Molecular characterisation and biochemical properties of phosphomannomutase/phosphoglucomutase (PMM/PGM) in the brown seaweed Saccharina japonica

Abstract

Saccharina japonica is one of the most important seaweed resources for alginate and fucoidan extraction, and these extracted chemical products are used in foods, textiles, cosmetics, pharmaceuticals, and other applications. The synthesis pathways of alginate and fucoidan are complicated and most of genes and their functions are not clearly understood. Phosphomannomutase is an important enzyme that is involved in the synthesis of both alginate and F-fucoidan in the kelp and also fulfils a phosphoglucomutase function in polysaccharide synthesis. In this study, the full sequence of phosphomannomutase/phosphoglucomutase gene (Sjpmm/Sjpgm) was isolated from S. japonica and characterised. The open reading frame (ORF) of Sjpmm/Sjpgm was 756 bp long, encoded 252 amino acids, and belonged to the haloacid dehydrogenase (HAD) superfamily. The fusion enzyme SjPMM/SjPGM was expressed and purified, and its phosphomannomutase (PMM) and phosphoglucomutase (PGM) activities were determined. The optimal temperature for both PMM and PGM activities was 30 °C, and the optimal pH values were 7.5 and 7.0, respectively. The influences of Mg2+, Mn2+, Co2+, and Ni2+ on the activation of SjPMM/SjPGM were measured, and the maximum activity occurred with Mg2+. Under the optimal conditions, the Km values for the substrates glucose-1-phosphate (G1P) and mannose-1-phosphate (M1P) were 0.08 and 1.15 mM, respectively. In addition, high-temperature (25 °C) and high-light intensity (55 ± 5 μmol photons m−2 s−1) both significantly increased the transcription of Sjpmm/Sjpgm (up to 1.7-fold and 4.6-fold, respectively). These results implied that the SjPMM/SjPGM was closely related to the synthesis of alginate and F-fucoidan and was directly involved in the acclimatisation of S. japonica to light and temperature stress.