing the expression of key enzymes and using engineered strains, high yield and high purity propionate production has been achieved, along with demonstrating an efficient sequential fermentation-biotransformation process.