Integrating Chemocatalysis and Enzyme Engineering for Coproduction of Bioplastic Precursors Butanediol and Succinic Acid from Xylose

Polybutylene succinate (PBS) is one of the most important biobased plastics, based on the amount produced. Owing to its high melting point and resemblance to petroleum-based plastics (i. e. PP), PBS becomes one of the emerging bioplastics with an array of applications. PBS is manufactured by polymerization of 1,4-butanediol and succinic acid. Thus, it is of great importance to ensure the use of renewable resources to produce the PBS precursors. As the second most abundant carbohydrate monomer on Earth, D-xylose will be a suitable candidate for this purpose. In this work, we combined protein engineering with chemical oxidation by gold catalyst to enable transformation of D-xylose to 1,4-butanediol and succinic acid simultaneously. In silico docking studies and semi rational design were employed to create variants of the key enzyme, branched chain α-keto acid decarboxylase (KdcA) with higher affinity for the intermediates in the production of 1,4-butanediol and succinic acid. Direct enzymatic biotransformation would result in a production of both monomers with 3:1 ratio, thus not readily suitable for a direct polymerization to PBS. By developing a one-pot multi-step chemo-enzymatic approach with a gold catalyst to perform the first oxidation step, we could achieve a final product ratio of 1:1. Application of an engineered KdcA variant allowed us to achieve >98% yield after four hours transformation. In contrast, after 24 h transformation, >10% intermediate was still observed when the original variant was used. We anticipate this new approach could serve as an alternative route for biotechnological productions of PBS and its precursors.