Acrylic acid and DMSP lyases in the green algae Ulva

Abstract

Green macroalgae of the genus Ulva contain dimethylsulfoniopropionate (DMSP) produced from methionine through a transamination pathway. DMSP is converted by DMSP lyase enzymes (DLs) into acrylic acid (AA), a high-value compound with a vast market as its esters have applications in the production of plastic additives, textiles, sealants, adhesives and surface coatings. The levels of AA produced by Ulva species, and the processes to extract it from this biomass, are currently poorly understood. In this study an analytical method was implemented for the simultaneous measurement of AA, DMSP and the pathway intermediates (4-methylthio-2-oxobutyrate, 4-methylthio-2-hydroxybutyrate, and 4-dimethylsulfonio-2-hydroxybutyrate) in Ulva biomass. The amounts of these compounds – with a focus on AA – were then quantified after processing the macroalgae with different chemical, thermal, and enzymatic treatments. AA was extracted in the range of 0.22–0.45 % of the algal dry weight. Sequential enzyme treatments showed no higher yields compared to individual enzymatic treatments or chemico-physical extractions, an encouraging result for the application of the treatments in industry. Codon-optimised synthetic genes for two candidate DLs from Ulva mutabilis were expressed and purified in E. coli with solubility tags, and the biochemical characteristics of these recombinant lyases were investigated. Both enzymes UM021_MBP and UM030_Halo7 had lower activity than their microalgal counterpart Alma1, with whom they share 33.9 % and 39.7 % similarity at amino acid level respectively. UM030_Halo7 appeared more active than UM021_MBP, with Km and Vm of 7.10 mM and 5.99 mM/min/mg protein respectively. UM030_Halo7 showed a preferred pH of 6.0, an optimal of temperature of 20 °C, was inhibited by NaCl concentration equal or higher than 0.5 M but not by H2O2 up to 50 μM. These results advance our understanding of AA biosynthesis in Ulva at the molecular level, and could be useful to implement processes for extracting AA from Ulva species within a biorefinery framework.