Environmental Engineering Reference
In-Depth Information
Methyl-SeCys
Accumulation
Selenate
Tolerance
SeCys
methyl-
transferase
(
SMT
)
ATP sulfurylase
(
APS
)
Selenite
Accumulation
Tolerance
Se - cysteine
Alanine
+ Se
0
Se-cysteine
lyase
(
SL
)
Cystathionine
γ
-synthase
(
C
γ
S
)
Accumulation
Se-methionine
Accumulation
Tolerance
Volatilization
Tolerance
Dimethylselenide
FIGURE 14.1
Overview. of. genetic. engineering. approaches. that. have. been. used. successfully. to. enhance.
plant.Se.tolerance,.accumulation,.and/or.volatilization.
cotransporters.(Smith.et.al..1995,.Leustek.1996,.Yoshimoto.et.al..2002,.2003,.
Hawkesford.2003,.Maruyama-Nakashita.et.al..2004)..Selenate.assimilation.
takes.place.predominantly.in.the.leaf.chloroplasts.(Pilon-Smits.et.al..1999)..
Most. plants. supplied. with. selenate. accumulate. predominantly. selenate,.
while.plants.supplied.with.selenite.accumulate.organic.Se,.suggesting.that.
the.reduction.of.selenate.to.selenite.is.a.rate-limiting.step.in.the.Se.assimi-
lation.pathway.(de.Souza.et.al..1998)..The.conversion.of.selenate.to.selenite.
is. mediated. by.two. enzymes.(Figure.14.1).. ATP. sulfurylase.(APS).couples.
selenate. to. ATP,. forming. adenosine. phosphoelenate. (APSe). (Wilson. and.
Bandurski.1958)..APSe.is.subsequently.reduced.to.selenite.by.APS.reductase.
(APR)..The.further.reduction.of.selenite.to.selenide.may.happen.exclusively.
in. the. chloroplast. if. it. is. mediated. by. sulite. reductase,. in. analogy. with.
sulite. reduction.. However,. it. has. also. been. suggested. that. nonenzymatic.
reduction.by.reduced.glutathione.(GSH).may.be.the.predominant.mecha-
nism.for.selenite.reduction.(Anderson.1993,.Terry.et.al..2000)..Selenide.can.
subsequently.be.coupled.to.OAS.to.form.SeCys.by.means.of.OAS.thiol.lyase.
(also. called. cysteine. synthase).. OAS. is. synthesized. by. the. enzyme. serine.
acetyl.transferase.(SAT).and.also.functions.as.a.signal.molecule.that.upreg-
ulates.the.activity.of.sulfate.transporters.and.sulfate.assimilation.enzymes.
SeCys. can. be. converted. to. SeMet. by. means. of. three. enzymes.
(Figure. 14.1).. Cystathionine-gamma-synthase. (CgS). irst. couples. SeCys. to.
O-phosphohomoserine. (OPH). to. form. Se-cystathionine.. Cystathionine-
gamma-lyase. further. converts. Se-cystathionine. to. Se-homocysteine..
Se-homocysteine. is. then. converted. to. SeMet. by. Met. synthase.. SeCys. and.
SeMet.can.also.be.(mis)incorporated.into.proteins,.replacing.Cys.and.Met;.this.
is. thought. to. be. an. important. reason. for. the. toxicity. of. Se.. SeMet. has. mul-
tiple. other. possible. fates,. one. of. which. is. to. be. methylated. via. methionine.
methyltransferase.(MMT)..Methyl-SeMet.can.be.further.metabolized.to.vola-
tile.DMSe,.which.is.cleaved.off.of.the.intermediate,.dimethylselenopropionate.
(DMSeP),.by.DMSeP.lyase.
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