Food Chemistry

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. 919. 54. 71. 1
Loin (M. psoas maior)75. 321. 12. 41. 2
Cutlets, chopsa 54. 515. 229. 40. 8
Ham 75 20. 23. 61. 1
Side cuts 60. 317. 821. 10. 85
Beef Shank 76. 421. 80. 71. 2
Sirloin steaka 74. 622. 02. 21. 2
Chickenb Hind leg (thigh + drum stick) 73. 320. 05. 51. 2
Breast 74. 423. 31. 21. 1

aWith adhering adipose tissue.
bWithout skin.

of two large (Mrca. 200,000) and four small (Mr
ca. 20,000) subunits and is a very long molecule
(measurements 140×2 nm). The two large
subunits form a long, double-strandedα-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, Mr 150 ,000) and heavy
meromyosin (HMM, Mr 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 re-

peating heads in the same row or line is 42.9nm.

Their association is reversible under certain con-

ditions. Myosin is stabilized by titin during mus-

cle contraction (cf. Fig. 12.9b).

12.3.2.1.2 Titin

Apart from actin and myosin, titin is the third fil-

ament 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 (Mr= 3 × 106 ), it moves very slowly