Biomedical Engineering Reference
In-Depth Information
Issues.of.cargo.unloading.have.prompted.some.interesting.new.combinations.of.biologi-
cal.materials..An.example.is.the.attachment.of.complimentary.ssDNA.oligonucleotides.to.
microtubules.and.cargo.for.highly.selective.and.reversible.binding.in.loading.and.unload-
ing..The.strength.of.the.double-stranded.DNA.(dsDNA).bond.can.be.controlled.by.altering.
DNA.length.and.A-T.versus.G-C.content.of.complementary.strands..A.“zipper”.effect.can.
be.designed.in.to.allow.for.low.forces,.such.as.temperature.or.higher.bond.strength.of.a.
competing.oligonucleotide,.to.sever.the.combined.strands.at.a.desired.location.for.unload-
ing.(Hiyama.et.al.,.2009)..In.addition.to.cargo.loading.and.unloading,.DNA.can.be.used.to.
immobilize.microtubules.to.a.desired.surface.location..A.limitation.and.design.consider-
ation.to.these.creative.approaches.is.the.strength.of.kinesin.motors.that.can.easily.generate.
enough.force.to.shear.apart.shorter.DNA.strands.
Incorporating. effective. process. monitoring. into. nanodevice. design. is. also. important,.
and.new.strategies.are.constantly.being.engineered.and.quickly.becoming.available.com-
mercially..“Molecular.Beacons”.have.been.developed.that.are.ssDNA.stem-loop.structures.
labeled.with.luorophores.on.each.end.(Tyagi.and.Kramer,.1996;.Raab.and.Hancock,.2008)..
In.the.resting.state,.FRET.between.the.donor.and.acceptor.luorophores.quenches.luores-
cence..In.the.presence.of.a.complementary.ssDNA.oligonucleotide.that.binds.to.the.loop.
sequence,.the.donor.is.now.unquenched.and.luorescence.is.observed.. Another. promis-
ing.advance.for.many.applications.that.include.cargo.attachment.and.release,.regulation,.
and.monitoring.is.the.incorporation.of.aptamers.(Shi.et.al.,.2007;.Syed.and.Pervaiz,.2010)..
Aptamers.can.be.bound.to.dyes,.such.as.malachite.green,.to.create.conjugates.that.function.
similar.to.molecular.beacons.(Hirabayashi.et.al.,.2006).
4.4.7 Designer Motor Proteins
The. use. of. motor. proteins. in. nanoengineering. design. becomes. important. when. active.
transport. over. distances. is. required.. For. small. distances. and. small. particles,. passive.
transport.by.diffusion.is.preferable.for.speed.and.simplicity.(Hess.and.Vogel,.2001)..In.cells,.
the.viscous.gel-like.interior.saturated.with.proteins.results.in.sharply.decreased.diffusion.
rates.as.the.size.of.cargo.increases..The.need.to.transport.larger.particles.while.navigating.
an. increasingly. viscous. and. complex. cell. interior. likely. contributed. to. the. evolution. of.
motor.proteins.in.cells.(Luby-Phelps,.1994)..Kinesin.is.highly.evolved.to.handle.the.cellular.
environment.. It. undergoes. highly. processive. 8.nm. steps. against. an. opposing. force. of.
6.pN.and.is.able.to.drag.cargo.through.this.gel.with.a.mesh.size.of.roughly.50.nm..The.
typical.chemical.eficiency.of.the.kinesin.motor.is.50%,.as.measured.by.the.ratio.between.
performed.work.and.free.energy.of.ATP.(Kawaguchi.and.Ishiwata,.2000)..In.the.task.of.
inding.cargo,.the.stalk.domain.of.kinesin.is.∼50.nm.long.(Song.and.Mandekow,.1994).and.
capable. of. high. rotational. lexibility. that. is. ideally. suited. for. capturing. cargo. when. not.
precisely. oriented. for. transport.. Speciicity. for. cargo. occurs. through. associated. adaptor.
proteins.. In. nanoengineering. applications,. truncated. forms. of. kinesin. or. Klps. may. be.
warranted.but.requires.that.speciic.parameters.of.each.motor.and.the.role.of.each.domain.
be. carefully. considered.. Table. 4.2. provides. a. comparison. of. typical. motor. parameters..
For.in.vitro.applications.with.motors,.the.tail.and.stalk.domains.are.frequently.truncated.
or. altered. or. functionalized. to. create. a. “designer. motor. protein.”. Chimeric. Klps. can. be.
generated.by.transferring.desired.functional.domains.from.one.motor.protein.to.another.
(Simeonov. et. al.,. 2009).. As. optimally. designed. motor. proteins. are. generated,. additional.
features.such.as.protein.stability.or.instability.can.be.engineered.in,.relying.on.insights.
from.extremophile.organisms.
