10.5. Vista: Other molecular motors[[Student version, January 17, 2003]] 395
Table 10.2:Examples of proteins that are believed to act as molecular motors. [From Vale, 1999.]10.5 Vista: Other molecular motors
New molecular machines are being discovered at a dizzying pace. Table 10.2 lists some of the
known examples from a recent review. Still other motors pack viral DNA into the virus head. Yet
another class of machines transport ions across membranes against their electrochemical gradient;
these “pumps” will play a key role in Chapter 11.
The big picture
Returning to the Focus Question, this chapter has uncovered two simple requirements for a molecu-
lar device to transduce chemical energy into useful mechanical work: The motor and track must be
asymmetrical, in order to select a direction of motion, and they must couple to a source of excess
free energy, for example a chemical reaction that is far from equilibrium. The following chapter
will introduce two other classes of molecular machines, ion pumps and the rotary ATP synthase.
Amechanochemical motor transduces chemical free energy to mechanical work. When the
relative concentrations of fuel and waste differ from equilibrium, that’s a form of order, analogous
to the temperature differential that ran our heat engine in Section 6.5.3. It may seem surprising
that the motors in this chapter can work inside a single, well-mixed chamber; in contrast, a heat
engine must sit at thejunctionbetween a hot reservoir and a cold one. But if there is an activation
barrier to the spontaneous conversion of fuel to waste, then even a well-mixed solution has an
invisible wall separating the two, like a dam on a river. It’s really not in equilibrium at all. The
motor is like a hydroelectric plant on that dam: It offers a low-barrier pathway to the state of lower
μ. Molecules will rush down that pathway, even if they are required to do some work along the
way,just as water rushes to drive the turbine of the hydroelectric plant.