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of conditions that have allowed not only a better understanding of the microbial composition and
functioning in the environment, but have also provided important tools for searching for new
molecules, such as enzymes and metabolites, for potential biotechnological use, such as in biocatalysts
and antibiotics [16].
Considering the fact that toxic OPs are not natural products, the bacterial enzymes seemingly have
evolved, within a relatively short period, to specifically hydrolyze associated functional groups [17].
The microbial cells, with the action of some enzymes, present detoxification activities for bioremediation
purposes. If a nerve agent has been used, OP-degradation enzymes have the potential to promote a
fast hydrolysis compared to traditional chemical methods, with less impact to the environment [9].
The way in which the microorganism’s soil community metabolizes OPs is still under investigation.
Some pathways in both Gram-positive and Gram-negative bacteria under aerobic conditions are
understood, and the general tendency is that degradation models for most OPs tend to share the same
characteristics, with hydrolytic enzymes playing a fundamental role in the process [18].
It has been reported in soil treated with OPs a rising activity of a variety of microorganisms’
enzymes capable of detoxifying OPs through hydrolysis, such as, acid and alkaline phosphatase,
phosphodiesterase, phosphotriesterase (PTE) and dehydrogenase [19,20]. Among them, enzymes
listed in the category of phosphoric triester hydrolases have special importance in that they work
over a wide temperature and pH range, and are capable of degrading many OP substrates, acting
in the metabolism of compounds such as nerve agents and pesticides [21,22]. These enzymes were
further broken down into two subgroups —the aryldialkylphosphatases (EC 3.1.8.1) (also referred to
as organophosphorus hydrolases and PTEs) that prefer substrates bearing P–O or, alternatively, P–S
bonds; and the diisopropyl-fluorophosphatases (EC 3.1.8.2) (also including organophosphorus acid
anhydrolase (OPAA)), which are more active against OP compounds with P–F or P–CN bonds [21].
PTE is encoded by the organophosphorus-degrading (opd) gene and this is part of the
amidohydrolase superfamily [9,21]. All PTEs are metal-dependent hydrolases, that is, there is a
requirement for a divalent metal, which directly binds to the substrate to favor the catalysis process [23].
Zinc was found to be the native metal; however, other bivalent cations, such as Cd2 and Co , can
+
2+
also support activity [24 25]. The evaluation of kinetic constants obtained for paraoxon (PXN) by
,
metal-substituted PTEs, including Zn/Zn, Co/Co, Cd/Cd, Cd/Co, and Cd/Zn, shows that the Cd/Cd
PTE has the highest Km and pKa values and the lowest kcat/Km correlation [24].
PTE presents a strong hydrolysis for a range of insecticides such as phosphothioesters and
phosphorofluoridates, including chemical weapons such as sarin and soman, and is the only
enzyme capable of hydrolyzing P–S bonds in OPs [22]. The best well-characterized PTE comes
from Brevundimonas diminuta, and is a homodimer (35 kDa per monomer), with the overall folding
pattern consisting of an α/β barrel with eight strands of a parallel β-pleated sheet. The two divalent
metal ions are situated at the C-terminal portion of the TIM (triosephosphate isomerase) barrel motif [26].
A histidine-rich region (residues His55, His57, His201, His230, Asp301, and Lys169) facilitates binding
of the metals in the active site. Lys169, which is carboxylated, and a water molecule (or hydroxide ion)
serve as bridging ligands between two metal cations, which are essential for nucleophilic attack to the
phosphorus center of the OPs [21].
It can be affirmed that biodegradation of OPs is an important process to protect living beings and
the environment of the hazardous effects of this compound. As the hydrolysis is the principal process
involved, the degradation will be better comprehended through the study of the behavior of hydrolytic
enzymes in the media [9]. It is known that there are different ways of estimating biodegradation, with
most of them being structure-based methods [27]. However these models are limited. There is also a
search for ways and models to predict and analyze the natural behavior of different compounds. It is a
consensus that the most of the degradation process follows the first-order kinetics but defining the
set of parameters that can influence most in the observed degradation pattern is a challenge that, in
general, demands a huge experimental effort [28,29].