Food Chemistry

52 1 Amino Acids, Peptides, Proteins

Fig. 1.22.Right-handedα-helix

Fig. 1.23.φ,ψ-Diagram with marked helix param-
eters n (---) and d (----). (according toSchulzand
Schirmer, 1979)

Table 1.22.β-Turns in the peptide chain of egg white

lysozyme

Residue Number Sequence

20–23 Y R G Y

36–39 S N F N

39–42 N T Q A

47–50 T D G S

54–57 G I L E

60–63 S R W W

66–69 D G R T

69–72 T P G S

74–77 N L C N

85–88 S S D I

100–103 S D G D

103–106 D G M N

1.4.2.2.3 ReverseTurns

An important conformational feature of globular

proteins are the reverseturnsβ-turns andβ-bends.

They occur at “hairpin” corners, where the

peptide chain changes direction abruptly. Such

corners involve four amino acid residues often

including proline and glycine. Several types of

turns are known; of greatest importance are type I

(42% of 421 examined turns), type II (15%) and

type III (18%); see Fig. 1.24.

In type I, all amino acid residues are al-

lowed, with the exception of proline in

position 3. In type II, glycine is required

in position 3. In type III, which corre-

sponds to a 3 10 -helix, all amino acids are

allowed. The sequences of the β-bends

of lysozyme are listed in Table 1.22 as an

example.

1.4.2.2.4 Super-Secondary Structures

Analysis of known protein structures has

demonstrated that regular elements can exist in

combined forms. Examples are the coiled-coil

α-helix (Fig. 1.25, a), chain segments with

antiparallel β-structures (β-meander structure;

Fig. 1.25, b) and combinations ofα-helix and

β-structure (e. g.,βαβαβ;Fig.1.25c).