Novel oligomeric SGNH-arylesterases from Sinorhizobium mellioti, SM23, SM24A and SM24B, were characterized using biochemical and biophysical methods. A sequence comparison of Sm23 with other SGNH members confirmed the presence of the catalytic triad (Ser10, Asp187, and His190) and oxyanion holes (Ser10-Gly50-Asn90). SM24A has a catalytic triad (Ser13, Asp187, and His190), but it does not have a consensus blockⅡ. SM24B as well as Sm23 have four conserved sequence blocks containing the catalytic triad (Ser15, Asp192, and His195), and oxyanion hole residues (Ser15-Gly57-Asn97). All these Enzymes were able to hydrolyze p-nitrophenyl acetate, α-, and β-naphthyl acetate, while the S10A of Sm23 mutant completely losts its activity. Structural properties of Sm23 were investigated using circular dichroism (CD), fluoroscence, dynamic light scattering (DLS), chemical cross-linking, electron microscopy (EM), and time of flight (TOF) mass spectrometry. Spherical or globular aggregates were observed with 1-butyl-3-methylimidazolium tetrafluoroborate, while amorphous aggregates were formed with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.
