Metabolic Pathways of Alginate-Derived Monomers in the Marine Bacterium Saccharophagus Degradans 2-40T

Marine plant biomass such as algae is a potential source for the sustainable production of biofuels and bio-based chemicals. Alginate, a major component of brown macroalgae, consists of two uronate monomers, mannuronic acid and guluronic acid, which are further non-enzymatically converted to 4-deoxy-L-erythro-5-hexoseulose uronic acid (DEH). DEH is known to be metabolized by a catabolic pathway consisting of DEH reductase, 2-keto-3-deoxy-d-gluconate (KDG) kinase, and 2-keto-3-deoxy-phosphogluconate (KDPG) aldolase, yielding two glycolytic intermediates, D-glyceraldehyde-3-phosphate and pyruvate. However, functional enzymes for the DEH pathway have rarely been identified. In the present study, a DEH catabolic pathway was investigated in Saccharophagus degradans 2-40^T, one of marine bacteria fermenting alginate. The genes encoding putative DEH reductase, KDG kinase, and KDPG aldolase were identified and individually overexpressed in Escherichia coli. Through in vitro tests assisted by GC-MS and GC/TOF MS, the purified enzymes were functionally confirmed and annotated as dehR, kdgK, and kdpgA, respectively. In a future study, the three genes will be used to build an engineered host expressing a complete DEH metabolic pathway for brown macroalgae-based bioconversion processes.