| Knowledge of thermodynamic parameters enables an understanding of how chemical energy gets converted into mechanical work. Myosins are molecular motors that use the chemical energy of ATP hydrolysis to perform mechanical work on actin filaments. The affinity of myosin for actin varies depending upon the state of the nucleotide bound; ADP bound myosin has a strong actin affinity while ATP and ADP.Pi bound myosin exhibit weak actin binding properties. Myosins that are bound to actin for most of their overall ATPase cycle are high duty ratio motors. High duty ratios are necessary for processive motility, which is the ability of myosin to undergo multiple ATPase cycles per encounter with an actin filament. Vesicle transport over long distances is only possible with such processive myosins. Processive myosins are high duty ratio motors because their ATPase cycling rate is limited by one step in the ATPase cycle, the release of ADP, when the myosin is strongly bound to actin. However, the thermodynamics that account for the differences in ADP affinities between myosin classes are unknown. The energetics of ADP binding to two processive myosins are described in this thesis to understand the thermodynamic basis (DeltaG°, DeltaH°, DeltaS°, DeltaCp°) of the duty ratio. The data obtained provide insights into the physical adaptations responsible for the processive motility exhibited by some myosins. |
