The duty ratio for a motor protein is defined as the fraction of time in each cycle of activity of the motor in which the head is bound with high affinity to its cytoskeletal track. Which of the following is reasonable given this definition?
A) Myosin V has a higher duty ratio than myosin II, because Pᵢ release is the rate-limiting step in its cycle.
B) Myosin II has a higher duty ratio than myosin V, because Pᵢ release is the rate-limiting step in its cycle.
C) Myosin V has a higher duty ratio than myosin II, because ADP release is the rate-limiting step in its cycle.
D) Myosin II has a higher duty ratio than myosin V, because ADP release is the rate-limiting step in its cycle.

Question

Quizplus · Accepted Answer

Myosin V is a processive motor and, not surprisingly, its duty ratio is significantly higher than that of myosin II. This is consistent with ADP release or ATP binding being slow (and rate-limiting) in the myosin V cycle, making the motor stay longer in the attached post-stroke state. In myosin II, inorganic phosphate (Pᵢ) release is rate-limiting instead, keeping the motor in the released or cocked state for a longer fraction of the cycle.

The Duty Ratio for a Motor Protein Is Defined as the Fraction