Handling Copper: A Paradigm for the Mechanisms of Metal Ion Homeostasis - Environmental Contamination: Health Risks and Ecological Restoration - page 266

both.proteins.that.can.oxygenate.substrates.by.transferring.four.electrons.
to. a. dioxygen. molecule,. resulting. in. two. water. molecules. (Bhattacharya.
2005).. Laccase. is. a. multicopper-containing. glycoprotein. that. has. been.
found. in. arthropods,. fungi,. and. higher. plants.. This. protein. can. act. as. a.
polyphenol. oxidase. and. has. been. most. widely. implicated. in. lignin. syn-
thesis. (Gavnholt. and. Larsen. 2002).. In. plants,. laccases. have. a. predicted.
N-terminal. signal. peptide. sequence. that. is. for. the. secretory. pathway,.
which. could. lead. to. integration. into. the. cell. wall. (Gavnholt. and. Larsen.
2002).. This. is. consistent. with. previous. research. that. has. isolated. laccase.
from.cell.walls.
The.biochemical.function.of.ascorbate.oxidase.is.to.oxidize.alpha.ascorbic.
acid. to. dehydroascorbic. acid. (Bhattacharya. 2005);. however,. the. biological.
function.is.not.well.understood..In.plants,.ascorbate.oxidase.is.found.pre-
dominately. in. the. apoplast. because. dehydroascorbic. acid. is. more. readily.
taken.up.by.the.cell.than.the.charged.form.of.the.molecule.(Horemans.et.al..
2000)..Ascorbic.acid.has.been.highly.studied.in.plants.and.the.major.biologi-
cal.function.is.in.oxidative.stress.defense.mechanisms.with.high.levels.found.
in. chloroplasts. (Smirnoff. 1996).. Even. though. ascorbate. oxidase. function. is.
unknown,.transcript.levels.are.increased.in.the.light.(Pignocchi.et.al..2003)..
More.recently,.it.has.been.suggested.that.one.role.could.include.cell.elonga-
tion.through.cell.wall.loosening.mediated.by.the.hormone.auxin.(Kato.and.
Esaka.2000)..Interestingly,.plants.contain.a.group.of.ascorbate.oxidases.that.
do. not. include. a. copper-binding. site.. These. two. groups. of. oxidases. have.
approximately. 25%-30%. sequence. similarity;. however,. it. is. expected. that.
the.proteins.lacking.copper.have.a.different.biological.function.(Nakamura.
and.Go.2005)..Both. Brassica napus .and. Arabidopsis .contain.ascorbate.oxidases.
lacking. copper. with. functions. in. Brassica . involving. pollen. tube. growth.
(Hulzink. et. al.. 2002). and. directional. root. growth. in. Arabidopsis . (Sedbrook.
et.al..2002).
The. most. notable. electron. transfer. proteins. containing. copper. are.
cytochrome. c . oxidase. and. plastocyanin.. Cytochrome. c . oxidase. is. found.
in. aerobic.bacteria.and. mitochondria.of. all.eukaryotes.. This. large,.trans-
membrane. protein. is. the. terminal. oxidase. in. the. cellular. respiration.
system.. The. biochemical. function. of. cytochrome. c . oxidase. is.to.use.elec-
trons.from.cytochrome. c .to.reduce.dioxygen.to.produce.water.and.at.the.
same.time.pump.protons.(Michel.et.al..1998)..The.enzyme.is.located.in.the.
mitochondrial.inner.membrane.and.contributes.to.the.electrochemical.gra-
dient.of.protons.across.the.membrane..This.gradient.drives.the.synthesis.
of. adenosine-5′-triphosphate. (ATP),. which. is. the. primary. energy. source.
for.living.organisms..Most.cytochrome. c .oxidases.have.three.core.subunits.
that.are.highly.conserved.across.organisms..There.are.two.copper.binding.
sites.(Cu A .and.Cu B ).with.a.total.of.three.copper.atoms.(Steffens.et.al..1987)..
The. large. C-terminal. domain. of. subunit. II. in. the. intermembrane. space.
contains.a.Cu A -center.that.functions.as.an.electron.conductor..In.contrast,.