Interrogation of the Substrate Profile and Catalytic Properties of the Phosphotriesterase from Sphingobium sp. Strain TCM1: An Enzyme Capable of Hydrolyzing Organophosphate Flame Retardants and Plasticizers

The most familiar organophosphorus compounds are the neurotoxic insecticides and nerve agents. A related group of organophosphorus compounds, the phosphotriester plasticizers and flame retardants, has recently become widely used. Unlike the neurotoxic phosphotriesters, the plasticizers and flame retardants lack an easily hydrolyzable bond. While the hydrolysis of the neurotoxic organophosphates by phosphotriesterase enzymes is well-known, the lack of a labile bond in the flame retardants and plasticizers renders them inert to typical phosphotriesterases. A phosphotriesterase from Sphingobium sp. strain TCM1 (Sb-PTE) has recently been reported to catalyze the hydrolysis of organophosphorus flame retardants. This enzyme has now been expressed in Escherichia coli, and the activity with a wide variety of organophosphorus substrates has been characterized and compared to the activity of the well-known phosphotriesterase from Pseudomonas diminuta (Pd-PTE). Structure prediction suggests that Sb-PTE has a β-propeller fold, and homology modeling has identified a potential mononuclear manganese binding site. Sb-PTE exhibits catalytic activity against typical phosphotriesterase substrates such as paraoxon, but unlike Pd-PTE, Sb-PTE is also able to effectively hydrolyze flame retardants, plasticizers, and industrial solvents. Sb-PTE can hydrolyze both phosphorus–oxygen bonds and phosphorus–sulfur bonds, but not phosphorus–nitrogen bonds. The best substrate for Sb-PTE is the flame retardant triphenyl phosphate with a kcat/Km of 1.7 × 106 M–1 s–1. Quite remarkably, Sb-PTE is also able to hydrolyze phosphotriesters with simple alcohol leaving groups such as tributyl phosphate (kcat/Km = 40 M–1 s–1), suggesting that this enzyme could be useful for the bioremediation of a wide variety of organophosphorus compounds.