Environmental Engineering Reference
In-Depth Information
Another paper published by Paul et al. (2006) demonstrated for the irst time that bio-
augmentationofcontaminatedsoilwith A. protophormiae stimulatedcompletedegradation
ofp-nitrophenolunderieldconditions.Ortiz-Hernándezetal.(2003)describedbiotrans-
formationby Flavobacterium sp.ofthefollowingorganophosphatepesticides:phorate,tet-
rachlorvinphos, methyl-parathion, terbufos, trichloronate, ethoprophos, phosphamidon,
fenitrothion,dimethoate,andDEF(S,S,S-tributylphosphorotrithioate).The Flavobacterium
sp.ATCC27551strainbearingtheorganophosphate-degradationgenewasused.Bacteria
wereincubatedinthepresenceofeachpesticideforadurationof7days.Parentpesticides
wereidentiiedandquantiiedbymeansofagas-chromatographymassspectrometersys-
tem(GC-MS).Theactivitywasdeinedastheamount(μmol)ofeachpesticidedegradedby
Flavobacterium sp.Pesticideswerehydrolyzedatthebondbetweenphosphorusandhetero-
atom,producingphosphoricacidandthreemetabolites.Enzymaticactivity(Equation6.7)
wasexpressedbythefollowingmultiplelinearrelationship(Ortiz-Hernándezetal.2003).
(
)
Enzymatic activity
=
162 2 9 5 dihedral angle energy
.
.
(
)
(
)
25
.0
to
tal energy
0.
51 molecular weight
(6.7)
Posphotriesterase (PTE) isolated from bacteria can catalyze the cleavage of the P—O
bondinavarietyoforganophosphatetriesters(Tsugawaetal.2000)andisthereforehighly
eficient in hydrolyzing this kind of pesticides (Masson et al. 1998). Assuming that the
enzymaticprocesswasdrivenbythe Flavobacterium sp.PTE,thefollowingmechanismto
hydrolyzepesticides,dividedintothreestepsforillustrativepurposeonly,wasproposed
byOrtiz-Hernándezetal.(2003).Consideringthatthetwoatomsofzinc,naturallypres-
ent in PTE (Benning et al. 2001), are participating in pesticide hydrolysis, the irst step
startswiththecoordinationoftheelectronegativeatomX(oxygenorsulfur)withonezinc,
resultinginapolarizationofthedoublebondP=X(Figure6.3a;Ortiz-Hernándezetal.
2003).Thereafter,phosphorusundergoesanucleophilicattackbythebridginghydroxyl,
producingapenta-coordinatedgeometryundergoingaregenerationofthedoublebond
P=XbyeliminatingasubstituentRX-inordertostabilizeitscharges(Figure6.3b;Ortiz-
Hernández et al. 2003). Thereafter, an OH- obtained from the ambient is coordinated
withthemetallicatomwhilethepesticideremainsbondedtothePTE(Figure6.3c;Ortiz-
Hernández et al. 2003). At the second step ( Figure 6.5 ; Ortiz-Hernández et al. 2003), the
nucleophilicattackisrepeateduntilallRX-groupsareeliminatedfromthephosphorus
(Figure6.4athrough4c;Ortiz-Hernándezetal.2003).Finally,thelaststepofthemecha-
nismofactionofPTEisdependentontheheteroatomX(Ortiz-Hernándezetal.2003).If
thepesticidecontains
• Oxygen, the hydrolyzed pesticide is delivered fromthe activecenter tothe PTE
(Figure6.5a)
• Sulfur, a last nucleophilic attack occurs to the phosphorus (Figure 6.5b) and a
penta-coordinatedintermediatecompoundisformedagain(Figure6.5c).
The aerobic degradation of ethion by mesophilic bacteria isolated from contaminated
soilssurroundingthedisusedcattledipsiteswasinvestigatedbyFosteretal.(2004).Two
isolates,identiiedas Pseudomonas and Azospirillum species,werecapableofbiodegrada-
tionofethionwhencultivatedinminimalsaltmedium.Theabiotichydrolyticdegrada-
tionproductsofethionsuchasethiondioxon,ethionmonoxon,O,O-diethylthiophosphate,
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