2009_Book_FoodChemistry

1.4 Proteins 53
Fig. 1.24. Turns of the peptide chains (β-turns), types I–III. = carbon,
of the amino acid residues are marked 1–4. X = no side chain allowed
= nitrogen, = oxygen. The α-C atoms
1.4.2.3.2 Globular Proteins
Regular structural elements are mixed with randomly
extended chain segments (randomly coiled
structures) in globular proteins. The proportion of
regular structural elements is highly variable: 20–
30% in casein, 45% in lysozyme and 75% in myoglobin
(Table 1.23). Five structural subgroups are
known in this group of proteins: (1) α-helices
Table 1.23. Proportion of “regular structural elements”
present in various globular proteins
Protein α-Helix β-Structure n G n %
Fig. 1.25. Superhelix secondary structure (according to
Schulz and Schirmer, 1979). a coiled-coil α-helix, b β-
meander, c βαβαβ-structure
1.4.2.3 Tertiary and Quaternary Structures
Proteins can be divided into two large groups on
the basis of conformation: (a) fibrillar (fibrous)
or scleroproteins, and (b) folded or globular proteins.
1.4.2.3.1 Fibrous Proteins
The entire peptide chain is packed or arranged
within a single regular structure for a variety of
fibrous proteins. Examples are wool keratin (αhelix),
silk fibroin (β-sheet structure) and collagen
(a triple helix). Stabilization of these structures
is achieved by intermolecular bonding (electrostatic
interaction and disulfide linkages, but
primarily hydrogen bonds and hydrophobic interactions).
Myoglobin 3–16 a 14
20–34 15
35–41 7
50–56 7
58–77 20
85–93 9
99–116 18
123–145 23
151 173 75
Lysozyme 5–15 11
24–34 11
41–54 14
80–85 6
88–96 9
97–101 5
109–125 7
129 63 49
α s1 -Casein 199 ca. 30
β-Casein 209 ca. 20
a Position number of the amino acid residue in the
sequence.
n G : Total number of amino acid residues.
n: Amino acid residues within the regular structure.
%: Percentage of the amino acid residues present in regular
structure.