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enzyme induces
mechanical stressenzyme holds
substrates in
alignmentenzyme induces
charged regions
on substratea
b
c
Figure 10.27: (Schematic.) Three mechanisms for an enzyme to assist a reaction. (a)The enzyme may exert
mechanical forces on the substrate. (b)The enzyme may change the substrate’s reactivity by altering its ionic
environment. (c)The enzyme may hold two substrate molecules in the precise orientation needed for a joining bond
to form, reducing the entropic part of the free-energy barrier to the desired reaction. All of these induced deviations
from the substrate’s normal distribution of states can be considered as forms of strain, pushing the substrate closer
to its transition state.
occurring at extremely low concentrations of substrate. In this case there will be large random
variations in the arrival times of substrate molecules at E. We interpret these variations as the
times to hop over the first bump in Figure 10.17b. Since this bump is large whencSis low, this
contribution to the randomness of the process can be as important as the usual one (hopping over
the middle bump of the figure). See Svoboda et al., 1994 for a discussion of this effect in the context
of kinesin.
10.4.3′
- The assumptions outlined in Section 10.4.3 were chosen to discourage any shortcuts across the
reaction diagram. For example, after state EP the trailing head Kacould in principle release its
ADP, remain bound to the microtubule, then bind and split another ATP—afutile hydrolysis,as
there would be no forward motion. The strain from binding the forward head makes this outcome
less likely than the alternative shown (the head retains ADP but lets go of the microtubule), and
so helps make the motor tightly coupled. Interestingly, a large, externally applied force in the
backward direction could cancel the effect of strain, leading toaabreakdown of tight coupling at
athreshold load force. The motor would then “slip,” as imagined in Figure 10.9 (see Idea 10.1 on
page 362). Schnitzer and coauthors measured and analyzed the force at which the motor stalls, and
argued that stalling reflects slipping (or futile hydrolysis), not thermal equilibrium.
It’s also possible for the trailing kinesin head to hydrolyze ATP and release Piprior to step ES 2 ,
allowing the entire kinesin dimer to detach from the microtubule and reducing its processivity. The
transition to ES 2 (binding of the forward head) is normally rapid enough to make this process rare.