Regioselective Oxidation of D-Galacturonic Acid to Provide Crystallized Mucic Acid Using Engineered Gluconobacter...

Mucic acid (MA) is used as a chelating agent or as a building block for bio-based polymers. MA can be produced by regioselective oxidation of D-galacturonic acid (GA). Gluconobacter oxydans is known for the partial oxidation of various substrates via membrane-bound dehydrogenases. As the wild-type strain shows only low oxidation activity towards GA, the engineered multideletion strain G. oxydans BP9.1 pta-mGDH, overexpressing a membrane-bound glucose dehydrogenase from Pseudomonas taetrolens, was used in buffered whole-cell batch biotransformations with GA as the sole substrate. Initial cell-specific MA formation rates elevated with rising educt concentrations up to 63 g L^-1. At pH 4, full GA conversion was only achieved with an initial GA concentration of 10 g L^-1. Complete conversion of 94 g L^-1 of GA was achieved at pH 5 with 3.4 g L^-1 of G. oxydans BP9.1 pta-mGDH within 48 h, resulting in >100 g L^-1 of MA, corresponding to a yield of >99% (mol/mol). Isolation of MA (purity > 90%) was achieved after cell separation, followed by cooling crystallization and drying, with a yield of 94%. Complete, full-yield GA conversion using non-growing cells of engineered G. oxydans in simple phosphate buffer yielded high product concentrations and enabled simple, high-yield product isolation, thus resulting in cost-effective and sustainable bioproduction of MA.