Organophosphate hydrolase interacts with ferric-enterobactin and promotes iron uptake in association with TonB-dependent transport system

Our previous studies have shown the existence of organophosphate hydrolase (OPH) as a part of the inner membrane associated Ton complex (ExbB/ExbD and TonB) of Sphingobium fuliginis. We now show its involvement in iron uptake by establishing direct interactions with ferric-enterobactin. The interactions between OPH and ferric-enterobactin were not affected even when the active site architecture is altered by substituting active site aspartate with either alanine or asparagine. Protein docking studies further substantiated these findings and predicted the existence of ferric-enterobactin binding site that is different from the catalytic site of OPH. A lysine residue (82K) found at the predicted ferric-enterobactin binding site facilitated interactions between OPH and ferricenterobactin. Substitution of lysine with alanine did not affect triesterase activity, but it abrogated OPH ability to interact with both ferric-enterobactin and ExbD, strengthening further the fact that the catalytic site is not the site for binding of these ligands. In the absence of interactions between OPHK82A and ExbD, OPHK82A failed to target membrane in E. coli cells. The Sphingobium fuliginis TonB-dependent transport ((Sf)TonBDT) system was reconstituted in E. coli GS027 cells generated by deleting the exbD and tonB genes. The E. coli GS030 cells having (Sf)TonBDT system with OPH showed increased iron uptake. Such an increase was not seen in E. coli GS029, cells having (Sf)TonBDT system generated either by omitting OPH or by including its variants, OPHD301A, OPHD301N suggesting a role for OPH in enhanced iron uptake.