2009_Book_FoodChemistry

578 12 Meat
Table 12.6. Amino acid composition of muscle proteins (values are in g/16g N)
Beef muscle Poultry muscle Myosin Actin
Amino acid total total a (calf skin) Collagen Elastin
Aspartic acid 9.7–9.9 9.7–11.0 10.9 10.4 5.4 1.0
Threonine 4.8 3.5–4.5 4.7 6.7 2.1 1.1
Serine 4.1–4.5 – 4.1 5.6 2.9 0.9
Glutamic acid 15.8–16.2 16–18 21.9 14.2 9.7 2.4
Proline 3.0–4.1 – 2.4 4.9 13.0 11.6
Hydroxyproline 10.5 1.5
Glycine 4.6–6.1 4.6–6.7 2.8 4.8 22.5 25.5
Alanine 6.1–6.3 – 6.7 6.1 8.2 21.1
Cystine 1.3–1.5 – 1.0 1.3 0 0.3
Valine 4.8–5.5 4.7–4.9 4.7 4.7 2.9 16.5
Methionine 4.1–4.5 – 3.1 4.3 0.7 Trace
}
Isoleucine 5.2 4.6–5.2 5.3 7.2 4.8 b 3.7
Leucine 8.1–8.7 7.3–7.8 9.9 7.9 8.6
Tyrosine 3.8–4.0 – 3.1 5.6 1.2 1.3
Phenylalanine 3.8–4.5 3.7–3.9 4.5 4.6 2.2 5.9
Lysine 9.2–9.4 8.3–8.8 11.9 7.3 3.9 0.5
Hydroxylysine 1.1 –
Histidine 3.7–3.9 2.2–2.3 2.2 2.8 0.7 0.1
Arginine 5.3–5.5 5.7–6.1 6.8 6.3 7.6 1.2
Tryptophan – – 0.8 2.0 0 –
a Chicken, duck, turkey: average values.
b Sum of isoleucine and leucine.
ferent connective tissue layers of muscular tissue
(cf. 12.2.1). An overview is presented in Table
12.7. The amino acid sequence of an α 1 -chain
of collagen type I of mammalian skin is shown in
Table 12.8. It is typical that every third residue
in this sequence is glycine. Deviations from this
regularity have been observed only at the ends of
a chain. A frequently recurring sequence is:
Gly − Pro − Hyp − .
As a result of the specificity of the hydroxylating
enzyme in vertebrates, hydroxyproline is always
located, as shown in the sequence (Table 12.8) before
glycine.
Collagen consists of three peptide chains which
can be different or identical, depending on the
type (cf. Table 12.7). The three peptide chains,
each of which has a helical structure, form
together a triple-stranded helix which has a structure
corresponding to that of polyglycine II.
A triple helix of this type is shown in Fig. 12.17.
The basic structural unit of collagen fibers is
called tropocollagen. It has a molecular weight of
approx. 30 kdal. With a length of approx. 280 nm
and a diameter of 1.4–1.5 nm, collagen is one
Fig. 12.17. Schematic representation of the conformation
of tropocollagen (period R = 8.7 nm, pseudoperiod
R ′ = 2.9nm)
of the longest proteins. Tropocollagen fibers
associate in a specific way to form collagen
fibers, as presented schematically in Fig. 12.18.
The association of adjacent rows is not in
register, but is displaced by about one-fourth of
the tropocollagen length (a “quarter staggered”
array). This is responsible for cross-striations in
the collagen fibers.
Fig. 12.18. Build up of a collagen fiber (b) from
tropocollagen (a) molecules