Proposed undecaprenyl-pyrophosphate phosphatase reaction mechanism in BacA. a Empty active site. Glu21 and Arg174 form a salt bridge as indicated by a blue dashed line. b Michaelis complex with C55PP. Glu21 serves as a proton acceptor that facilitates the nucleophilic attack of the hydroxyl group of Ser27 at the β-phosphate of C55PP. c Collapse of the pentavalent intermediate, phosphorylation of Ser27 and cleavage of the pyrophosphate bond. Upon disruption of the salt bridge between Glu21 and Arg174, the latter can stabilize the pentavalent intermediate in close proximity via ionic interactions. d Product complex. Hydrolysis of the phosphate group at Ser27 is mediated by a water molecule. Stabilization of the pentavalent intermediate by Arg174 potentially facilitates the exit of the product C55P from the active site. e Collapse of the second pentavalent intermediate. f Empty active site ready to undergo another reaction cycle. Calcium likely plays a role in catalysis by coordinating with the polar head groups of C55P and C55PP and the pyrophosphate pentavalent intermediate. Electron lone pairs are shown as double dots. Red curved arrows indicate electron flow. Dashed blue lines denote ionic interactions. R represents the polyisoprenyl chain of C55 with the double bond configuration of bacterial Z8,E2,ω-undecaprenol (structure in inset at bottom). See also Supplementary Movie 1 and Supplementary Movie 2
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