Fig. 5: Proposed catalytic mechanism of PimE.

a Schematic representation of the active site of PimE with bound substrates, showing the positioning of PPM and Ac₁PIM4. D58, located at the tip of JM1, acts as the catalytic base. b Proposed catalytic mechanism, showing the role of D58 in deprotonating the 2-OH group of Ac₁PIM4, initiating a nucleophilic attack on the anomeric carbon of PPM. This leads to the formation of an α(1 → 2) glycosidic bond between the mannose moiety of PPM and Ac₁PIM4. The phosphate group of PPM is cleaved and stabilized by K195 and H322, forming the product Ac₁PIM5 and the by-product PP. The tetra-acylated acceptor Ac₂PIM4 and its corresponding glycosylated product Ac₂PIM5 are not shown here for the sake of simplicity.