Structure of Contractile Proteins

Thin Myofilaments

•  Actin: Actin is a polymer composed of monomers called globular actin (G-actin). These G-actin molecules join together in the presence of Magnesium ions to form a double helix structure known as F-actin (filamentous actin). Two F-actin filaments twirl around each other to form a thin filament. Each actin monomer has a binding site for myosin. These actin filaments form a helical arrangement around the thick myofilaments.
• Tropomyosin: Tropomyosin is a long, filamentous protein that runs along the length of the actin filaments. It covers the myosin binding sites on actin in a relaxed muscle state to prevent the interaction between actin and myosin.
• Troponin Complex: The troponin complex consists of three subunits: troponin C, troponin I, and troponin M. 

Thick Myofilaments

Myosin: Myosin is a polymer formed by 300-400 units of meromyosin. Meromyosin consists of three major parts – the head region, the arm region, and the tail region. The head region is globular and contains the ATP-binding site and the actin-binding site. The arm region is short. Head and arm are also known as Heavy Meromyosin (HMM). The tail region is a long, coiled structure that stabilizes the myosin molecule. Tail is also known as Light Meromyosin (LMM). The myosin molecules aggregate together to form thick filaments, with the heads projecting outward.

In muscle contraction, the myosin heads attach to the actin filaments, forming cross-bridges. The heads then undergo a conformational change, causing the actin filaments to slide past the myosin filaments, resulting in muscle contraction. This is explained by the sliding filament theory. 

Contractile proteins are a group of proteins responsible for the contraction and movement of muscles in living organisms. These proteins work together in a highly coordinated manner to enable muscle cells to shorten and generate force, allowing for various types of movements. In a muscle cell, there are two types of myofilaments-Thin Myofilaments and Thick Myofilaments.