  The performance of a molecular motor, characterized by its power output and
energy efficiency, is investigated in the motor design space spanned by the
stepping rate function and the motor-track interaction potential. Analytic
results and simulations show that a gating mechanism that restricts forward
stepping in a narrow window in configuration space is needed for generating
high power at physiologically relevant loads. By deriving general
thermodynamics laws for nonequilibrium motors, we find that the maximum torque
(force) at stall is less than its theoretical limit for any realistic
motor-track interactions due to speed fluctuations. Our study reveals a
tradeoff for the motor- track interaction: while a strong interaction generates
a high power output for forward steps, it also leads to a higher probability of
wasteful spontaneous back steps. Our analysis and simulations show that this
tradeoff sets a fundamental limit to the maximum motor efficiency in the
presence of spontaneous back steps, i.e., loose-coupling. Balancing this
tradeoff leads to an optimal design of the motor-track interaction for
achieving a maximum efficiency close to 1 for realistic motors that are not
perfectly coupled with the energy source.Comparison with existing data and
suggestions for future experiments are discussed.
