354 Chapter 10. Enzymes and molecular machines[[Student version, January 17, 2003]]
given as much substrate as it can handle. In the case of catalase, the numbers given in the previous
paragraph reflect the saturated case, so the maximum turnover number is the quantity you found
in Your Turn 10a.
Catalase is a speed champion among enzymes. A more typical example is fumarase, which
hydrolyzes fumarate tol-malate,^2 with maximum turnover numbers somewhat above 1000s−^1.
This is still an impressive figure, however: It means that a liter of 1mMfumarase solution can
process up to about a mole of fumarase per second, many orders of magnitude faster than a similar
reaction catalyzed by an acid.
10.1.3 All eukaryotic cells contain cyclic motors
Section 6.5.3 made a key observation, that the efficiency of a free energy transduction process is
greatest when the process involves small, controlled steps. Though we made this observation in
the context of heat engines, still it should seem reasonable in the chemically driven case as well,
leading us to expect that Nature should choose to build even its most powerful motors out of many
subunits, each made as small as possible. Indeed, early research on muscles discovered a hierarchy
of structures on shorter and shorter length scales (Figure 10.1). As each level of structure was
discovered, first by optical and then by electron microscopy, each proved not to be the ultimate force
generator, but rather a collection of smaller force-generating structures, right down to the molecular
level. At the molecular scale, we find the origin of force residing in two proteins:myosin(golf-club
shaped objects in Figure 10.1) and actin (spherical blobs in Figure 10.1). Actin self-assembles from
its globular form (G-actin)intothin filaments (F-actin,the twisted chain of blobs in the figure),
forming a track to which myosin molecules attach.
The direct proof that single actin and myosin molecules were capable of generating force came
from a remarkable set of experiments, called single-moleculemotility assays. Figure 10.2 sum-
marizes one such experiment. A bead attached to a glass slide carries a small number of myosin
molecules. A single actin filament attached at its ends to other beads is maneuvered into position
overthe stationary myosin using optical tweezers. The density of myosin on the bead is low enough
to ensure that at most one myosin engages the filament at a time. When the fuel molecule ATP is
added to the system, the actin filament is observed to take discrete steps in one definite direction
awayfrom the equilibrium position set by the optical traps; without ATP, no such stepping is seen.
This directed, non-random motion occurs without any external macroscopic applied force (unlike,
say, electrophoresis).
Muscles are obvious places to look for molecular motors, because they generate macroscopic
forces. Other motors are needed as well, however. In conrast to muscle myosin, many other
motors do not work in huge teams but rather alone, generating tiny, piconewton-scale forces. For
example, Section 5.3.1 described how locomotion inE. colirequires a rotary motor joining the
flagellum to the body of the bacterium; Figure 5.9 on page 157 shows this motor as an assembly of
macromolecules just a few tens of nanometers across. In a more indirect argument, Section 4.4.1
argued that passive diffusion alone could not transport proteins and other products synthesized
at one place in a cell to the distant places where they are needed; instead some sort of “trucks
and highways” are needed to transport these products actively. Frequently the “trucks” consist of
bilayer vesicles. The “highways” are visible in electron microscopy as long protein polymers called
(^2) Fumarase plays a part in the citric acid cycle (Chapter 11), splitting a water molecule and attaching the fragments
to fumarate, converting it to malate.