The Work Performed By Biological Motors

Molecular motors are enzymes that perform work (F {middle dot} x) when they move along a track a distance x against a constant force F. This work is performed through intermediate chemical steps in a motors ATPase reaction cycle, each step having a free energy change associated with it that is a sum of chemical, {Delta}{micro}chem, and mechanical, {Delta}{micro}ext, potentials. Defining {Delta}{micro}ext is fundamental to our understanding of how molecular motors work, yet after decades of study the definition of {Delta}{micro}ext remains disputed. Some postulate that {Delta}{micro}ext is a function of both F and x, while others assume that {Delta}{micro}ext is a function of neither F nor x, and still others argue that {Delta}{micro}ext is a function of F but not x. Here we evaluate these models and conclude that only the latter - a mechanochemical model proposed by A.V. Hill in the 1930s - describes molecular motor mechanochemistry.