8 Chapter 1
each) and two sets of light chains (14,000 –
20,000 daltons). One of the light chains is
required for enzymatic activity, and the other
has regulatory functions.
Actin is the second - most abundant protein
in the myofi bril, accounting for approxi-
mately 20% of the total protein in the myo-
fi bril. Actin is a globular protein (G - actin)
that polymerizes to form fi laments (F - actin).
G - actin has a molecular weight of approxi-
mately 42,000. There are approximately
400 actin molecules per thin fi lament. Thus
the molecular weight of each thin fi lament
is approximately 1.7 × 1 0^7 (Squire 1981 ).
The thin fi laments (F - actin polymers) are
1 μ m in length and are anchored in the
Z - line.
Two other proteins that are important in
muscle contraction and are associated with
the thin fi lament are tropomyosin and tropo-
nin. Tropomyosin is the second - most abun-
dant protein in the thin fi lament and makes
up about 7% of the total myofi brillar protein.
Tropomyosin is made up of two polypeptide
chains (alpha and beta) The alpha chain has
an approximate molecular weight of 34,000,
and the beta chain has a molecular weight of
approximately 36,000. These two chains
interact with each other to form a helix. The
native tropomyosin molecule interacts with
the troponin molecule to regulate contrac-
tion. Native troponin is a complex that con-
sists of three subunits. These are termed
troponin I (MW 23,000), troponin C (MW
18,000), and troponin T (MW 37,000).
Troponin C has the ability to bind calcium
released from the sarcoplasmic reticulum,
troponin I can inhibit the interaction between
actin and myosin, and troponin T binds very
strongly to tropomyosin. The cooperative
action of troponin and tropomyosin in
response to calcium increases in the sarco-
plasm regulates the interaction between actin
and myosin and thus is a major regulator of
contraction. Calcium that is released from the
sarcoplasmic reticulum is bound to the tropo-by proteinacious fi laments, known as inter-
mediate fi laments. Outermost myofi brils are
attached to the cell membrane (sarcolemma)
by intermediate fi laments that interact not
only with the Z - line, but also with structures
at the sarcolemma known as costameres
(Robson et al. 2004 ).
Myofi brils are made up of many myofi la-
ments, of which there are two major types,
classifi ed as thick and thin fi laments. There
is also a third fi lament system composed pri-
marily of the protein titin (Wang et al. 1979 ;
Wang 1984 ; Wang et al. 1984 ; Wang and
Wright 1988 ; Wang et al. 1991 ; Ma et al.
2006 ;). With respect to contraction and rigor
development in postmortem muscle, it is the
interdigitating thick and thin fi laments that
supply the “ machinery ” needed for these pro-
cesses and give skeletal muscle cells their
characteristic appearance (Squire 1981 ).
Within the myofi bril, the less dense I - band is
made up primarily of thin fi laments, while
the A - band is made up of thick fi laments and
some overlapping thin fi laments (Goll et al.
1984 ). The backbone of the thin fi laments is
made up primarily of the protein actin, while
the largest component of the thick fi lament is
the protein myosin. Together, these two pro-
teins make up nearly 70% of the proteins in
the myofi bril of the skeletal muscle cell.
Myosin is the most abundant myofi brillar
protein in skeletal muscle, making up approx-
imately 50% of the total protein in this organ-
elle. Myosin is a negatively charged protein
with an isoelectric point of 5.3. Myosin is
a large protein (approximately 500,000
daltons) that contains six polypeptides.
Myosin consists of an alpha helical tail (or
rod) region that forms the backbone of the
thick fi lament and a globular head region that
extends from the thick fi lament and interacts
with actin in the thin fi lament. The head
region of myosin also has ATPase activity,
which is important in the regulation of con-
traction. Each myosin molecule contains two
heavy chains (approximately 220,000 daltons