570 12 Meat
Fig. 12.9.Schematic repre-
sentations of (a) a myosin
molecule (according to
Lehninger, 1975), (b)ar-
rangement of the Z line(1),
actin(2), titin(3)and
myosin(4)in the sarcom-
ere;(I)stretched muscle,
(II)contracted muscle
in an electric field and, consequently, was not
noticed for a long time in electrophoretic sepa-
rations of muscle proteins. Titin is the largest
known protein until now. Its sequence consists
of 26,926 amino acid residues. In fact, 90% of
the molecule consists of globular domains, a large
part of which bind to other proteins, especially to
myosin.
12.3.2.1.3 Actin
Actin is the main constituent of the thin filament.
It makes up ca. 22% of the total protein of the
contractile apparatus. It is substantially less
soluble than myosin, probably because it is
fixed to substances in the Z line. Actin can be
isolated, for example, by extraction of pulverized,
acetone-dried muscle tissue with an aqueous ATP
solution.
The monomer G-actin (globular actin) consists
of 375 amino acids, has a molecular weight
of 42,000 and is able to bind ATP and a doubly
charged cation. G-actin exists only at low ionic
strengths. The addition of singly and doubly
charged cations starts the polymerization to
F-actin (fibrillar actin) with the cleavage of
ATP to ADP, which remains in the bound
state.
F-actin in the thin filaments (1ā¼1000 nm,dā¼
8 nm) is in the form of a double-stranded helix
in which the G-actin beads are stabilized by two