Phosphotriesterase (PTE) from soil bacteria is known for its ability to catalyze the detoxification of organophosphate pesticides and chemical warfare agents. Most of the organophosphate chemical warfare agents are a mixture of two stereoisomers at the phosphorus center and the SP-enantiomers are significantly more toxic than the RP-enantiomers. In previous investigations PTE variants were created through the manipulation of the substrate binding pockets and these mutants were shown to have greater catalytic activities for the detoxification of the more toxic SP-enantiomers of nerve agent analogs for GB, GD, GF, VX, and VR than the less toxic RP-enantiomers. In this investigation alternate strategies were employed to discover additional PTE variants with significant improvements in catalytic activities relative to the wild type enzyme. Screening and selection techniques were utilized to isolate PTE variants from randomized libraries and site specific modifications. The catalytic activities of these newly identified PTE variants towards the SP-enantiomers of chromophoric analogs of GB, GD, GF, VX, and VR have been improved up to 15,000 fold relative to the wild-type enzyme. The X-ray crystal structures of the best PTE variants were determined. Characterization of these mutants with the authentic G-type nerve agents has confirmed the expected improvements in catalytic activity against the most toxic enantiomers of GB, GD, and GF. The values of kcat/Km for the H257Y/L303T (YT) mutant for the hydrolysis of GB, GD, and GF were determined to be 2 × 106 M−1 s−1, 5 × 105 M−1 s−1, and 8 × 105 M−1 s−1, respectively. The YT mutant is the most proficient enzyme reported thus far for the detoxification of G-type nerve agents. These results support a combinatorial strategy of rational design and directed evolution as a powerful tool to discover more efficient enzymes for the detoxification of organophosphate nerve agents.

The Met receptor tyrosine kinase and its ligand hepatocyte growth factor (HGF) play an important role in mediating both tumor progression and tissue regeneration. The N-terminal and first Kringle domains (NK1) of HGF comprise a naturally occurring splice variant that retains the ability to activate the Met receptor. However, NK1 is a weak agonist and is relatively unstable, limiting its therapeutic potential. Here, we engineered NK1 mutants with improved biochemical and biophysical properties that function as Met receptor agonists or antagonists. We first engineered NK1 for increased stability and recombinant expression yield using directed evolution. The NK1 variants isolated from our library screens acted as weak Met receptor antagonists due to a mutation at the NK1 homodimerization interface. We introduced point mutations that restored this NK1 homodimerization interface to create an agonistic ligand, or that further disrupted this interface to create more effective antagonists. The rationally engineered antagonists exhibited melting temperatures up to approximately 64°C, a 15°C improvement over antagonists derived from wild-type NK1, and approximately 40-fold improvement in expression yield. Next, we created disulfide-linked NK1 homodimers through introduction of an N-terminal cysteine residue. These covalent dimers exhibited nearly an order of magnitude improved agonistic activity compared to wild-type NK1, approaching the activity of full-length HGF. Moreover, covalent NK1 dimers formed from agonistic or antagonistic monomeric subunits elicited similar activity, further signifying that NK1 dimerization mediates agonistic activity. These engineered NK1 proteins are promising candidates for therapeutic development and will be useful tools for further exploring determinants of Met receptor activation.

Hepatocyte growth factor (HGF), through activation of the c-MET receptor, mediates biological processes critical for tissue regeneration; however, its clinical application is limited by protein instability and poor recombinant expression. We previously engineered a HGF fragment (eNK1) that possesses increased stability and expression yield, and developed a c-MET agonist by coupling eNK1 through an introduced cysteine residue. Here, we further characterize this eNK1 dimer, and show it elicits significantly greater c-MET activation, cell migration, and proliferation than the eNK1 monomer. The efficacy of the eNK1 dimer was similar to HGF, suggesting its promise as a c-MET agonist.

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