High substrate specificity of 3,6-anhydro-l-galactose dehydrogenase indicates its essentiality in the agar catabolism of a marine bacterium

3,6-Anhydro-l-galactose (AHG) is a rare sugar found in red macroalgae. The key metabolic steps in AHG catabolism involve its oxidation into 3,6-anhydrogalactonate (AHGA), followed by cycloisomerization of AHGA into 2-keto-3-deoxy-galactonate. These steps were recently discovered in a marine bacterium Vibrio sp. strain EJY3. In this study, we characterized NAD(P)⁺-dependent AHG dehydrogenase (AHGD) involved in the first step of AHG catabolism. AHGD displayed high substrate specificity for AHG, but showed no catalytic activity toward other aldehyde sugars, including D-form of AHG, glucose, and galactose. This high substrate specificity of AHGD may be associated with the unique chemical structure of its substrate AHG. Unlike other common aldehyde sugars, AHG mainly exists in its hydrated form under aqueous conditions. Growth of EJY3 in presence of AHG, agar, and Gelidium amansii but glucose, as the sole carbon source resulted in a significant increase in the AHGD activity of cell-free EJY3 lysates. Amino acid sequence analysis revealed that AHGD is highly homologous to other aldehyde dehydrogenases from agar-degrading bacteria, suggestive of its key role in agar-related metabolism in marine bacteria utilizing red macroalgae. Therefore, AHGD may serve as an important enzyme involved in the bioconversion of red macroalgal biomass to value-added chemicals.