Protein purification, crystallization and preliminary X-ray diffraction analysis of L-arabinose isomerase from Lactobacillus fermentum CGMCC2921

L-Arabinose isomerase (AI) catalyzes the isomerization of L-arabinose to L-ribulose, as well as that of D-galactose to D-tagatose. A thermophilic AI derived from Lactobacillus fermentum CGMCC2921 (LFAI) was overexpressed in Escherichia coli BL21 (DE3). This enzyme was purified to over 95% purity by nickel affinity, Mono-Q ion-exchange and size-exclusion chromatography. The LFAI protein was crystallized from either 0.1 M bis-tris pH 6.5, 23% PEG 3350, 0.3 M NaCl (form 1 crystals) or 0.1 M bis-tris pH 6.0, 25% PEG monomethyl ether 5000 (form 2 crystals). Diffraction data from form 1 LFAI crystals were collected to 2.80 Å resolution using synchrotron radiation. The form 1 crystals belonged to the orthorhombic space group P2₁2₁2₁, with unit-cell parameters a = 85.11, b = 184.57, c = 186.26 Å, α = β = γ = 90°. The asymmetric unit contained six LFAI subunits, corresponding to a calculated Matthews coefficient of 2.29 ų Da⁻¹ and a solvent content of 46.22%.