L-guluronan-specific alginate lyase from a marine bacterium associated with Sargassum

The major extracellular alginate lyase activities secreted by a Gram-negative, facultative bacterium associated with actively growing Sargassum fluitans have been resolved an examined for substrate specificity. A fraction excluded from Sephadex G-75 was equally active toward (1----4)-beta-D-mannuronan, (1----4)-alpha-L-guluronan, and alginate with the formation of di- and tri-saccharides as apparent limit products and oligo-saccharides indicative of an endolytic mechanism. A second fraction which was included during G-75 filtration was inactive toward D-mannuronan and 4 times more active toward L-guluronan than native alginate. Proton magnetic resonance spectrometry identified the primary product of this enzyme as O-(4-deoxy-alpha-L-erythro-hex-4-enopyranosyluronic acid)-(1----4)-O-(alpha-L-gulopyranosyluronic acid)-(1----4)-O-alpha-L-gulopyranuronic acid. The L-guluronan-specific enzyme requires 0.5 M NaCl for maximal activity and has been purified as a monomeric protein having an apparent molecular mass of 38 kD and an approximate pI of 4.5. The predominant formation of trisaccharide over the course of a reaction showed a primarily exolytic mechanism, indicating an enzyme activity unique from any previously reported.