2009_Book_FoodChemistry

12.3 Muscle Tissue: Composition and Function 569
Fig. 12.8. Schematic representation of a sarcomere in the relaxed (a) and contracted (b) state (according to
Gault, 1992). 1 I band, 2 A band, 3 Z line, 4 M line, 5 thin filament, 6 thick filament, 7 H zone. Cross section:
I thin filaments near the Z line, II overlapping thick and thin filaments, III thick filaments, IV M line
Table 12.4. Average composition of meat (%)
Meat Cut Moisture Protein Fat Ash
Pork Boston butt (M. subscapularis) 74.9 19.5 4.7 1.1
Loin (M. psoas maior) 75.3 21.1 2.4 1.2
Cutlets, chops a 54.5 15.2 29.4 0.8
Ham 75 20.2 3.6 1.1
Side cuts 60.3 17.8 21.1 0.85
Beef Shank 76.4 21.8 0.7 1.2
Sirloin steak a 74.6 22.0 2.2 1.2
Chicken b Hind leg (thigh + drum stick) 73.3 20.0 5.5 1.2
Breast 74.4 23.3 1.2 1.1
a With adhering adipose tissue.
b Without skin.
of two large (M r ca. 200,000) and four small (M r
ca. 20,000) subunits and is a very long molecule
(measurements 140 × 2 nm). The two large
subunits form a long, double-stranded α-helical
rod with a double head of globular protein, both
heads being joined at the same end of the coil
(head dimensions, 5 × 20 nm) (Fig. 12.9a). The
myosin ATPase activity is localized in the heads
and is required for the interaction of the heads
with actin, the protein constituent of the thin
filaments. Myosin is cleaved by trypsin into two
fragments: light (LMM, M r 150,000) and heavy
meromyosin (HMM, M r 340,000). The HMM
fraction contains the globularheaded region and
has the ATPase activity and the ability to react
with actin. Further proteolysis of HMM yields
two subfragments S1 and S2, which correspond
to the actual head and neck. The four smaller
subunits mentioned above are found in the head
region.
Up to 400 myosin molecules are arranged in the
thick filaments (l ∼ 1500 nm, d ∼ 12 nm). By
bringing the tails together, a major cord is formed
and on its surface the heads are spirally located.
The distance between two adjacent heads on such
aspiralis14.3 nm, and that between the two repeating
heads in the same row or line is 42.9nm.
Their association is reversible under certain conditions.
Myosin is stabilized by titin during muscle
contraction (cf. Fig. 12.9b).
12.3.2.1.2 Titin
Apart from actin and myosin, titin is the third filament
in the sarcomere (Fig. 12.9b). It connects
the myosin filaments with the Z line and forms
an “elastic” region with actin. Therefore, titin
is the “backbone” of the sarcomere. As a result
of its size (M r = 3 ×10 6 ), it moves very slowly