Probing of the reaction pathway of human UDP-xylose synthase with site-directed mutagenesis

Engineering Bifunctional Galactokinase/Uridyltransferase Chimera for Enhanced UDP-d-Xylose Production

The biotechnological production of uridine diphosphate-d-xylose (UDP-d-xylose), the glycosyl donor in enzymatic for d-xylose, is an important precursor for advancing glycoengineering research on biopharmaceuticals such as heparin and glycosaminoglycans. Leveraging a recently discovered UDP-xylose salvage pathway, we have engineered a series of bifunctional chimeric biocatalysts derived from Solitalea canadensis galactokinase/uridyltransferase, facilitating the conversion of d-xylose to UDP-d-xylose. This study elucidates the novel assembly of eight fusion protein constructs, differing in domain orientations and linker peptide lengths, to investigate their functional expression in Escherichia coli, resulting in the synthesis of the first bifunctional enzyme that orchestrates a direct transformation from d-xylose to UDP-d-xylose. Fusion constructs with a NH2-GSGGGSGHM-COOH peptide linker demonstrated the highest expression and catalytic tenacity. For the highest catalytic conversion from d-xylose to UDP-d-xylose, we established an optimum pH of 7.0 and a temperature optimum of 30 °C, with an optimal fusion enzyme concentration of 3.3 mg/mL for large-scale UDP-d-xylose production. Insights into ATP and ADP inhibition further helped to optimize the reaction conditions. Testing various ratios of unfused galactokinase and uridyltransferase biocatalysts for UDP-xylose synthesis from d-xylose revealed that a 1:1 ratio was optimal. The K cat/K m value for the NH2-GSGGGSGHM-COOH peptide linker showed a 10% improvement compared with the unfused counterparts. The strategic design of these fusion enzymes efficiently routes for the convenient and efficient biocatalytic synthesis of xylosides in biotechnological and pharmaceutical applications.

EvdS6 is a bifunctional decarboxylase from the everninomicin gene cluster

The everninomicins are bacterially produced antibiotic octasaccharides characterized by the presence of two interglycosidic spirocyclic ortho-δ-lactone (orthoester) moieties. The terminating G- and H-ring sugars, L-lyxose and C-4 branched sugar β-D-eurekanate, are proposed to be biosynthetically derived from nucleotide diphosphate pentose sugar pyranosides; however, the identity of these precursors and their biosynthetic origin remain to be determined. Herein we identify a new glucuronic acid decarboxylase from Micromonospora belonging to the superfamily of short-chain dehydrogenase/reductase enzymes, EvdS6. Biochemical characterization demonstrated that EvdS6 is an NAD⁺-dependent bifunctional enzyme that produces a mixture of two products, differing in the sugar C-4 oxidation state. This product distribution is atypical for glucuronic acid decarboxylating enzymes, most of which favor production of the reduced sugar and a minority of which favor release of the oxidized product. Spectroscopic and stereochemical analysis of reaction products revealed that the first product released is the oxidatively produced 4-keto-D-xylose and the second product is the reduced D-xylose. X-ray crystallographic analysis of EvdS6 at 1.51 Å resolution with bound co-factor and TDP demonstrated that the overall geometry of the EvdS6 active site is conserved with other SDR enzymes and enabled studies probing structural determinants for the reductive half of the net neutral catalytic cycle. Critical active site threonine and aspartate residues were unambiguously identified as essential in the reductive step of the reaction and resulted in enzyme variants producing almost exclusively the keto sugar. This work defines potential precursors for the G-ring L-lyxose and resolves likely origins of the H-ring β-D-eurekanate sugar precursor.