Chapter 35: Molecular Motors

A significant portion of the discussion focuses on the actomyosin system, detailing how myosin heads undergo conformational changes—specifically a lever-arm rotation powered by phosphate release—to traverse polar actin filaments. This mechanism is contextualized within the sliding-filament model of muscle contraction, where the organized arrangement of sarcomeres, thick filaments, and thin filaments facilitates force generation regulated by calcium-dependent tropomyosin and troponin interactions. The chapter further explores the cytoskeleton by examining microtubules, hollow cylinders composed of alpha-tubulin and beta-tubulin that display dynamic instability. Here, the processive, hand-over-hand transport mechanism of kinesin is contrasted with the non-processive cycle of myosin. Finally, the text shifts to prokaryotic motility, explaining how bacterial flagella function as rotary propellers driven by a transmembrane proton gradient rather than direct ATP hydrolysis. This section elucidates the molecular architecture of the flagellar motor (including MotA, MotB, and the MS ring) and describes the signal transduction pathway of chemotaxis, where the phosphorylation state of the protein CheY modulates the direction of flagellar rotation to bias bacterial movement toward chemoattractants.