The OP (organophosphate)-degrading enzyme from Agrobacterium
radiobacter (OpdA) is a binuclear metallohydrolase able
to degrade highly toxic OP pesticides and nerve agents into
less or non-toxic compounds. In the present study, the effect
of metal ion substitutions and site-directed mutations on the
catalytic properties of OpdA are investigated. The study shows
the importance of both the metal ion composition and a hydrogenbond
network that connects the metal ion centre with the
substrate-binding pocket using residues Arg254 and Tyr257 in the
mechanism and substrate specificity of this enzyme. For theCo(II)
derivative of OpdA two protonation equilibria (pKa1 ∼5; pKa2
∼10) have been identified as relevant for catalysis, and a terminal hydroxide acts as the likely hydrolysis-initiating nucleophile. In
contrast, the Zn(II) and Cd(II) derivatives only have one relevant
protonation equilibrium (pKa ∼4–5), and theμOHis the proposed
nucleophile. The observed mechanistic flexibility may reconcile
contrasting reaction models that have been published previously
and may be beneficial for the rapid adaptation of OP-degrading
enzymes to changing environmental pressures
