Biological Physics: Energy, Information, Life

394 Chapter 10. Enzymes and molecular machines[[Student version, January 17, 2003]]

0

5

10

15

20

25

0 0.5 1 1.5 2

b

number

displacement, μm

2-headed

0.5 s kinesin

1 s

2 s

0

8

16

-0.5 0 0.5 1

number

displacement, μm

a 1-headed

kinesin

0.5 s

1 s

2 s

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4

0 12345

squared displacement,

μm

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time, s

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2-headed

1-headed

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0.1

0.2

0.3

0.4

012345

d

diffusive term,

μm

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2-headed

1-headed

time, s

Figure 10.26: (Experimental data.) Analysis of the movement of single kinesin molecules. (a)Data for C351, a
single-headed form of kinesin. The graphs give the observed distributions of displacementxfrom the original binding
site, at three different times. The solid curves show the best Gaussian fit to each dataset. Notice that even at 2sa
significant fraction of all the kinesins has made netbackwardprogress. (b)The same data as (a), but for conventional
two-headed kinesin. None of the observed molecules made net backward progress. (c)Mean-square displacement,
〈x(t)^2 〉,asafunction of time for single-headed (open circles) and two-headed (solid dots) kinesin. The solid curves
show the best fits to the predicted random-walk law (see text). (d)The same data and fits as (c), after subtracting
the (vt)^2 term (see text). [Data from Okada & Hirokawa, 1999.]

eukaryotic cells, the “GTP-binding proteins” (orG-proteins); they play a number of intracellular
signalling roles, including a key step in the detection of light in your retina. It seems reasonable to
suppose that the first, primitive motors were G-proteins whose binding targets were polymerizing
proteins, like tubulin. Interestingly, G-proteins have indeed turned out to have close structural
links to both kinesin and myosin, perhaps reflecting a common evolutionary ancestry.
T 2 Section 10.4.4′on page 402 gives some quantitative analysis of the model and compares it to
the experimental data.