Catalytic versatility and backups in enzyme active sites: The case of serum paraoxanase 1Moshe Ben-David, Mikael Elias, Jean-Jacques Filippi, Elisabet Dunach, Israel Silman, Joel Sussman and Dan Tawfik, PhD [1] Molecular TourThe PON1 enzyme has theoretical biological importance as well as application for treatment of neurotoxins. Questions of the origins of enzyme promiscuity or the evolution of protein diversity may be illuminated by PON1's accidental low-level phosphotriesterase activity, and by the unintuitive effect of switching one amino acid in PON1 whereby it changes from a lactonase to a phosphotriesterase. Practically, a potent neurotoxin, "Paraoxon", and therefore a biochemical warfare threat, can be neutralized by phosphotriesterases. See also Serum Paraoxonase for a general review of issues relating to PON1 and the overall class of Serum Paraoxonases. A initial structural tour begins there as well. We experimentally solved two critical new PON1 structures. Previously solved in non-physiological conditions of pH 4.5, we have solved PON1 in physiological conditions of pH 6.5. While generally similar, as expected, there are some key differences. The side-chain of V346 within the active site pocket is rotated relative to the pH 4.5 structure, and the side-chains of F347 and H348 in the active site's 'second shell' adopted completely different rotamers. Next, we crystallized PON1 in complex with 2-hydroxyquinoline (2HQ), which is a lactone analog. As expected, this structure was also generally similar to the one at pH 4.5 and pH 6.5. We could now see an active site loop, residues 71-81, most of which had not been seen at either pH 4.5 or 6.5. The first segment of the active site loop, and residues Y71 and I74 in particular,comprises part of PON1's active-site wall. Further, 2HQ's carbonyl oxygen and NH moiety overlap with the phosphate oxygens in the apo structure. This overlap supports the notion that both the phosphate ion and 2HQ mimic the binding mode of substrates and/or reaction intermediates. In addition to interacting with the catalytic calcium, 2HQ interacts with the side-chains of H115, D269, E53 and N168. Importantly, while the bound 2HQ is in contact with the side-chains of Y71, in the absence of ligand Y71 is either disordered (pH 6.5), or (pH 4.5) positioned outside the binding pocket . PDB references: Serum paraoxonase-1 by directed evolution at pH 6.5, 3sre; Serum paraoxonase-1 by directed evolution at pH 6.5 in complex with 2-hydroxyquinoline, 3srg.
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