17.2.1. Structure of Contractile Proteins

Each actin (thin) filament is made of two ‘F’ (filamentous) actins helically wound to each other. Each ‘F’ actin is a polymer of monomeric ‘G’ (Globular) actins. Two filaments of another protein, tropomyosin also run close to the ‘F’ actins throughout its length. A complex protein Troponin is distributed at regular intervals on the tropomyosin. In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments.

Actin filaments, which are crucial components in muscle contraction, are composed of two intertwined structures called F-actins. These F-actins themselves are made of smaller, globular units known as G-actins, which are polymerized to form the longer F-actin filament. Additionally, the tropomyosin protein runs alongside these actin filaments and plays a regulatory role. The troponin complex, associated with tropomyosin, periodically acts to cover the binding sites on actin for myosin, preventing premature muscle contraction until the muscle receives the appropriate signals.

Think of actin as the flexible strands of a rope, where each strand (F-actin) is made of small beads (G-actins). The tropomyosin acts like a protective sheath that covers parts of the rope to prevent someone from grabbing it. When you want to pull the rope, a mechanism lifts the sheath (troponin), allowing people to grasp the rope securely and pull it toward them.

Each myosin (thick) filament is also a polymerised protein. Many monomeric proteins called Meromyosins constitute one thick filament. Each meromyosin has two important parts, a globular head with a short arm and a tail, the former being called the heavy meromyosin (HMM) and the latter, the light meromyosin (LMM). The HMM component, i.e.; the head and short arm projects outwards at regular distance and angle from each other from the surface of a polymerised myosin filament and is known as cross arm. The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.

Myosin filaments are made up of multiple units known as meromyosins. Each of these units has two distinct parts: a globular head (heavy meromyosin) that is critical for the muscle's contractile function and a tail (light meromyosin) that helps to form the overall structure of the thick filament. This head contains enzymes and binding sites that interact with ATP, which provides the energy needed for muscular movement, as well as binding sites that connect with actin to facilitate contraction. The spatial orientation of these heads stems from their arrangement, forming what we recognize as the cross arms of myosin within muscles.

Imagine myosin as a set of baseball players on a field. The players (meromyosins) form a long line (the thick filament), and each player has a unique role. The head of each player (heavy meromyosin) has the ability to throw the ball (bind to actin) and needs a ball (ATP) to throw effectively. When the entire team works together, they can move as one, similar to how muscle fibers contract when myosin heads pull on actin.

--