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Myosin
The
main constituent of the thick filaments is myosin. Each
thick filament is composed of about 250 molecules of myosin.
Myosin has two important roles: a structural one as the
building block for the thick filaments; and a functional
one as the catalyst of the breakdown of ATP during contraction
and in its interaction with actin as part of the force generator
of muscle. The individual myosin molecule has a molecular
weight (based on the weight of a hydrogen atom as one) of
about 500,000; it contains two major protein chains and
four small ones, the entire molecule being about 160 nanometres
in length and asymmetrically shaped. The rodlike tail region,
about 120 nanometres long, consists of two chains of protein
each wound into what is known as an ?-helix together forming
a coiled-coil structure. At the other end of the molecule
the two protein chains form two globular headlike regions
that have the ability to combine with the protein actin
and carry the enzymatic sites for ATP hydrolysis.
The
myosin molecule can be split by proteolytic cleavage to
produce two fragments, light meromyosin (LMM) and heavy
meromyosin (HMM). The LMM portion, about 80 nanometres long,contains
most of the ?-helical tail region of the original myosin
molecule. The HMM portion, also about 80 nanometres long,
consists of two globular head regions attached to a part
of the tail region of the myosin molecule. It forms the
cross bridges that protrude from the thick filaments (see
below) and interacts with the thin actin-containing filaments.
The HMM portion contains the enzymatic site that catalyzes
the splitting of ATP. Under appropriate conditions single
myosin heads (subfragment-1, or S-1) and actin binding ability
also can be obtained.
There
are some structural differences in myosins isolated from
different muscles within the same animal and from muscles
at different stages of development. These so-called isoforms
are products of genes belonging to a family. Some differences
in corresponding protein components are due to different
ways of processing the messenger RNA produced by the same
gene. While the basic pattern of myosin structure is preserved,
there are many differences in the sequence of amino acids
(primary structure). Many other proteins involved in muscle
contraction (e.g., actin, troponin) also have isoforms characteristic
of different types of muscle.
