Abstract
Terephthalic acid (TPA) is an important industrial chemical currently produced by energy intensive and potentially hazardous p-xylene (pX) oxidation process. Here we report the development of metabolically engineered Escherichia coli system for biological transformation of pX into TPA. The engineered E. coli strain harbours a synthetic TPA pathway optimized through manipulation of expression levels of upstream and downstream modules. The upstream pathway converts pX to p-toluic acid (pTA) and the downstream pathway transforms pTA to TPA. In a two-phase partitioning fermentation, the engineered strain converts 8.8 g pX into 13.3 g TPA, which corresponds to a conversion yield of 96.7 mol%. These results suggest that the E. coli system presented here might be a promising alternative for the large-scale biotechnological production of TPA and lays the foundations for the future development of sustainable approaches for TPA production.
Highlights
Terephthalic acid (TPA) is an important industrial chemical currently produced by energy intensive and potentially hazardous p-xylene oxidation process
We first designed a synthetic metabolic pathway converting pX to TPA (Fig. 1), which can be divided into two parts: upstream pathway responsible for converting pX to p-toluic acid and downstream pathway responsible for converting pTA to TPA
The upstream pathway is found in the initial three steps of natural degradation of pX (Supplementary Fig. 1a), which are the sequential oxidation of one of the two methyl groups of pX to its corresponding alcohol (p-tolualcohol, pTALC), aldehyde (p-tolualdehyde, pTALD) and pTA catalysed by xylene monooxygenase (XMO), benzyl alcohol dehydrogenase (BADH) and benzaldehyde dehydrogenase (BZDH), respectively
Summary
Terephthalic acid (TPA) is an important industrial chemical currently produced by energy intensive and potentially hazardous p-xylene (pX) oxidation process. In a two-phase partitioning fermentation, the engineered strain converts 8.8 g pX into 13.3 g TPA, which corresponds to a conversion yield of 96.7 mol%. In comparison with chemical processes, biotransformation processes for chemical production offer several advantages such as reaction operation under milder conditions (for example, ambient temperature and pressure), use of fewer and less toxic chemicals and no need to use expensive and often toxic heavy metal catalysts[6] Given these evident process advantages, we were motivated to produce TPA through whole-cell biotransformation of pX. We report the development of a metabolically engineered E. coli strain capable of oxidizing pX into TPA, based on detailed studies on pathway reactions, and reconstruction of synthetic pathway for TPA production. TPA production was improved through manipulation of enzyme expression levels for increasing the metabolic flux towards TPA biosynthesis, while reducing byproducts formation, and developing a two-phase partitioning fermentation process
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