12.3 Muscle Tissue: Composition and Function 571Table 12.5.Muscle proteins
Proteins Percentagea
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 extracellu-
lar 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
aAverage 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 fila-
ment. Six F-actin strands surround a thick fila-
ment; 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 as-
sumed to be a double-strandedα-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 con-
trols the contact between the filaments of myosin
and actin during muscle contraction by means of
aCa^2 ⊕concentration-dependent change in con-
formation (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 en-
zyme 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 ProteinsApart 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 isconnectin(ca. 10% of
the contractile proteins), an insoluble protein (Mr
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 ismyomesin(subunit: Mr= 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 myofib-
rils. 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(Mr= 200 ,000),
desmin(Mr= 55 ,000),vimentin(Mr= 58 ,000)
andsynemin(Mr= 23 ,000) are localized in the
Z lines. Desmin appears to connect neighboring
myofibrils.
AN line protein(Mr= 60 ,000) has been isolated
from the N lines which run parallel to the Z lines
on both sides and through the I bands.