Protein Chemistry. Structure, Properties, and Research Methods - Shendryk A.N. 2022

Proteins of the circulatory and muscular tissues
Proteins of the contractile system
Functions of muscle tissue proteins - Actin

Actin is a Water-soluble protein with a Molecular Weight of approximately 42–46 kDa. It exists in two known forms: G-actin (globular protein) and F-actin (fibrillar protein). G-actin has been isolated in a highly purified state. It consists of a single peptide chain folded into a globule with a diameter of 5.5–6.0 nm, and its Primary Structure has been fully elucidated. The G-actin molecule contains seven Cysteine residues, a high proportion of Proline residues, and a single residue of an Unusual amino acid, $N^ au$-methyllysine:

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The G-actin molecule tightly binds one calcium ion (2+) and one molecule of ATP or ADP. ATP binding is typically accompanied by aggregation leading to The formation of F-actin. This aggregation process is described by the following scheme:

F-actin consists of two G-actin chains wound around each other into a superhelix. The average diameter of this coiled F-actin filament is about 60 Å, which is close to the diameter of thin filaments in Muscle I-bands. In addition to F-actin, thin filaments comprise Tropomyosin and troponin (see Fig. 3.6).

Fig. 3.6. Actin (thin) filament formed by actin, tropomyosin, and troponin molecules

In the thin filament, actin, tropomyosin, and troponin are present in a specific stoichiometric ratio: one molecule of troponin and one molecule of tropomyosin per seven actin molecules.