Integrated Bioprocessing for the pH-Dependent Production of 4- Levulinate in Pseudomonas putida KT2440

Enzymes are powerful biocatalysts capable of performing specific chemical transformations under mild conditions, yet as catalysts they remain subject to the laws of thermodynamics, namely, that they cannot catalyze chemical reactions beyond equilibrium. Here we report the phenomenon and application of using extracytosolic enzymes and medium conditions, such as pH, to catalyze metabolic pathways beyond their intracellular catalytic limitations. This methodology, termed “integrated bioprocessing” because it integrates intracellular and extracytosolic catalysis, was applied to a lactonization reaction in Pseudomonas putida for the economical and high-titer biosynthesis of 4-valerolactone from the inexpensive and renewable source levulinic acid. Mutant paraoxonase I (PON1) was expressed in P. putida, shown to export from the cytosol in Escherichia coli and P. putida using an N-terminal sequence, and demonstrated to catalyze the extracytosolic and pH-dependent lactonization of 4-hydroxyvalerate to 4-valerolactone. With this production system, the titer of 4-valerolactone was enhanced substantially in acidic medium using extracytosolically expressed lactonase versus an intracellular lactonase: from