2009_Book_FoodChemistry

52 1 Amino Acids, Peptides, Proteins
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 Reverse Turns
Fig. 1.22. Right-handed α-helix
An important conformational feature of globular
proteins are the reverseturns β-turns and β-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 allowed,
with the exception of proline in
position 3. In type II, glycine is required
in position 3. In type III, which corresponds
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
Fig. 1.23. φ,ψ-Diagram with marked helix parameters
n (---) and d (----). (according to Schulz and
Schirmer, 1979)
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 of α-helix and
β-structure (e. g., βαβαβ;Fig.1.25c).