Structural determinants of the high thermal stability of SsoPox from the hyperthermophilic archaeon Sulfolobus solfataricus

Organophosphates (OPs) constitute the largest class of insecticides used worldwide and certain of them are potent nerve agents. Consequently, enzymes degrading OPs are of paramount interest, as they could be used as bioscavengers and biodecontaminants. Looking for a stable OPs catalyst, able to support industrial process constraints, a hyperthermophilic phosphotriesterase (PTE) ( Sso Pox) was isolated from the archaeon Sulfolobus solfataricus and was found to be highly thermostable. The solved 3D structure revealed that Sso Pox is a noncovalent dimer, with lactonase activity against “quorum sensing signals”, and therefore could represent also a potential weapon against certain pathogens. The structural basis of the high thermostability of Sso Pox has been investigated by performing a careful comparison between its structure and that of two mesophilic PTEs from Pseudomonas diminuta and Agrobacterium radiobacter . In addition, the conformational stability of Sso Pox against the denaturing action of temperature and GuHCl has been determined by means of circular dichroism and fluorescence measurements. The data suggest that the two fundamental differences between Sso Pox and the mesophilic counterparts are: (a) a larger number of surface salt bridges, also involved in complex networks; (b) a tighter quaternary structure due to an optimization of the interactions at the interface between the two monomers.