Interplay between amino acid residues at positions 192 and 115 in modulating hydrolytic activities of human paraoxonase 1

Human paraoxonase 1 (h-PON1) is a Ca2+-dependent serum enzyme that catalyzes the hydrolysis of different types of substrates. The crystal structure of h-PON1 is not solved yet and the molecular details of how the enzyme catalyzes different types of reactions are not clear. Literature suggests that the amino acid residues at positions 192 and 115 are important for various hydrolytic activities of h-PON1. It is proposed that catalytic residue H115 (and H134) mediates the lactonase and the arylesterase activities of the enzyme while the amino acid residue at position 192 modulates various other hydrolytic activities of the enzyme. However, the relationship between these two residues in the hydrolytic activities of h-PON1 is not studied in detail. In this study, we have expressed and purified the wild-type recombinant h-PON1 (rh-PON1(wt)) and its point mutants differing in the amino acid residues at positions 192 and/or 115 using an Escherichia coli expression system. The hydrolytic activities of the purified enzymes were compared using enzymatic assays. Our results, for the first time, show that (a) the presence of a particular amino acid residue at position 192 differentially alters the effect of the H115W substitution, and (b) H115 residue is not always needed for the lactonase and arylesterase activities of the enzyme. The results also suggest that the amino acid residues at position 192 and 115 act in conjunction in modulating the hydrolytic activities of the enzyme.