The myosin head is a microscopic, yet powerful, component found within cells throughout the body. It acts as a molecular motor, converting chemical energy into mechanical force. This structure is a fundamental part of the larger myosin protein, playing a central role in generating movement at the cellular level. Its ability to produce force and motion makes it a key player in various biological processes.
Understanding the Myosin Head’s Structure
The myosin head is a globular domain located at one end of the myosin heavy chain. This region of the myosin protein contains specific sites for distinct functions. One is the ATP (adenosine triphosphate) binding site, where the cell’s energy currency attaches. This site is distinct from the actin filament binding site, also located on the myosin head.
The actin binding site allows the myosin head to physically connect with actin filaments, which are thin protein strands found in cells. The interaction between these two proteins is fundamental to how force is generated. The ability of the myosin head to bind both ATP and actin filaments enables its function as a molecular motor, facilitating the cyclical interactions necessary for movement.
How Myosin Heads Drive Movement
The movement generated by myosin heads is described as the “cross-bridge cycle” or “power stroke.” This cycle begins when ATP binds to the myosin head, causing it to detach from the actin filament. The ATP is then hydrolyzed into ADP (adenosine diphosphate) and an inorganic phosphate (Pi) by an enzyme called ATPase located on the myosin head.
The energy released from ATP hydrolysis changes the shape of the myosin head, moving it into a “cocked” or high-energy position, similar to pulling back a spring. At this point, ADP and Pi remain attached to the myosin head. When the binding sites on the actin filament become accessible, the myosin head forms a connection, known as a cross-bridge, with the actin.
The release of the inorganic phosphate (Pi) triggers the “power stroke,” a conformational change where the myosin head pivots and pulls the actin filament. This pulling action moves the actin filament approximately 10 nanometers (nm) towards the center of the muscle unit, known as the M-line, generating force and shortening the structure. Following the power stroke, ADP is released from the myosin head. The myosin head remains attached to the actin in a low-energy state until a new ATP molecule binds, initiating the cycle anew.
Myosin’s Diverse Roles Beyond Muscle Contraction
While commonly associated with muscle contraction, myosin heads are involved in a wide array of cellular activities. These molecular motors are present in nearly all cell types, contributing to various forms of cellular movement and organization. They play a role in processes like cell crawling, where cells move across surfaces.
Myosin heads are also important for cell division, specifically during cytokinesis, which is the final stage where one cell divides into two. In this process, a contractile ring composed of actin filaments and myosin II assembles, pinching the cell in half. Myosin heads are also involved in intracellular transport, facilitating the movement of various cargoes, such as vesicles and organelles, along actin filaments within the cell. This broad involvement highlights the importance of myosin as a versatile molecular motor across different biological contexts.
When Myosin Head Function Goes Awry
When the function of myosin heads is disrupted, it can lead to health conditions, particularly affecting muscle performance. Genetic mutations in the MYH7 gene, which codes for a specific type of myosin heavy chain, are a common cause. These mutations can alter the structure of the myosin protein, leading to improper function.
One condition linked to MYH7 gene mutations is hypertrophic cardiomyopathy, where the heart muscle thickens abnormally. These mutations can change single amino acids in the β-myosin heavy chain protein, impairing its function within the heart. Other conditions, such as Laing distal myopathy and myosin storage myopathy, also arise from MYH7 gene defects, resulting in progressive muscle weakness and the accumulation of abnormal proteins in muscle fibers. These examples underscore the precision required for myosin head function and the consequences when that precision is lost.