Food Chemistry

1.4 Proteins 53

Fig. 1.24.Turns of the peptide chains (β-turns), types I–III.= carbon, = nitrogen,= oxygen. Theα-C atoms
of the amino acid residues are marked 1–4. X = no side chain allowed

Fig. 1.25.Superhelix secondary structure (according to
SchulzandSchirmer, 1979).acoiled-coilα-helix,bβ-
meander,cβαβαβ-structure

1.4.2.3 TertiaryandQuaternaryStructures.........................

Proteins can be divided into two large groups on
the basis of conformation: (a) fibrillar (fibrous)
or scleroproteins, and (b) folded or globular pro-
teins.

1.4.2.3.1 FibrousProteins

The entire peptide chain is packed or arranged
within a single regular structure for a variety of
fibrous proteins. Examples are wool keratin (α-
helix), silk fibroin (β-sheet structure) and colla-
gen (a triple helix). Stabilization of these struc-
tures is achieved by intermolecular bonding (elec-
trostatic interaction and disulfide linkages, but
primarily hydrogen bonds and hydrophobic inter-
actions).

1.4.2.3.2 GlobularProteins

Regular structural elements are mixed with ran-

domly 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 myo-

globin (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 nG n%

Myoglobin 3–16a 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

αs1-Casein 199 ca. 30

β-Casein 209 ca. 20

aPosition number of the amino acid residue in the

sequence.

nG: Total number of amino acid residues.

n: Amino acid residues within the regular structure.

%: Percentage of the amino acid residues present in reg-

ular structure.