Biomedical Engineering Reference
In-Depth Information
tracks..While.space.limitations.here.prevent.detailed.discussion,.we.briely.mention.three.
interesting. and. well-studied. motor. complexes.. ATP. synthase. is. responsible. for. the. syn-
thesis.of.energy-rich.ATP.that.is.needed.in.a.multitude.of.cellular.processes.(for.review,.
see.Boyer,.1997;.García-Trejo.and.Morales-Ríos,.2008)..The.600.kDa.multiprotein.complex.is.
docked.within.the.phospholipid.membrane.of.cells.and.acts.as.a.drive.shaft.(Davies.et.al.,.
2011)..To.drive.the.rotating,.axial.motion,.a.chemical.gradient.of.hydrogen.atoms.across.the.
complex.is.used.(for.review,.see.Yoshida.et.al.,.2001).
Another. complex. motor. is. the. ribosome. that. is. associated. with. the. perinuclear. rough.
endoplasmic. reticulum. in. cells,. which. functions. to. decode. mRNA. into. a. polypeptide.
protein. sequence. (Kurkcuoglu. et. al.,. 2010).. Large. and. small. ribonucleoprotein. subunits.
assemble.around.a.mRNA.molecule.to.generate.three.active.sites.termed.aminoacyl.(A-),.
peptidyl.(P-),.and.exit.(E-).that.relect.speciic.assembly.line.roles.in.the.translation.process.
(Valle.et.al.,.2003)..The.start.codon.of.a.protein-encoding.mRNA.sequence.contains.a.special.
cap.structure.that.is.recognized.by.the.smaller.ribosome.subunit.and.initiates.the.sequence.
of.events..A.guanosine.triphosphate.(GTP)-dependent.process.cranks.the.mRNA.through.
the. assembled. ribosome. complex. in. a. one-dimensional. fashion,. similar. to. a. mechanical.
ratchet.(Julián.et.al.,.2008;.Spirin,.2009)..The.ratchet.mechanism.safeguards.the.cell.from.
slip. errors. by. ensuring. that. the. mRNA. polymer. does. not. dissociate. or. slip. backward,.
generating. information. that. is. redundant. or. incorrect,. which. would. compromise. the.
protein.sequence.and.inluence.its.stability.and.function.
A. rotary. drive. system. that. provides. cellular. motility. among. certain. prokaryotes. and.
eukaryotes.is.the.lagella.(Vincensini.et.al.,.2011)..This.symmetrically.bundled.arrangement.
of. microtubules. extend. from. the. cellular. cortex. to. drive. rotation. in. both. clockwise. or.
counterclockwise.directions.with.the.help.of.a.number.of.associated.proteins.that.include.
dynein. motors. (Vogel,. 2005;. Heuser. et. al.,. 2009).. Rotation. in. the. clockwise. directions.
produces.an.erratic.“tumble”.style.of.locomotion,.while.rotation.in.the.opposite.direction.
winds.the.lagellar.structure.into.a.corkscrew-like.conformation.that.produces.a.smooth.
“run”.of.motion.(Minamino.et.al.,.2011).
An. appreciation. of. the. mechanistic. beauty. of. these. large. motor. assemblies. has.
evolved. through. numerous. studies. that. include. the. generation. of. crystal. structures,.
biochemistry,.genetics,.cell.biology,.and.mathematical.modeling..Continued.studies.will.
allow. their. future. use. or. the. implementation. of. key. design. elements. into. applications.
of.nanotechnology.(for.review,.see.Soong.et.al.,.2000;.Van.den.Heuvel.and.Dekker,.2007;.
Hess,.2011;.Sanchez.et.al.,.2011).
4.2.8 Chemical Motors
Devices. that. operate. on. submicron. levels. (whether. they. be. nanoscale. motors. or. the.
microscale.complexes.they.regulate).are.subject.to.and.manipulated.by.a.range.of.forces.
typically.different.from.those.experienced.in.our.macroscopic.world..In.the.nanoworld.of.
proteins,.inertia.wanes.in.comparison.with.the.random.kinetic.motion.of.molecules.and.
viscous. forces. that. dominate.. The. term. Brownian. motion. describes. the. seemingly. cha-
otic.random.movement.within.the.cellular.environment..Though.viewed.as.inconvenient.
for. motor. proteins. in. regard. to. maximum. eficiency,. chemical. motors. may. harness. this.
kinetic.movement.in.the.form.of.chemical.gradients..When.analyzing.directional.drift.of.a.
chemical.motor,.such.motion.must.be.evaluated.after.superimposition.onto.a.background.
governed.by.Brownian.motion..This.sets.limitations.on.how.small.a.chemical.motor.may.
be.(Shi.et.al.,.2009)..It.is.dificult.to.determine.directional.movement.if.the.motor.size.is.
much.smaller.than.a.fraction.of.a.micron..Self-propelled.chemical.motors.are.expected.to.
