Protein Chemistry – Part 2 – Selected Topics in Special Protein Chemistry - Ashmarin, I. P. 1968

Contractile Proteins of Muscle Tissue
Actin

Actin can exist in two forms: as a monomer known as globular actin (G-actin) and as a polymer known as fibrillar actin (F-actin). G-actin is a protein with a relatively low Molecular Weight of 56,000–57,000 and a fairly asymmetric molecule measuring 290х24 Å. The Amino Acid Composition of actin has no features worthy of special mention, as it contains all known Amino Acids. G-actin consists of a single polypeptide chain. The N-terminal group of actin belongs to an acetylated aspartic acid residue. The sequence histidyl—isoleucyl—phenylalanine has been determined at the carboxyl end of the protein moiety. The Introduction/11.html">Secondary Structure of actin is characterized by the fact that about 40% of its constituent polypeptide chains are in an a-helical conformation.

G-actin is readily soluble in Water. The viscosity of its solutions is insignificant. Near its isoelectric point (pH 4.7), it readily precipitates. The native G-actin molecule typically contains one molecule of ATP, which is bound rather tightly and can only be removed by prolonged dialysis.

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Fig. 7. Electron micrograph of an F-actin preparation (Hanson a. Lowey, 1963).

Polymerization of G-actin and F-actin. In the presence of neutral salts (over a wide range of concentrations), rapid polymerization of G-actin occurs, which is manifested by a sharp increase in the viscosity of its solutions, the appearance of thixotropy, and significant Flow Birefringence. The process begins a few minutes after The addition of salts and is completed by the 10th minute under optimal conditions—in the presence of 0.1–0.15 M potassium chloride or sodium chloride; magnesium chloride salts also promote polymerization. Calcium Ions inhibit polymerization induced by potassium, sodium, and magnesium cations, although they can induce polymerization on their own in the absence of the latter. During polymerization, actin dimers with a molecular weight of 112,000–117,000 are initially formed, which then combine into filamentous polymers—F-actin—with a molecular weight equal to or exceeding 3∙10⁶.

Fig. 8. Scheme of actin polymerization and depolymerization.

F-actin filaments are clearly visible under an Electron microscope (Fig. 7); their thickness is about 80 Å, and their length is very substantial—exceeding 3000 Å and variable. Monomers in F-actin are bound to each other by Hydrogen Bonds formed between the nitrogen atoms of The polypeptide chains, on the one hand, and SH and OH groups, on the other. In addition, during The formation of F-actin, a-structures are converted into ß-structures. Of particular note is the fact that during polymerization, the ATP contained in native G-actin is converted into ADP. At the same time, the strength of the bond between the nucleotide and the protein increases. ADP cannot be completely removed from F-actin even by prolonged dialysis. This conversion of ATP is not merely an accompanying process; it is obligatory for polymerization. Prior removal of ATP from G-actin renders it incapable of polymerization. The reverse process, depolymerization of F-actin, can proceed upon the addition of Reagents that disrupt hydrogen bonds, such as urea. Depolymerization also occurs upon the addition of potassium iodide (0.5 M), as well as during prolonged dialysis in a mildly alkaline solution in the presence of magnesium ions. However, if ATP is not added in the process, a defective G-actin is formed, which in turn is incapable of polymerization. It should be noted that if we consider the entire cycle of actin polymerization and depolymerization, the return to the initial state is ultimately achieved through the Cleavage of one ATP molecule to ADP per monomer. No external ATP is consumed in this process, and actin can therefore be considered a unique type of enzyme (sometimes referred to as a semi-enzyme). The General scheme of interconversions between G- and F-actin is presented in Fig. 8.