Pseudomonas aeruginosa PfpI is a methylglyoxalase

Pseudomonas aeruginosa is an opportunistic pathogen, commonly associated with human airway infections. Based on its amino acid sequence similarity with Pyrococcus furiosus protease I, P. aeruginosa PfpI was originally annotated as an intracellular protease. In this work, we show that PfpI is a methylglyoxalase. The X-ray crystal structure of the purified protein was solved to 1.4 Å resolution. The structural data indicated that PfpI shares the same constellation of active site residues (including the catalytic Cys112 and His113) as those seen in a well-characterized bacterial methylglyoxalase from Escherichia coli, YhbO. Using NMR, we confirmed that PfpI qualitatively converted methylglyoxal into lactic acid. Quantitation of lactate produced by the methylglyoxalase activity of PfpI yielded a k_cat of 102 min^-1 and a K_M of 369 μM. Mutation of Cys112 and His113 in PfpI led to complete loss of methylglyoxalase activity. To investigate the functional impact of PfpI in vivo, a ΔpfpI deletion mutant was made. Quantitative proteomic analyses revealed a pattern of changes consistent with perturbation of ribosomal function, Zn²⁺ limitation, C1 metabolism, and glutathione metabolism. These findings are consistent with PfpI being a glutathione-independent methylglyoxalase. Previously, transposon insertion (pfpI::Tn) mutants have been reported to exhibit phenotypes associated with antibiotic resistance, motility, and the response to oxidative stress. However, the ΔpfpI mutant generated in this study displayed none of these phenotypes. Whole-genome sequencing of the previously described pfpI::Tn mutants revealed that they also contain a variety of other genetic changes that likely account for their observed phenotypes.