Identification and characterization of 3,6-anhydro-L-galactose dehydrogenases belonging to the aldehyde dehydrogenase superfamily from marine and soil microorganisms

Abstract

The complete hydrolysis of agarose produces its monomeric sugars, D-galactose and 3,6-anhydro-Lgalactose (L-AnG). Although enzymes of D-galactose metabolism are well characterized, those involved in L-AnG metabolism have not yet been investigated. In this study, we report the identification and characterization of L-AnG dehydrogenase (L-AnGDH), an aldehyde dehydrogenase (ALDH), catalyzing the first step of L-AnG degradation. To compare substrate and cofactor specificities of L-AnGDH, two L-AnGDH genes obtained from the marine bacterium Postechiella marina (Pm_L-AnGDH) and the soil bacterium Streptomyces coelicolor (Sc_L-AnGDH) were cloned and expressed in E. coli. Whereas the recombinant Pm_LAnGDH and Sc_L-AnGDH were similar in their oligomeric state (homotetramer) and optimum reaction conditions (30℃, pH 8.0), the two enzymes were distinguishable by their substrate and cofactor specificities. Sc_L-AnGDH catalyzed the oxidation of L-AnG using both NAD+ and NADP+, with a preference for NAD+. It also catalyzed the dehydrogenation of L-glyceraldehyde, glycolaldehyde, and L-lactaldehyde in the presence of NAD+. On the other hand, Pm_L-AnGDH showed exclusive selectivity towards NADP+ and did not oxidize aldehydes other than L-AnG and L-glyceraldehyde. The phylogenetic analysis of amino sequences indicated that L-AnGDH belongs to a novel subfamily within the ALDH superfamily. To our knowledge, this is the first report on the characterization of L-AnGDH.