Myosin heavy chain
Argopecten irradians (bay scallop)
Muscles are extremely important for the human body and can be found in many individual organs. Perhaps the most important of them all is the heart. The rhythmic contraction and relaxation of the heart muscle, triggered like all muscle action by electrical impulses, is the pump that lets blood circulate throughout the body. The body contains several kinds of muscles: skeletal muscles of the arms and legs, smooth muscles of digestive organs and blood vessels, and the cardiac muscle of the heart. The structure shown here belongs to a skeletal or striated muscle. Muscles are aggregate fibers of cells, containing long cylinders of muscle proteins, called myofibrils. They contain two different types of filaments, thin filaments and thick filaments. Six thin filaments surround the thick filaments. The thin filaments consist of molecules of the protein actin (see PDB:1ATN) and the thick filaments consist of molecules of the protein myosin. Myosin and actin interact with one another and this interaction causes motion that lets the muscle contract. The motion is controlled by ATP, adenosine triphosphate, the chemical essential to many cellular processes. Initially adenosine triphosphate (ATP) binds to myosin in one conformation. The ATP is then split into adenosine diphosphate (ADP) and a phosphate group p. After cleavage of ATP myosin changes its shape and forms a connection with actin. Once this connection is form, the myosin head, the molecule that you see here, bends. The force of this movement makes the actin slide past the myosin and during this stage ADP and P are released. Myosin then binds another molecule of ATP, leading to the disconnection between actin and myosin and another round of the same cycle.
The structure here shows a molecule of myosin with a molecule of ADP bound to it. The myosin structure itself shows two quite distinct domains: a compact, globular domain about 50-70 angstrom in diameter and another stalk-like cylinder-shaped domain, somewhere between 30-40 angstrom in diameter, but about 100 angstrom long. It is easy to imagine a ratchet-like movement of this shape driving muscle contraction. The actin filament contains two other proteins, troponin (see PDB:1J1D) and tropomyosin, controlling in more detail the interaction between myosin and actin, influence by binding or release of calcium.
Protein Data Bank (PDB)
Houdusse, A. Kalabokis, V.N. Himmel, D. Szent-Gyorgyi, A.G. Cohen, C.; "Atomic structure of scallop myosin subfragment S1 complexed with MgADP: a novel conformation of the myosin head."; Cell; (1999) 97:459-470 PubMed:10338210.
author: Arno Paehler