Handling Copper: A Paradigm for the Mechanisms of Metal Ion Homeostasis - Environmental Contamination: Health Risks and Ecological Restoration - page 267

subunit.I.is.the.site.of.oxygen.reduction,.containing.two.atoms.of.copper.at.
the.Cu B .site.and.two.heme.groups.(Steffens.et.al..1987).
Plastocyanin. is. a. small,. blue,. copper-containing. protein. found. in. pho-
tosynthetic.organisms.. It. is. deined. as. a. blue.protein. because. of. the.form.
of. copper. bound. (Cu. II). and. the. characteristics. of. the. binding. site. which.
consists.of.two.histidines,.one.cysteine,.and.one.methionine.(Bhattacharya.
2005)..Plastocyanin.contains.one.copper.ion,.is.approximately.10.kDa,.and.
is. located. inside. the. lumen. of. plant. chloroplasts. as. well. as. some. cyano-
bacteria.and.green.algae..The.biochemical.function.is.to.transfer.electrons.
between.cytochrome. b 6. f .complex.and.PSI.in.the.light-mediated.reaction.of.
photosynthesis.(Pessarakli.2005)..The.structure.of.plastocyanin.is.an.eight.
strand,. antiparallel. β-barrel. (Sigfridsson. 1998). and. the. copper. binding.
site.is.highly.conserved.across.higher.plant.systems.as.well.as.some.algae.
at. HIS 42 ,. CIS 92 ,. HIS 95 ,. and. MET 100 . (Pessarakli. 2005).. The. photosynthetic.
machinery. of. photosystem. II. and. photosystem. I. are. located. within. the.
membrane.of.the.thylakoid..Similar.to.cellular.respiration.in.mitochondria,.
this. machinery. drives. an. electrochemical. gradient. that. produces. ATP. as.
well.as.the.reduced.form.of.nicotinamide.adenine.dinucleotide.phosphate.
(NADPH)..The.products.from.this.light-dependent.pathway.are.then.used.
to.convert.carbon.dioxide.to.sugar,.for.the.plant.to.use.as.food.(Pessarakli.
2005)..This.carbon.ixation,.via.the.Calvin.cycle,.occurs.in.the.stroma.of.the.
chloroplast.
The. third. type. of. catalytic. copper. protein. is. the. superoxide. dismutase.
enzyme.which.requires.a.metal.cofactor.for.proper.function..There.are.three.
different.kinds.of.superoxide.dismutases,.manganese,.iron,.and.copper,.zinc.
(Linder.and.Goode.1991)..This.enzyme.is.responsible.for.reducing.oxidative.
stress.caused.by.superoxides.(O 2 − ).and.highly.reactive.hydroxyl.radials.(OH).
within.the.cell.by.mediating.the.following.reaction:

− − + +

O + O 2 H → H O + O

.

2 2 2 2 2

Hydrogen.peroxide.(H 2 O 2 ).is.then.converted.to.H 2 O.via.catalases.and.per-
oxidases. (Bowler. et. al.. 1992,. Kliebenstein. et. al.. 1998).. Manganese. and. iron.
superoxide.dismutases.(SOD).are.structurally.similar.with.the.metal.ion.not.
having.a.structural.role..Copper,.zinc.SOD.has.two.equal.subunits.with.each.
subunit. containing. both. one. copper. and. one. zinc. ion. (Bhattacharya. 2005)..
The.presence.of.types.of.SOD.varies.between.species,.but.many.plants.have.
a.mitochondrial.MnSOD,.a.cytosolic.CuZnSOD,.and.a.chloroplastic.FeSOD.
and/or.CuZnSOD.(Bowler.et.al..1992)..In.plants,.all.three.kinds.of.SOD.are.
nuclear.encoded.with.targeting.sequences.that.directed.them.to.their.subcel-
lular.locations.
The.last.major.group.of.copper.containing.proteins,.and.a.major.focus.of.
this.review,.functions.in.copper.transport.and.sequestration..This.group.of.
proteins.is.described.in.detail.in.the.following.section.