356 Chapter 10. Enzymes and molecular machines[[Student version, January 17, 2003]]
02460 05 1.0 15 2.0
time, slaser beammoving
beadglass slidemyosinactin filament
force,pNab
fixed beadFigure 10.2: (Schematic; experimental data.) Force production by a single myosin molecule. (a)Beads are
attached to the ends of an actin filament. Optical tweezers are used to manipulate the filament into position above
another, fixed bead coated with myosin fragments. Forces generated by a myosin fragment pull the filament to the
side, displacing the beads. The optical trap generates a known spring-like force opposing this displacement, so that
the observed movement of the filament gives a measure of the force generated by the motor. (b)Force generation
observed in the presence of 1μMATP,showing how the motor takes a step, then detaches from the filament. [After
Finer et al., 1994.]
ure 2.21 on page 49): Often just one kinesin molecule carries an entire transport vesicle toward
its destination. Many other organized intracellular motions, for example the separation of chro-
mosomes during cell division, also imply the existence of motors to overcome viscous resistance to
such directed motion. These motors too have been found to be in the kinesin family.^3
Foramore elaborate example of a molecular machine, recall that each cell’s genetic script is
arranged linearly along a long polymer, the DNA. The cell must copy (or replicate) the script (for
cell division) as well as transcribe it (for protein synthesis). Your experience with cassette tapes,
videotapes, and other linear storage media should make it clear that an efficient way to perform
these operations is to have a single readout machine through which the script is physically pulled.
The pulling of a copy requires energy, just as a motor is needed to pull the tape across the read
heads of a tape player. The corresponding machines are known as DNA or RNA polymerases for
the cases of replication or transcription respectively (see Section 2.3.4 on page 57). Section 5.3.5
has already noted that some of the chemical energy used by a DNA polymerase must be spent
opposing rotational friction of the original DNA and the copy.
10.1.4 One-shot motors assist in cell locomotion and spatial organization
Myosin, kinesin, and polymerases are all examples of cyclic motors; they can take an unlimited
number of steps without any change to their own structure, as long as “fuel” molecules are available
in sufficient quantities. Other directed, nonrandom motions in cells do not need this property, and
for them, simpler one-shot motors can suffice.
Translocation Some products synthesized inside cells must not only be transported some dis-
tance inside the cell, but must actually pass across a bilayer membrane to get to their destination.
Forexample, mitochondria import certain proteins that are synthesized in the surrounding cell’s
(^3) Wenow know that both “kinesin” and “myosin” are really large families of related molecules; human cells express
about 40 varieties of each. For brevity we will use these terms to denote the best-studied members in each family:
muscle myosin and “conventional” kinesin.