Identification and Characterization of a Novel Alginate Lyase from Mesonia hitae R32 Exhibiting High Thermal Stability and Potent Antioxidant Oligosaccharide Production

Alginate lyases are of great importance in biotechnological and industrial processes, yet research on these enzymes from Mesonia genus bacteria is still limited. In this study, a novel PL6 family alginate lyase, MhAly6, was cloned and characterized from the deep-sea bacterium Mesonia hitae R32. The enzyme, composed of 797 amino acids, contains both PL6 and GH28 catalytic domains. Phylogenetic analysis revealed its classification into the subfamily 1 of the PL6 family. MhAly6 showed optimal activity at 45°C and pH 9.0, retaining over 50% activity after 210 min of incubation at 40°C, highlighting its remarkable thermal stability. The enzyme exhibited degradation activity toward sodium alginate, poly M, and poly G, with the highest affinity for its natural substrate, sodium alginate, producing alginate oligosaccharides (AOS) with degrees of polymerization (DP) ranging from 2 to 7. Molecular docking identified conserved catalytic sites (Lys241/Arg262) and calcium-binding sites (Asn202/Glu234/Glu236), while the GH28 domain played an auxiliary role in substrate binding. Antioxidant assays revealed that MhAly6-derived AOS showed potent radical-scavenging activity, achieving 80.64% and 95.39% inhibition rates against DPPH and ABTS radicals, respectively, highlighting their potential as natural antioxidants. This work provides a promising candidate for developing efficient alginate lyases and functional AOS, while expanding the application potential of Mesonia genus bacteria.