The movements of single motor-protein molecules can be analyzed directly. Using

Question

The movements of single motor-protein molecules can be analyzed directly. Using polarized laser light, it is possible to create interference patterns that exert a centrally directed force, ranging from zero at the center to a few piconewtons at the periphery (about 200 nm from the center). Individual molecules that enter the interference pattern are rapidly pushed to the center, allowing them to be captured and moved at the experiment's discretion. Using such "optical tweezers, " single kinesin molecules can be positioned on a microtubule that is fixed to a cover-slip. Although a single kinesin molecule cannot be seen optically, it can be tagged with a silica bead and tracked indirectly by the following the bead (Figure Q16-2A). In the absence of ATP, the kinesin molecule remains at the center of the interference pattern, but with ATP it moves toward the plus end of the microtubule. As kinesin moves along the microtubule, it encounters the force of the interference pattern, which simulates the load kinesin carries during its actual function in the cell. Moreover, the pressure against the silica bead counters the effects of Brownian (thermal) motion, so that the position of the bead more accurately reflects the position of the kinesin molecule on the microtubule. Traces of the movements of a kinesin molecule along microtubule are shown in Figure Q16-2B. Movement of kinesin along a microtubule (Problem 16-8). (A) Experimental setup with kinesin linked to a silica bead, moving along a microtubule. (B) Position of kinesin (as visualized by position of silica bead) relative to center of interference pattern, as a function of time of movement along the microtubule. The jagged nature of the trace results from Brownian motion of the bead. A. As shown in Figure Q16-2B, all movement of kinesin is in one direction (toward the plus end of the microtubule). What supplies the free energy needed to ensure a unidirectional movement along the microtubule? B. What is the average rate of movement of kinesin along the microtubule? C. What is the length of each step that a kinesin takes as it moves along a microtubule? D. From other studies it is known that kinesin has two globular domains that each can bind to beta-tubulin, and that kinesin moves along a single protofilament in a microtubule. In each protofilament the beta-tubulin subunit repeats at 8-nm intervals. Given the step length and the interval between beta-tubulin subunits, how do you suppose a kinesin molecule moves along a microtubule? E. Is there anything in the data in Figure Q16-2B that tells you how many ATP molecules are hydrolyzed per step?

Explanation / Answer

Answer:

16. A) ATP hydrolysis supplies free energy needed to ensure unidirectional movement along with the microtubule. Kinesin is a motor protein, the microtubule is polar and the heads only bind to the microtubule in one orientation, while ATP binding gives each step its direction through a process known as neck linker zippering.
16. B) As per the graph:
although its a Brownian motion but approx Kinesin covers 8 nm /seconds distance.(in 2 secs 16 nm , 4 secs 32nm, 6 secs 48 nm and so on)
rate of movement=8 nm/sec

C) For each step kinesin takes less than a second time.
D) Conventional kinesin is a protein machine that steps along the surface of a microtubule as it carries a membrane-bound organelle toward the periphery of a cell. The size of the steps is 8 nm . This is the distance between consecutive binding sites along the microtubule protofilament, and a single kinesin molecule can take hundreds of steps without detaching, even against opposing loads as high as 6 pN . The steps are driven by the hydrolysis of ATP; kinesin is an ATPase whose speed of movement increases linearly with ATP concentration until it approaches a maximum of about 800 nm/s, and AMP-PNP, a nonhydrolyzable analog of ATP, arrests movement.

E) The processivity of ATP hydrolysis is ten molecules per site at low salt concentration but is reduced to one ATP per site at higher salt concentration.

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