Biomedical Engineering Reference
In-Depth Information
sequence-dependent.structural.transitions.with.a.resolution.up.to.10.bp.when.integrating.
optical.tweezers.and.interferometry.(Bockelmann.et.al..2002)..A.similar.approach.was.also.
used.to.study.the.folding/unfolding.of.short.RNA.hairpins.(Liphardt.et.al..2001)..These.
single.molecule.experiments.are.expected.to.beneit.our.understanding.on.the.replication,.
transcription,.and.translation.processes.of.polynucleotide.acids.
19.4.1.2 Mechanics of Other Biomolecules
Besides. DNA. and. RNA,. optical. tweezers. are. also. applied. to. study. the. mechanics. of.
other.biopolymers.including.various.microtubules,.proteins,.and.molecular.motors..The.
stretching-relaxation.mechanics.of.these.biomolecules.help.in.discovering.their.highly.
ordered. structures. and. assembly. mechanisms.. For. example,. the. folding. and. unfold-
ing. dynamics. of. the. giant. multiglobular. protein. titin. (a. sarcomeric. protein). has. been.
studied. with. optical. tweezers. (Kellermayer. et. al.. 1997;. Tskhovrebova. et. al.. 1997).. As.
shown.in.Figure.19.7c,a.titin.protein.was.stretched.irst.with.a.large.force.(>100.pN).to.a.
certain.extension.and.that.deformation.was.maintained..The.molecular.expansion.of.the.
protein.from.the.unfolding.of.the.multiple.domains.of.the.protein.results.in.a.gradual.
decrease. of. the. sensing. force.. In. this. way,. scientists. discovered. several. intermediate.
states.and.the.real-time.kinetics.of.the.folding/unfolding.process.of.titin..With.the.help.
of.DNA.molecules.to.link.RNase.H.with.the.155-residue.single.domain.and.two.optically.
trapped. microparticles,. scientists. found. jumps. between. unfolded. and. intermediated.
states.(Cecconi.et.al..2005)..These.biomechanics.studies.opened.the.windows.to.reveal.
the.assembly.or.fatigue.dynamics.of.other.important.proteins.inside.cells.
19.4.1.3 Mechanics of Molecular Motors
Optical.tweezers.are.also.widely.used.to.study.the.intricate.mechanics.of.molecular.motors.
and. packaging. in. viruses.. Molecular. motors. are. single. molecules. or. macromolecule.
complexes.that.can.move.along.nucleic.acids.or.cytoskeleton.ibers..By.coating.on.dielectric.
beads. (e.g.,. silica. particles),. molecular. motor. molecules. can. be. concisely. positioned. on.
their. polymer. track. by. optical. tweezers.. By. tracking. the. positions. of. silica. particles,. the.
movement.of.the.coated.molecular.motors.is.conveniently.monitored..For.a.common.linear.
motor,. kinesin,. scientists. found. that. these. motor. molecules. could. go. through. multiple.
reaction. cycles. before. leaving. their. polymer. tracks.. Their. average. on-track. traveling.
distance.is.∼1.4.μm,.while.they.might.briely.detach.midway.with.an.average.discrete.step.
of. 8.nm,. about. the. length. of. a. tubulin. dimer. (Figure. 19.7d).. For. other. linear. molecular.
motors.from.myosin.families.or.dynein.families,.their.behaviors.vary.with.their.different.
transport. mechanisms. and. on-track. steps. (Mallik. et. al.. 2004).. For. nucleic. acid-binding.
molecular.motors,.such.as.RNA.polymerase.(RNAP),.the.average.discrete.step.is.about.3.7.
Å.(Abbondanzier.et.al..2005)..These.RNAP.molecular.motors.move.along.the.DNA.template.
at.a.speed.of.a.few.nucleotides.per.second..There.are.many.pauses.during.their.journey,.
distributed.unevenly.along.the.DNA.template..These.pauses.are.believed.to.depend.on.the.
speciic.nucleotide.sequence.and.control.the.actual.polymerization.rate.of.mRNA.
By.imposing.an.opposite.force,.the.maximum.resistive.load.for.molecular.motors.can.also.
be. determined.. These. measurements. typically. require. very. sensitive. detection. systems.
and.complicated.statistical.analysis.to.get.rid.of.thermal.noise.and.luctuations..The.force.
needed.to.stop.molecular.motors.often.depends.on.its.applied.direction..For.example,.the.
stall. force. for. kinesin. is. about. 5-7.pN. when. adding. backward,. while. the. same. force. if.
adding. from. the. side. or. forward. hardly. affects. its. movement.. The. maximum. resistance.
