Contractile Proteins and the Sliding Filament Theory

Each sarcomere is built of two distinct kinds of protein filament that lie parallel to the long axis of the myofibril and partly overlap one another: a thin filament and a thick filament.

The thin filament is formed mainly of the globular protein actin. Many individual G-actin (globular actin) monomers polymerise to form a long, double-helical chain called F-actin (filamentous actin), and it is this double helix that constitutes the backbone of the thin filament. Lying along the groove of the actin double helix is a second, rod-shaped protein, tropomyosin, and at regular intervals along the tropomyosin strand is a third protein complex, troponin, made of three subunits: troponin C, which is able to bind calcium ions; troponin I, which helps keep the myosin-binding site on actin covered in the resting state; and troponin T, which binds the whole troponin complex to tropomyosin. Together, tropomyosin and troponin are not structural filler but active regulatory proteins -- they control, moment to moment, whether or not a myosin head is permitted to attach to actin.

The thick filament is formed of many molecules of the protein myosin bundled together. Each myosin molecule has a long, rod-shaped tail (by which many myosin molecules associate to build the shaft of the thick filament) and a globular, projecting head -- the cross-bridge -- that points outward towards the neighbouring thin filaments. Crucially, the myosin head is itself an enzyme: it has ATPase activity, meaning it can bind and hydrolyse ATP, and the energy released by that hydrolysis is what powers the head's movement during contraction. …