Rational design of organophosphorus hydrolase for the degradation and detection of neurotoxic pesticides and chemical warfare agents
Reeves, Tony Elvern
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It is critical to consider the balance between the catalytic capabilities of an enzyme and the inherent structural stability of the protein when developing enzymes for specific applications. Rational site directed mutagenesis has been used to explore the role of residues 254 and 257 in the global stability and catalytic specificities of organophosphorus hydrolase (OPH, EC 188.8.131.52). Substitution of residues H254 and H257, which are located near the active site, had a marked effect on both the global stability and substrate specificity of the enzyme. For example, the for the double mutation CoTG????2+ H254R H257L (RL) enzyme variant was 19.6 kcal/mol, 5.7 kcal/mol less than that of the wild type enzyme. At the same time, the altered enzyme was catalytically more effective against VX and VR (Russian VX), as compared to the wild type enzyme. Limited proteolysis verified the importance of residues 254 and 257 for functional stability, evidenced by enhanced resistance to irreversible unfolding associated with thermal denaturation. It has been possible to construct third generation OPH variants, which are more stable than the wild type enzyme, with a 10 ????C increase in the apparent melting temperature (TM app), yet retained desirable catalytic properties. It appeared that aromatic stacking and cation-???? interactions involving near active site residues not only affected activity but significantly contributed to the chemical and thermal stability of OPH. Rational design was used to develop an enzyme with an optimized orientation on a catalytically active biosensor surface. In these studies, lysine side chains located on the surface of OPH were used to create attachment sites to a surface plasmon resonance sensor resulting in an ensemble of enzyme orientations. Some of these orientations could be functionally restrictive if the active site is oriented toward the sensor surface. Substitution of a lysine near the active site resulted in 20% more activity with 53% less enzyme immobilized, thus increasing the specific activity of the decorated surface 2.5 fold.