Structure of the Bacillus anthracis dTDP- l -rhamnose-biosynthetic enzyme dTDP-4-dehydrorhamnose 3,5-epimerase (RfbC)

The exosporium layer of Bacillus anthracis spores is rich in l-rhamnose, a common bacterial cell-wall component, which often contributes to the virulence of pathogens by increasing their adherence and immune evasion. The biosynthetic pathway used to form the activated l-rhamnose donor dTDP-l-rhamnose consists of four enzymes (RfbA, RfbB, RfbC and RfbD) and is an attractive drug target because there are no homologs in mammals. It was found that co-purifying and screening RfbC (dTDP-6-deoxy-d-xylo-4-hexulose 3,5-epimerase) from B. anthracis in the presence of the other three B. anthracis enzymes of the biosynthetic pathway yielded crystals that were suitable for data collection. RfbC crystallized as a dimer and its structure was determined at 1.63-Å resolution. Two different ligands were bound in the protein structure: pyrophosphate in the active site of one monomer and dTDP in the other monomer. A structural comparison with RfbC homologs showed that the key active-site residues are conserved across kingdoms.The crystal structure of dTDP-6-deoxy-d-xylo-4-hexulose 3,5-epimerase (RfbC) from B. anthracis was determined at 1.63-Å resolution in complex with dTDP and pyrophosphate. RfbC is the third enzyme of the dTDP-l-rhamnose-biosynthetic pathway and only crystallized in the presence of the other three members of the pathway RfbA, RfbB and RfbD.