Growth-(un)coupled biotransformation of D-xylose to D-xylonic acid by Pseudomonas putida EM42.

Co-production of two or more desirable compounds from low-cost substrates by a single microbial catalyst could greatly improve the economic competitiveness of many biotechnological processes. However, reports demonstrating the adoption of such co-production strategy are still scarce. In this study, the ability of genome-edited strain Psudomonas putida EM42 to simultaneously valorise D-xylose and D-cellobiose -two important lignocellulosic carbohydrates -by converting them into the platform chemical D-xylonic acid and medium chain length polyhydroxyalkanoates, respectively, was investigated. Biotransformation experiments performed with P. putida resting cells showed that promiscuous periplasmic glucose oxidation route can efficiently generate extracellular xylonate with high yield reaching 0.97 g per g of supplied xylose. Xylose oxidation was subsequently coupled to the growth of P. putida with cytoplasmic {beta}-glucosidase BglC from Thermobifida fusca on D-cellobiose. This disaccharide turned out to be a better co-substrate for xylose-to-xylonate biotransformation than monomeric glucose. This was because unlike glucose, cellobiose did not block oxidation of the pentose by periplasmic glucose dehydrogenase Gcd, but, similarly to glucose, it was a suitable substrate for polyhydroxyalkanoate formation in P. putida. Co-production of extracellular xylose-born xylonate and intracellular cellobiose-born medium chain length polyhydroxyalkanoates was established in proof-of-concept experiments with P. putida grown on the disaccharide. This study highlights the potential of P. putida EM42 as a microbial platform for the production of xylonic acid, identifies cellobiose as a new substrate for mcl-PHA production, and proposes a fresh strategy for the simultaneous valorisation of xylose and cellobiose.