Choline sulfatase from Ensifer(Sinorhizobium) meliloti: Characterization of the unmodified enzyme

Ensifer(Sinorhizobium) melilotiis a nitrogen-fixing α-proteobacterium able to biosynthesize the osmoprotectant glycine betaine from choline sulfate through a metabolic pathway that starts with the enzyme choline-O-sulfatase. This protein seems to be widely distributed in microorganisms and thought to play an important role in their sulfur metabolism. However, only crude extracts with choline sulfatase activity have been studied. In this work, Ensifer(Sinorhizobium) meliloticholine-O-sulfatase was obtained in a high degree of purity after expression in Escherichia coli. Gel filtration and dynamic light scattering experiments showed that the recombinant enzyme exists as a dimer in solution. Using calorimetry, its catalytic activity against its natural substrate, choline-O-sulfate, gave a kcat=2.7×10−1s−1and a KM=11.1mM. For the synthetic substrates p-nitrophenyl sulfate and methylumbelliferyl sulfate, the kcatvalues were 3.5×10−2s−1and 4.3×10−2s−1, with KMvalues of 75.8 and 11.8mM respectively. The low catalytic activity of the recombinant sulfatase was due to the absence of the formylglycine post-translational modification in its active-site cysteine 54. Nevertheless, unmodified Ensifer(Sinorhizobium) meliloticholine-O-sulfatase is a multiple-turnover enzyme with remarkable catalytic efficiency.