Biomolecular Machines - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 174

using ATP hydrolysis as fuel, and rotary motors. Examples of rotary motors are flag-
ella, which are complex structures that contain approximately 20 parts and propel
bacteria fueled by an ion flux ( Berg 2003 ), and the F 1 F 0 -type ATP synthase enzyme,
which controls energy interconversion in cells and consists of two interconnected
rotary motors ( Capaldi and Aggeler 2002 ). Linear motors include myosins, which
glide on actin filaments, and kinesins and dyneins, which move on microtubules
( Schliwa and Woehlke 2003 ). Myosin is involved in muscle contraction. To provide
the large forces needed to contract skeletal muscles, for example, myosin motors
and actin filaments work collectively. Kinesin motors transport cargoes in the
form of vesicles or organelles within a biological cell along microtubule filaments,
toward their plus ends, in 8-nm steps resulting from an asymmetric hand-over-
hand mechanism that involves an alternation between two conformational states
( Yildiz and Selvin 2005 ). The mechanical energy necessary for these enzymatic
biomotors, each generating forces of 5-6 pN, is provided by conversion of the
chemical energy derived from adenosine triphosphate. The interest for kinesin
motors in nanotechnology derives from their ability to work outside biological
cells, in solutions with controlled temperature and pH. Microtubules, which are
cylindrical hollow protein filaments with an outer diameter of 24 nm and a length of
a few micrometers, can also self-assembly in vitro from tubulin heterodimers.
Artificial molecular motors constitute an emerging research area. In many cases,
artificial motors do not involve biological molecules [see Kay et al. ( 2007 )fora
review], but the focus of this chapter is on synthetic biomolecular motors. These
motors should benefit from a detailed theoretical study of the natural biological
motors. A common denominator of the latter is that they are small, out-of-
equilibrium, and stochastic systems. Several models have been devised to study
the operation modes and thermodynamics of biomolecular motors, with particular
emphasis on kinesin. Among these, we mention the minimal ratchet model for
fluctuating systems ( Lacoste et al. 2008 ), a model that uses the general network
representation for the motor and includes the energetics of ATP hydrolysis and
the separation of time scales of mechanical and chemical processes ( Liepelt and
Lipowsky 2009 ), and a model based on the overdamped Langevin equation that
incorporates experimentally accessible parameters such as ATP concentration, the
external load, or the step of the motor ( Ciudad et al. 2005 ). The collective dynamics
of kinesin motors has been detailed in Hendricks et al. ( 2009 ). Rotary biomotors
have also received attention, being modeled as nanoelectromechanical systems that
convert the transmembrane electrochemical proton gradient into mechanical energy
( Smirnov et al. 2008 ).

6.1 Biological Actuators and Switches

The biological machines included in this section are based on conformation changes.
An example of a photon-driven nanomotor consisting of a single molecule that
contains a hairpin DNA structure, which incorporates azobenzene moiety, has been