16-8 The movements of single motor-protein molecules can be analyzed directly. U

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16-8 The movements of single motor-protein molecules can be analyzed directly. Using polarized laser light, it is pos sible 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 cen ter). Individual molecules that enter the interference pat tern are rapidly pushed to the center, allowing them to be captured and moved at the experimenter's discretion. Using such "optical tweezers single kinesin molecules s can be positioned on a microtubule that is fixed to a cover slip. Although a single kinesin molecule cannot be seen n optically, it can be tagged with a silica bead and tracked indi- rectly by 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 micro tubule, 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 a microtubule are in Figure Q16-2B

Explanation / Answer

Ans a) It is seen that movement of the Kinesin is in one direction and move towards the end of the microtubule. The position and movement of the Kinesin molecule was tagged with silica bead as it helped in tracking the movement. It was seen that Kinesin molecule remains at center of the interference pattern but only with help of ATP, it moves forward and towards the plus end of the microtubule.

Ans b) It is evident that movement of Kinesin along microtubule is coupled with ATP hydrolysis. The average rate of movement or step size is around 80 Å. It moves along the microtubule at speed of 6400 Å per second.

Ans c) The length of each step is 8nm for Kinesin for each ATP it hydrolyses as it moves along the microtubule.

Ans d) Kinesin has two globular sub-units that can bind to -Tubulin. A single kinesin molecule will typically take 100 or more steps toward the plus end of a microtubule in a period of seconds before the molecule becomes detached from the microtubule. These measurements also revealed that the average step size is approximately 80 Å, a value that corresponds to the distance between consecutive - or -tubulin subunits along each protofilament. The addition of ATP increases the affinity of kinesin for microtubules and with its hydrolysis, it tends to move along the microtubule.

Ans e) There are 10 ATP molecules hydrolysed as per the diagram as the distance moved is 80nm. Per 8nm one ATP is hydrolysed which makes it 10 ATP per 80 nm of the microtubule.

Ans a) It is seen that movement of the Kinesin is in one direction and move towards the end of the microtubule. The position and movement of the Kinesin molecule was tagged with silica bead as it helped in tracking the movement. It was seen that Kinesin molecule remains at center of the interference pattern but only with help of ATP, it moves forward and towards the plus end of the microtubule.

Ans b) It is evident that movement of Kinesin along microtubule is coupled with ATP hydrolysis. The average rate of movement or step size is around 80 Å. It moves along the microtubule at speed of 6400 Å per second.

Ans c) The length of each step is 8nm for Kinesin for each ATP it hydrolyses as it moves along the microtubule.

Ans d) Kinesin has two globular sub-units that can bind to -Tubulin. A single kinesin molecule will typically take 100 or more steps toward the plus end of a microtubule in a period of seconds before the molecule becomes detached from the microtubule. These measurements also revealed that the average step size is approximately 80 Å, a value that corresponds to the distance between consecutive - or -tubulin subunits along each protofilament. The addition of ATP increases the affinity of kinesin for microtubules and with its hydrolysis, it tends to move along the microtubule.

Ans e) There are 10 ATP molecules hydrolysed as per the diagram as the distance moved is 80nm. Per 8nm one ATP is hydrolysed which makes it 10 ATP per 80 nm of the microtubule.

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