Spinning sugars in antigen biosynthesis: a direct study of the Coxiella burnetii and Streptomyces griseus TDP-sugar epimerases

The sugars streptose and dihydrohydroxystreptose (DHHS) are unique to the bacteria Streptomyces griseus and Coxiella burnetii respectively. Streptose forms the central moiety of the antibiotic streptomycin, whilst DHHS is found in the O-antigen of the zoonotic pathogen C. burnetii. Biosynthesis of these sugars has been proposed to follow a similar path to that of TDP-rhamnose, catalysed by the enzymes RmlA/RmlB/RmlC/RmlD. Streptose and DHHS biosynthesis unusually require a ring contraction step that might be performed by the orthologues of RmlC or RmlD. Genome sequencing of S. griseus and C. burnetii proposed the StrM and CBU1838 proteins respectively as RmlC orthologues. Here, we demonstrate through both coupled and direct observation studies that both enzymes can perform the RmlC 3,5 double epimerisation activity; and that this activity supports TDP-rhamnose biosynthesis in vivo. We demonstrate that proton exchange is faster at the 3 position than the 5 position, in contrast to a previously studied orthologue. We solved the crystal structures of CBU1838 and StrM in complex with TDP and show that they form an active site highly similar to previously characterised enzymes. These results further support the hypothesis that streptose and DHHS are biosynthesised using the TDP pathway and are consistent with the ring contraction step being performed on a double epimerised substrate, most likely by the RmlD paralogue. This work will support the determination of the full pathways for streptose and DHHS biosynthesis.