  The precise details of how myosin-V coordinates the biochemical reactions and
mechanical motions of its two head elements to engineer effective processive
molecular motion along actin filaments remain unresolved. We compare a
quantitative kinetic model of the myosin-V walk, consisting of five basic
states augmented by two further states to allow for futile hydrolysis and
detachments, with experimental results for run lengths, velocities, and dwell
times and their dependence on bulk nucleotide concentrations and external loads
in both directions. The model reveals how myosin-V can use the internal strain
in the molecule to synchronise the motion of the head elements. Estimates for
the rate constants in the reaction cycle and the internal strain energy are
obtained by a computational comparison scheme involving an extensive
exploration of the large parameter space. This scheme exploits the fact that we
have obtained analytic results for our reaction network, e.g. for the velocity
but also the run length, diffusion constant and fraction of backward steps. The
agreement with experiment is often reasonable but some open problems are
highlighted, in particular the inability of such a general model to reproduce
the reported dependence of run length on ADP. The novel way that our approach
explores parameter space means that any confirmed discrepancies should give new
insights into the reaction network model.
