Structure of the Bacillus anthracis dTDP- l -rhamnose-biosynthetic enzyme dTDP-4-dehydrorhamnose reductase (RfbD)

Bacillus anthracis is the causative agent of the deadly disease Anthrax. Its use in bioterrorism and its ability to re-emerge have brought renewed interest in this organism. B. anthracis is a Gram-positive bacterium that adds l-rhamnose to its cell-wall polysaccharides using the activated donor dTDP-β-l-rhamnose. The enzymes involved in the biosynthesis of the activated donor are absent in humans, which make them ideal targets for therapeutic development to combat pathogens. Here, the 2.65 Å resolution crystal structure of the fourth enzyme in the dTDP-β-l-rhamnose-biosynthetic pathway from B. anthracis, dTDP-4-dehydro-β-l-rhamnose reductase (RfbD), is presented in complex with NADP⁺. This enzyme catalyzes the reduction of dTDP-4-dehydro-β-l-rhamnose to dTDP-β-l-rhamnose. Although the protein was co-crystallized in the presence of Mg²⁺, the protein lacks the conserved residues that coordinate Mg²⁺.The crystal structure of dTDP-4-dehydro-β-l-rhamnose reductase (RfbD) from Bacillus anthracis was determined at 2.65 Å resolution in complex with NADP⁺. RfbD is the fourth enzyme of the dTDP-β-l-rhamnose-biosynthetic pathway and provides evidence that RfbD homologs in Gram-positive bacteria exhibit key differences in the residues important for oligomerization and metal utilization compared with those from many Gram-negative bacteria.