Fig. 3. Vitamin C coordination.

For clarity, only the substrate-coordinating residues different between pmUlaA and ecUlaA are shown here. a, b Substrate coordination in the outward-facing structure of ecUlaA. Vitamin C is entirely coordinated by the residues from the core domain. c, d Substrate coordination in the occluded structure of ecUlaA. Ser59 from V motif 1 contributes a hydrogen bond to the O3 atom of vitamin C, supposed to stabilize the charge balance of the substrate. e, f Substrate coordination in the inward-facing structure of pmUlaA. Arg288 from V motif 2 forms a hydrogen bond with the O3 atom of vitamin C