2009_Book_FoodChemistry

12.3 Muscle Tissue: Composition and Function 573
12.3.2.1.7 Actomyosin
Solutions of F-actin and myosin at high ionic
strength (μ = 0.6) in vitro form a complex called
actomyosin. The formation of the complex is
reflected by an increase in viscosity and occurs
in a definite molar ratio: 1 molecule of myosin
per 2 molecules of G-actin, the basic unit of
the double-helical F-actin strand. It appears that
a spike-like structure is formed, which consists
of myosin molecules embedded in a “backbone”
made of the F-actin double helix. Addition of
ATP to actomyosin causes a sudden drop in viscosity
due to dissociation of the complex. When
this addition of ATP is followed by addition of
Ca 2+ , the myosin ATPase is activated, ATP is
hydrolyzed and the actomyosin complex again
restored after the ATP concentration decreases.
Upon spinning of an actomyosin solution into water,
fibers are obtained which, analogous to muscle
fibers, contract in the presence of ATP. Glycerol
extraction of muscle fibers removes all the
soluble components and abolishes the semipermeability
of the membrane. Such a model muscle
system shows all the reactions of in vivo muscle
contraction after the readdition of ATP and Ca 2+ .
This and similar model studies demonstrate that
the muscle contraction mechanism is understood
in principle, although some molecular details are
still not clarified.
of enzymes involved in ATP metabolism, e. g.,
creatine phosphokinase and ADP-deaminase
(cf. 12.3.6 and 12.3.8) are also present.
12.3.2.2.2 Myoglobin
Muscle tissue dry matter contains an average
of 1% of the purple-red pigment myoglobin.
However, the amounts in white and red meat vary
considerably.
Myoglobin consists of a peptide chain (globin)
of molecular weight of 16.8 kdal. It has known
primary and tertiary structures (Fig. 12.12). The
pigment component is present in a hydrophobic
pocket of globin and is bound to a histidyl (His 93 )
residue of the protein. The pigment, heme, is the
same as that in hemoglobin (blood pigment), i. e.
Fe 2+ -protoporphyrin (Fig. 12.13).
12.3.2.2 Soluble Proteins
Soluble proteins make up 25–30% of the total
protein in muscle tissue. They consist of
ca. 50 components, mostly enzymes and myoglobin
(cf. Table 12.5). The high viscosity of the
sarcoplasm is derived from a high concentration
of solubilized proteins, which can amount to
20–30%. The glycolytic enzymes are bound to
the myofibrillar proteins in vivo.
12.3.2.2.1 Enzymes
Sarcoplasm contains most of the enzymes
needed to support the glycolytic pathway and
the pentosephosphate cycle. Glyceraldehyde-
3-phosphate dehydrogenase can make up more
than 20% of the total soluble protein. A series
Fig. 12.12. Molecular model of myoglobin (a) and
a schematic representation of peptide chain course (b).
(from Schormueller, 1965)