Understanding the Composition of Thick Filaments in Sarcomeres: Myosin and its Role in Muscle Contraction

Muscle contraction is a complex biological process that involves the interaction between various protein filaments within muscle fibers. One of the key components responsible for this process are the thick filaments, which are primarily composed of myosin molecules. In this article, we delve into the intricacies of myosin, its structure, and its role in facilitating muscle contraction. We will explain the mechanism through which muscle fibers contract and relax, and the specific properties of myosin that make it an essential part of this process.

The Role of Myosin in Muscle Contraction

Myosin is a motor protein that plays a crucial role in muscle contraction. It harbors a long tail and a globular head, which makes it capable of binding to actin filaments (the thin filaments) during the contraction process. In skeletal muscles, the thick filaments are primarily made up of myosin II isoforms, which aggregate to form a filamentous structure that interacts with the thin filaments.

The Structure of Myosin

Myosin is a hexameric protein, with each molecule consisting of two heavy chains (MHC), two light chains (RLC and ELC), and two globular heads. The MHC and RLC/ELC are responsible for the regulatory and motor functions, respectively. Each myosin molecule has a length of approximately 1500 angstroms, while the thick filament itself is 1.6 μm. The two heads of the myosin molecule form cross-bridges with the actin filament, a process known as cross-bridging cycling.

Cross-Bridging Cycling in myosin

The mechanism of muscle contraction and relaxation is known as cross-bridging cycling. During this process, myosin heads bind to actin filaments, exert a force, and then release from the filament, repeating the cycle. This process involves several steps: attachment, power stroke, and detachment. The ATPase activity of the myosin head is crucial for the hydrolysis of ATP, which provides the necessary energy for the cycling.

Role of Myosin II Isoforms in Sarcomeres

The thick filament of sarcomeres is made primarily of myosin II isoforms, which are the motor proteins responsible for facilitating muscle contraction. These isoforms aggregate to form a filamentous structure that interacts with the thin filaments. The myosin II isoforms are regulated by MyBp-C complexes, which are formed by binding of myosin to C isoforms of myosin II. The thin filaments are tethered to the myosin heads through trompomodulin and leimodin, part of the long thin filament attached to the myosin heads. This complex interaction allows the myosin to slide on the actin filaments as the heads attach and detach.

The Composition and Function of Myosin Heads

The myosin head is a crucial component in the mechanism of muscle contraction. The head contains several domains, including the motor domain and the neck region, as well as light chains (RLC and ELC) that play regulatory roles. The motor domain is responsible for binding to actin filaments, while the neck region is involved in the regulation of the head's movement. The cross-bridges that form between myosin and actin are essential for the sliding filament theory of muscle contraction.

Figure: Myosin Head and Motor Domain

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Myosin II isoforms and their specific binding interactions with actin filaments are crucial for the contraction and relaxation of muscle fibers. Understanding the intricate details of myosin and its role in muscle contraction provides valuable insights into muscle physiology and potential therapeutic targets for muscle-related disorders.