Protein Chemistry - Part 2 - Selected Chapters in Special Protein Chemistry - Ashmarin I. P. 1968

Contractile Proteins of Muscle Tissue
Tropomyosin

Tropomyosin is a protein with a relatively low Molecular Weight of approximately 70,000, consisting of two polypeptide chains predominantly in the a-conformation. The molecule is nearly 400 Å in length. In its Amino Acid Composition, tropomyosin is quite similar to Myosin; the only significant difference is the absence of Tryptophan. Its C-terminal amino acid is isoleucine, just like in myosin, whereas no N-terminal amino acid has been detected. It is soluble in Water and dilute salt solutions; its isoelectric point lies at pH 5.1, and precipitation occurs over a broad range—at 45–70% ammonium sulfate saturation. Tropomyosin solutions are characterized by an extremely high viscosity. In salt-free solutions and at very low salt concentrations, the polar molecules of tropomyosin have a strong tendency to aggregate end-to-end, forming fibrous and reticular structures. As the Ionic strength of the solution increases, the viscosity drops rapidly. In this respect, tropomyosin resembles Actomyosin and differs significantly from Actin. Tropomyosin is also capable of forming complexes with actin. However, unlike myosin, tropomyosin lacks enzymatic activity, and its complexes with actin are incapable of superprecipitation in the presence of ATP.

The function of tropomyosin is not yet fully understood. Nevertheless, there is good reason to believe that tropomyosin is essential within the myofibril as a mechanistic link through which Calcium Ions influence the ATPase activity of actomyosin. Recent studies have established that tropomyosin performs its Functions regarding calcium ions not in isolation, but rather in a complex with a recently discovered protein, troponin. The relative content of the latter in Muscle is low, and data concerning its COMPOSITION AND PROPERTIES remain sparse.

It is also hypothesized that tropomyosin is necessary for maintaining the sustained contracted state of a muscle over extended periods—the so-called catch function of tropomyosin. This phenomenon is particularly pronounced in Mollusks, whose Muscles contain relatively large amounts of tropomyosin (25–30% of the total protein). Muscles exhibiting the capacity for prolonged tonic contraction are similarly rich in tropomyosin. Apparently, the Molecular Mechanism of the catch function of tropomyosin involves The formation of copolymers with filamentous F-actin molecules.