Functional and structural characterization of a novel L-fucose mutarotase involved in non-phosphorylative pathway of L-fucose metabolism

Abstract

Mutarotases catalyze the α-β anomeric conversion of monosaccharide, and play a key role in utilizing sugar as enzymes involved in sugar metabolism have specificity for the α- or β-anomer. In spite of the sequential similarity to l-rhamnose mutarotase protein superfamily (COG3254: RhaM), the ACAV_RS08160 gene in Acidovorax avenae ATCC 19860 (AaFucM) is located in a gene cluster related to non-phosphorylative l-fucose and l-galactose metabolism, and transcriptionally induced by these carbon sources; therefore, the physiological role remains unclear. Here, we report that AaFucM possesses mutarotation activity only toward l-fucose by saturation difference (SD) NMR experiments. Moreover, we determined the crystal structures of AaFucM in the apo form and in the l-fucose-bound form at resolutions of 2.21 and 1.75 Å, respectively. The overall structural folding was clearly similar to the RhaM members, differed from the known l-fucose mutarotase (COG4154: FucU), strongly indicating their convergent evolution. The structure-based mutational analyses suggest that Tyr18 is important for catalytic action, and that Gln87 and Trp99 are involved in the l-fucose-specific recognition.