Biomedical Engineering Reference
In-Depth Information
The operation timescale of molecular machines [point (5)] can range from micro-
seconds to seconds, depending on the type of rearrangement and the nature of the
components involved.
Finally, as far as point (6) is concerned, the functions that can be performed by
exploiting the movements of the component parts in molecular machines are various
and, to a large extent, still unpredictable. It is worth to note that the mechanical move-
ments taking place in molecular-level machines, and the related changes in the spec-
troscopic and electrochemical properties, usually obey binary logic and can thus be
taken as a basis for information processing at the molecular level. Artifi cial molecular
machines capable of performing logic operations have been reported (Balzani
2003
) .
2
Natural Systems
In the last few years, much progress has been made in elucidation of the moving
mechanisms of motor biomolecules, owing to the fact that - in addition to the estab-
lished physiological and biochemical methods - novel in vitro techniques have been
developed which combine optical and mechanical methods to observe the behavior
of a single protein.
The most important and best-known natural molecular motors are adenosine
triphosphate (ATP) synthases, myosin, and kinesin (
2003
; Schliwa and Woehlke
2003
). An ATP synthase is the ubiquitous enzyme that manufactures ATP and is a
rotary motor (Boyer
1993
; Walker
1998
). The enzymes of the myosin and the kine-
sin families are linear motors that move along polymer substrates (actin fi laments
for myosin and microtubules for kinesin), converting the energy of ATP hydrolysis
into mechanical work (Frey
2002
; Vale and Milligan
2000
). Motion derives from a
mechanochemical cycle, during which the motor protein binds to successive sites
along the substrate in such a way as to move forward on average.
Several other biological processes are based on motions, including protein fold-
ing/unfolding. Another example is RNA polymerase, which moves along DNA
while carrying out transcription, thus acting as a molecular motor. A short and sim-
plifi ed description of ATP synthase, myosin, and kinesin is reported below.
2.1
ATP Synthase
This enzyme consists of two rotary molecular motors attached to a common shaft,
each attempting to rotate in the opposite direction. The F
1
motor uses the free energy
of ATP hydrolysis to rotate in one direction while the F
0
motor uses the energy
stored in a transmembrane electrochemical gradient to turn in the opposite direction.
Which motor “wins” (i.e., develops more torque) depends on cellular conditions.
When F
0
takes over, which is the normal situation, it drives the F
1
motor in reverse
whereupon it synthesizes ATP from its constituents, adenosine diphosphate (ADP)
