Characterization of a novel D-lyxose isomerase from Cohnella laevoribosii RI-39 sp. nov.

A newly isolated bacterium, Cohnella laevoribosii RI-39, could grow in a defined medium with L-ribose as the sole carbon source. A 21-kDa protein isomerizing L-ribose to L-ribulose, as well as D-lyxose to D-xylulose, was purified to homogeneity from this bacterium. Based on the N-terminal and internal amino acid sequences of the purified enzyme obtained by N-terminal sequencing and quantitative time of flight mass spectrometry-mass spectrometry analyses, a 549-bp gene (lyxA) encoding D-lyxose (L-ribose) isomerase was cloned and expressed in Escherichia coli. The purified endogenous enzyme and the recombinant enzyme formed homodimers that were activated by Mn²⁺. C. laevoribosii D-lyxose (L-ribose) isomerase (CLLI) exhibits maximal activity at pH 6.5 and 70°C in the presence of Mn²⁺ for D-lyxose and L-ribose, and its isoelectric point (pI) is 4.2 (calculated pI, 4.9). The enzyme is specific for D-lyxose, L-ribose, and D-mannose, with apparent K_m values of 22.4 ± 1.5 mM, 121.7 ± 10.8 mM, and 34.0 ± 1.1 mM, respectively. The catalytic efficiencies (k_cat/K_m) of CLLI were 84.9 ± 5.8 mM^-1 s^-1 for D-lyxose (V _max, 5,434.8 U mg^-1), 0.2 mM^-1 s^-1 for L-ribose (V_max, 75.5 ± 6.0 U mg^-1), and 1.4 ± 0.1 mM^-1 s^-1 for D-mannose (V_max, 131.8 ± 7.4 U mg^-1). The ability of lyxA to permit E. coli cells to grow on D-lyxose and L-ribose and homology searches of other sugar-related enzymes, as well as previously described sugar isomerases, suggest that CLLI is a novel type of rare sugar isomerase.