Journal:JMB:2

Catalytic versatility and backups in enzyme active sites: The case of serum paraoxanase 1

Moshe Ben-David, Mikael Elias, Jean-Jacques Filippi, Elisabet Dunach, Israel Silman, Joel Sussman and Dan Tawfik, PhD ^[1]

Molecular Tour

The 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 , we have solved PON1 in . While , as expected, there are some key differences. The side-chain of V346 within the active site pocket is , and the side-chains of F347 and H348 in the active site's 'second shell' .

Next, we crystallized , which is a lactone analog. As expected, this structure was also and pH 6.5. We could now see an , most of which had not been seen at either pH 4.5 or 6.5. The 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 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. Importantly, while the bound 2HQ is in contact with the , in the absence of ligand Y71 is either disordered (pH 6.5), or (pH 4.5) .