2009_Book_FoodChemistry

12.3 Muscle Tissue: Composition and Function 571
Table 12.5. Muscle proteins
Proteins
Percentage a
Myofibrillar proteins 60.5
Myosin (H-, L-meromyosin,
various associated 29
components)
Actin 13
Titin 3.7
Tropomyosins 3.2
Troponins C, I, T 3.2
α, β-, γ-Actinins 2.6
Myomesin, N-line proteins etc. 3.7
Desmin etc. 2.1
Sarcoplasma proteins 29
Glyceraldehydephosphate
dehydrogenase 6.5
Aldolase 3.3
Creatine kinase 2.7
Other glycolytic enzymes 12.0
Myoglobin 1.1
Hemoglobin, other extracellular
proteins 3.3
Connective tissue proteins
Proteins from organelles 10.5
Collagen 5.2
Elastin 0.3
Mitochondrial proteins
(including cytochrome c
and insoluble enzymes) 5.0
a Average percentage of the total protein of a typical
mammalian muscle after rigor mortis and before other
post mortem changes.
tropomyosin fibrils (cf. 12.3.2.1.3), as shown in
Fig. 12.10. Altogether, it is a four-stranded filament.
Six F-actin strands surround a thick filament;
consequently, each F-actin strand adheres
to the heads of three thick filaments (Fig. 12.8a,
II).
12.3.2.1.4 Tropomyosin and Troponin
Tropomyosin (ca. 5% of the contractile proteins)
is a highly elongated molecule (2 × 45 nm) with
a molecular weight of about 68,000, and is assumed
to be a double-stranded α-helix. Although
each chain contains the same number of amino
acids, their sequences differ in 39 positions.
Tropomyosin contains no di-sulfide bridges and
no proline. Indeed, 100% of tropomyosin is an
α-helix. The monomer readily forms polymeric
fibrils which are bound to F-actin on the thin
filament.
Troponin (ca. 5% of the contractile proteins) sits
on the actin filaments (cf. Fig. 12.10) and controls
the contact between the filaments of myosin
and actin during muscle contraction by means of
aCa 2⊕ concentration-dependent change in conformation
(cf. 12.3.2.1.5). It is a complex of three
components, T, I, and C. Troponin T consists of
a peptide chain with 259 amino acid residues and
binds to tropomyosin. Troponin I (179 amino acid
residues) binds to actin and inhibits various enzyme
activities (ATPase). Troponin C (158 amino
acid residues) binds Ca 2⊕ ions reversibly through
a change in conformation.
12.3.2.1.5 Other Myofibrillar Proteins
Apart from the main components of sarcomeres,
myosin and actin, a series of cytoskeletal proteins
exist that are responsible for the stabilization
of the structure of the sarcomeres. The most
important component is connectin (ca. 10% of
the contractile proteins), an insoluble protein (M r
ca. 2,000,000) capable of forming fine filaments
(g-filaments, d = 2 nm) which start at the Z line
and proceed between the thick filaments of
neighboring sarcomeres. These g-filaments
greatly contribute to the elasticity and firmness of
meat because they can be stretched from 1.1µm
to a length of 3.0 µm. Another protein of the cell
skeleton is myomesin (subunit: M r = 165,000).
As the main component of the M line, myomesin
is involved in fixing the thick filaments of the
A band and in connecting neighboring myofibrils.
Since myomesin strongly binds to myosin, it
is possibly involved in the packing and cohesion
of the myosin molecules in the thick filaments as
well.
Among other proteins, α-actinin (M r = 200,000),
desmin (M r = 55,000), vimentin (M r = 58,000)
and synemin (M r = 23,000) are localized in the
Z lines. Desmin appears to connect neighboring
myofibrils.
A N line protein (M r = 60,000) has been isolated
from the N lines which run parallel to the Z lines
on both sides and through the I bands.