Expression, purification and enzymatic characterization of undecaprenyl pyrophosphate phosphatase from Vibrio vulnificus

Undecaprenyl pyrophosphate phosphatase (UppP), a cell membrane integral enzyme, catalyzes the dephosphorylation of undecaprenyl pyrophosphate to undecaprenyl phosphate, which is an essential carrier lipid in bacterial cell wall synthesis. We previously purified E. coli UppP and concluded that its catalytic site is likely located in the periplasm. To search for additional natural UppP homologs to elucidate what constitutes a common catalytic mechanism and to gain a better chance of obtaining high-resolution crystal structural information, we expressed and purified recombinant Vibrio vulnificus UppP using E. coli as a host. Mutagenesis analysis demonstrates that the proposed catalytic residues Gln-13, Glu-17, His-26 and Arg-166 are directly involved in enzyme catalysis. Additionally, mutations of most of the conserved serine and glycine residues within the proposed catalytic site (S22A, G163A and S165A) lead to complete inactivity, very low activity (<1.3% of the wild type) or no protein expression at all (G163R and G168A), whereas S23A and S167A retain enzyme activity (65% and 34%). Kinetic analysis indicates that S23A and S167A result in 1.4- and 5-fold decreases in k_cat, whereas the substrate K_mvalue exhibits only minor changes compared with wild-type UppP, implying that they are involved in enzyme catalysis. The structural modeling and molecular dynamics simulation analyses also provide a plausible structure of the catalytic core, centered on a conserved histidine (His-26) that initiates the hydrolysis of phosphate esters, rationalizing the mutagenesis data. This conclusion can be applied generally to all bacterial UppP enzymes.