384 Chapter 10. Enzymes and molecular machines[[Student version, January 17, 2003]]
Figure 10.21:(Schematic.) One plausible model for directed motion of two-headed kinesin. Each of the steps in
this cyclic reaction is described in the text starting on page 388. Several elements of this mechanism are still under
debate, including the hand-over-hand character of the cycle. The eight steps form a loop, to be read clockwise from
upper center. The rows of units labeledαβαβ.. .represent a microtubule, with its “+” end at the right. Strong
physical bonds are denoted by multiple lines, weak ones by single lines. The symbols K, T, D, and P denote kinesin,
ATP,ADP, and inorganic phosphate, respectively. The rapid isomerization step, ES 1 ES′ 1 ,isassumed to be
nearly in equilibrium. The states denoted ES 3 and EP are under internal strain, as described in the text. After one
complete loop the kinesin dimer has moved one step, or 8nm,tothe right, and the two kinesin heads have exchanged
roles. [Similar schemes, with some variations, appear in Gilbert et al., 1998; Rice et al., 1999; Schnitzer et al., 2000;
Vale & Milligan, 2000; Schief & Howard, 2001; Mogilner et al., 2001; Uemura et al., 2002.]
Later work showed that in fact two-headed kinesin is tightly coupled: It takes exactly one spatial
step for each ATP molecule it consumes, even under moderate load. From the discussion at the
start of this subsection, then, we may expect that two-headed kinesin would obey MM kinetics, with
load-dependent parameters. Several experimental groups confirmed this prediction (Figure 10.23).
Specifically, Table 10.1 shows thatvmaxdecreases with increasing load, whileKMincreases.
Your Turn 10e
The load forces tabulated in Table 10.1 reflect the force of the optical trap on the bead. But the
bead experiences another retarding force, namely that of viscous friction. Shouldn’t this force be
included when we analyze the experiments?Let us see what these results tell us about the details of the mechanism of force generation by