INDIRECT COUPLING OF PHOSPHATE RELEASE TO DE-NOVO TENSION GENERATION DURING MUSCLE-CONTRACTION

A key question in muscle contraction is how tension generation is coup led to the chemistry of the actomyosin ATPase. Biochemical and mechano chemical experiments link tension generation to a change in structure associated with phosphate release, Length-jump and temperature-jump ex periments, on the other hand, implicate phase 2(slow), a significantly faster, markedly strain-sensitive kinetic process in tension generati on, We use a laser temperature jump to probe the kinetics and mechanis m of tension generation in skinned rabbit psoas fibers-an appropriate method since both phosphate release and phase 2(slow) are readily pert urbed by temperature. Kinetics characteristic of the structural change associated with phosphate release are observed only when phosphate is added to fibers, When present, it causes a reduction in fiber tension ; otherwise, no force is generated when it is perturbed, We therefore exclude this step from tension generation, The kinetics of de novo ten sion generation by the temperature-jump equivalent of phase 2(slow) ap pear unaffected by phosphate binding. We therefore propose that phosph ate release is indirectly coupled to de novo tension generation via a steady-state flux through an irreversible step, We conclude that tensi on generation occurs in the absence of chemical change as the result o f an entropy-driven transition between strongly bound crossbridges in the actomyosin-ADP state, The mechanism resembles the operation of a c lock, with phosphate release providing the energy to tension the sprin g, and the irreversible step functions as the escapement mechanism, wh ich is followed in turn by tension generation as the movement of the h ands.