Biological Physics: Energy, Information, Life

10.4. Kinetics of real enzymes and machines[[Student version, January 17, 2003]] 385

time, s

0.0 0.5 1.0 1.5 2.0

bead

microtubule kinesin

+

∆x

bead position,

nm

-100

0

100

laser spot

Figure 10.22:(Experimental data, with schematic.) Sample data from a kinesin motility assay.Inset: An optical
tweezers apparatus pulls a 0.5μmbead against the direction of kinesin stepping (not drawn to scale). A feedback
circuit continuously moves the trap (gray hourglass shape) in response to the kinesin’s stepping, maintaining a fixed
displacement ∆xfrom the center of the trap and hence a fixed backward load force (“force clamping”). Graph:
Stepping motion of the bead under a load force of 6. 5 pN,with 2mMATP.The gray lines are separated by intervals
of 7.95nm;each corresponds to a plateau in the data. [After Visscher et al., 1999.]

0

0.05

0.1

0.15

0.2

0 0.2 0.4 0.6 0.8 1

1/

, v

s nm

-1

b

10

100

1000

110100 1000

velocity,

nm s

-1

a

1.05 pN

3.59 pN

5.63 pN

ATP concentration c, μM 1/c, μM-1

Figure 10.23:(Experimental data.) (a)Log-log plot of the speedvof kinesin stepping versus ATP concentration,
at various loads (see legend). For each value of load, the data were fit to the Michaelis–Menten rule, yielding the
solid curves with the parameter values listed in Table 10.1. (b)Lineweaver–Burk plot of the same data. [Data from
Visscher et al., 1999.]