Characteristics of the opd Gene Product and Other Bacterial OPA Anhydrolases

1 Characteristics of the opd Gene Product and Other Bacterial OPA Anhydrolases 

Currently under intense scrutiny, the protein product of the opd gene of Psuedomonas diminuta is perhaps the best studied of the bacterial OPA anhydrolases. It has been shown that the opd OPA anhydrase (also called 

Mechanism of hydrolysis of parathion by the opd OPA anhydrase as determined by Lewis et al. The reaction is a single displacement using a base at the active site to activate a water molecule. The activated water attacks the phosphorus, producing diethyl phosphate and 4-nitrophenol. The same active site is able to hydrolyze DFP (Dumas et al. 1989) and related organofluo-rophosphates (Dumas et al. 1990). (Modified after Lewis, V.E., W.J. Donarski, J.R. Wild, and F.M. Raushel. 1988. Biochemistry 27 1591-1597.) 

Horne et al. (2002) isolated an enzyme (OpdA) from Agrobacterium that hydrolyzes a variety of organophosphates. The gene (opdA) was sequenced and found to be 88% identical to the sequence for the opd gene. There are differences in substrate selectivity, with OpdA hydrolyzing some important organophosphate more rapidly than the opd gene product. 

Clearly, there is a diversity of related and unrelated OPA anhydrolases found in bacteria. The opd and opdA genes are obviously related sequences and share a common evolutionary ancestor. The OPAA2 enzyme is apparently quite different. The selection pressure resulting in enzymes with similar activities but quite different structure is not known. This situation mimics the situation in eukaryotic organisms where at least two very different enzymes are capable of hydrolyzing organophosphates. 

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