Application of chemical kinetics to ecological

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Application of chemical kinetics to ecological
and toxicological problems
I. Application of enzyme kinetics ?
Degradation of organophosphate pesticides
Enzymes can be used to catalyze degradation of
pesticides
While extremely beneficial for protection of
crops, pesticides can have serious environmental
impact possible deleterious consequences from
seepage of these otherwise helpful chemicals
into soil and ground water
Case study organophosphate pesticides ?
abundant ? highly toxic ? neutralized
via reaction
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The same enzyme catalysts which can neutralize
these pesticides can also be used to detoxify
chemical nerve agents.
? nerve agents are structurally similar to
organophosphate pesticides
? both contain organophosphate esters
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Recent studies have shown that enzymes, which
effectively degrade organophosphates, can be
incorporated into polymers -- specifically, foams
-- in order to aid in their practical application.
One enzyme under current investigation is
organophosphorus hydrolase (OPH) a.k.a.
phosphotriesterase. derived from Escherichia
coli
The degradation reaction of organophosphates
works via hydrolysis.
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Here, ENZ enzyme OPH (a.k.a.
phosphotriesterase)
The kinetics can be monitored by following the
production of p-nitrophenol, a yellow colored
chemical, spectroscopically.
Enzyme kinetics can be used to determine the
efficiency of OPH when it is integrated into a
polymeric foam compared to OPHs efficiency in
solution.
? evaluate KM and Vmax and compare
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Lets review how these parameters are derived in
enzyme kinetics
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First, lets look at the activity of OPH in
solution
To obtain Vmax and KM from this data
Method I ? extrapolate Vmax for the plot shown
above ? find Vmax/2 ? find substrate
corresponding to the x-value at Vmax/2 ? This is
KM
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Here, Vmax 0.063 mM/min and KM 0.08 mM
Or Method II plot 1/V versus 1/substrate (Line
weaver-Burk Plot)
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The line that fits the data has the following
form y 14.3 min/mM 1.4 min x
This yields Vmax 1/14.3(min/mM)-1 0.07
mM/min, and KM 1.4 Vmax 0.09 mM
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Now, lets see how well OPH hydrolyzes paraoxon
when it is encapsulated in foam.
Vmax
Vmax/2
KM
Again, Vmax can be found from the asymptote of
the curve, and KM is that substrate
concentration which corresponds to Vmax/2.
Vmax for OPH in foam 0.035 mM/min, while KM
0.19 mM.
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We get similar results from a Lineweaver-Burk
plot
from the y-intercept, Vmax 0.040 mM/min from
the slope, KM 0.24 mM
Now, we can compare these parameters for the
cases where OPH is in solution and integrated
into foam.
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Vmax decreases by a factor of 2 when the enzyme
OPH is encapsulated in foam and KM increases by
approximately the same factor.
What does this mean????
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The lower the value for KM, the better the enzyme
works (tighter binding) as a catalyst for the
hydrolysis of paraoxon.
OPH is a more efficient catalyst in solution than
in foam!!!
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Why is OPH less efficient as a catalyst when used
in a foam???
Lets look at Vmax for some clues Recall
where OPH0 is the total amount of enzyme
present.
There are 2 possible effects at work here
1. Inhibition effect
In order to get the enzyme into foam, certain
groups of atoms (called functional groups) within
the enzyme (e.g. NH2 and OH) react with the
foam, immobilizing a percentage of the total
enzyme.
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2. Change in the rate constant k2.
A decrease in k2 may occur when OPH is used in
foam, leading to a smaller Vmax as compared to
that parameter measured when OPH is in solution.
Is the decrease in catalytic efficiency when OPH
is used in a foam a negative result?
Not exactly ... ? for use on pesticides,
delivery of catalysts like OPH via foams is
likely not the best means to degrade organo-
phosphates -- reduced efficiency ? for use on
other toxins (e.g. nerve agents) delivery
in foams is extremely beneficial OPH is still
highly effective and delivery via foams is more
convenient
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Environmental Resources on the internet
Environmental Protection Agency more
information on organophosphates and other
environmental problems. http//www.epa.gov/ more
specific information on pesticides is located
at http//www.epa.gov/pesticides/ (for
organophosphates, append /op/)