Purification and characterization of d-Gal-6-sulfurylase from Eucheuma striatum

d-Gal-6-sulfurylase catalyzing the conversion of μ-carrageenan into κ-carrageenan was extracted from Eucheuma striatum and purified by ammonium sulfate precipitation, hydrophobic interaction chromatography and ion exchange chromatography. The purified enzyme was a monomeric protein with a molecular mass of about 65 kDa as shown in SDS–PAGE. The maximum activity of the enzyme was observed at pH 7.0 and temperature 40 °C. Km value for μ-carrageenan was 4.31 mM, and the corresponding Vmax was 0.17 mM min−1. The carrageenan treated with 10 U of the purified enzyme exhibited 7.1-fold increase in gel strength with a removal of 30% sulfate groups. 1H NMR spectral analysis of the control and enzyme treated carrageenan confirmed the conversion of μ- into κ-carrageenan and highlighted the specificity of Gal-6-sulfurylase for μ-carrageenan. This Gal-6-sulfurylase provides an eco-friendly and alternative for alkali treatment method to produce high gel strength κ-carrageenan.