Motor Proteins as Nanomachines: The Roles of Thermal Fluctuations in Generating Force and Motion

Abstract

Motor proteins are enzymes that convert chemical energy derived from the hydrolysis of a small molecule called ATP into mechanical work used to power directed movement along cytoskeletal filaments inside cells. Motor proteins have essential biological functions such as driving the contraction of muscle, the beating of sperm and cilia, and the transport of intracellular cargoes. Motor proteins are also interesting from a physical point of view because they do what no man-made engines do: they transduce chemical energy directly to mechanical work without using heat or electrical energy as an intermediate. A central issue in the mechanism of this chemomechanical transduction by motor proteins concerns the roles played by thermal fluctuations, diffusion and Brownian motion. In this lecture I discuss several molecular models for motor proteins, including so-called ratchet models, and compare predictions of these models to experimental results for the microtubule-based motor protein kinesin. I argue that kinesin, which has two motor domains or “heads,” walks using a “hand-over-hand” mechanism such that at least one head is bound to the microtubule. Diffusion likely plays an essential role by facilitating the search of the unbound head for the next binding site, a distance 8 nm away. During this diffusive phase, the bound head supports the load ensuring that forward motion can still take place even against loads up to several piconewtons.