Biochemical characterization and structural analysis of ulvan lyase from marine Alteromonas sp. reveals the basis for its salt tolerance

Marine macroalgae have gained considerable attention as renewable biomass sources. Ulvan is a watersoluble anionic polysaccharide, and its depolymerization into fermentable monosaccharides has great potential for the production of bioethanol or high-value food additives. Ulvan lyase from Alteromonas sp. (AsPL) utilizes a beta-elimination mechanism to cleave the glycosidic bond between rhamnose 3-sulfate and glucuronic acid, forming an unsaturated uronic acid at the non-reducing end. AsPL was active in the temperature range of 30-50 degrees C and pH values ranging from 7.5 to 9.5. Furthermore, AsPL was found to be halophilic, showing high activity and stability in the presence of up to 2.5 M NaCl. The apparent K(m )and k(cat) values of AsPL are 3.19 +/- 0.37 mg mL(-1) and 4.19 +/- 0.21 s(-1), respectively. Crystal structure analysis revealed that AsPL adopts a beta-propeller fold with four anti-parallel beta-strands in each of the seven propeller blades. The acid residues at the protein surface and two Ca2+ coordination sites contribute to its salt tolerance. The research on ulvan lyase has potential commercial value in the utilization of algal resources for biofuel production to relieve the environmental burden of petrochemicals. (C) 2019 Elsevier B.V. All rights reserved.