Silkmoth Chorion: A Natural Protective Amyloid - The Functional Fold: Amyloid Structures in Nature - page 66

a b c

β

Figure 4.11 (a) Antiparallel, twisted -sheet model proposed for the cA

peptide. Sequence should be read continuously, beginning
at the bottom. Invariant glycines (G) occupying the second
position in the

β

-turns are black boxed. Tentative type II'

β β

-strands. (b) A represen-
tation of the electrostatic surface potential of the antiparallel
twisted

-turns alternate with four-residue

-sheet model proposed for the cA peptide in (a),
calculated by the program DELPHI and displayed using GRASP
(Nicholls

β

., 1991; Nicholls, 1993). The calculations were
performed with the default charges file. Electrostatic potential
is shown from -10 kT (red) to +10 kT (blue). (c) A ribbon
model of the antiparallel twisted

et al

β

-sheet model structure
proposed for the cA peptide in (a), displayed using GRASP
(Nicholls , 1991). The side chains of the residues and the

carbonyl oxygens of the main chain are shown as “sticks”. View
perpendicular to a “face” of the

et al.

-sheet. Reprinted from ref.
18, copyright (2004), with permission from Elsevier. See also
Colour Insert.

β

Another possibility for the structure of the cA peptide might be
that of the left-handed parallel -helix (Fig. 4.12b), similar to that

found in the structure of UDP-N-acetylglucosamine acyltransferase

β

34

β

4

and other left-handed parallel This protein

shows hexapeptide sequence motifs (Fig. 4.12a). It is interesting
to note that right-handed, parallel

-helical proteins.

β

-helices, similar to those found
in the pectate lyases, have been postulated as the main molecular
components of amyloid protofibrils, although no detailed molecular
models were presented.

35

Characteristic hexapeptide periodicities of
both glycine and hydrophobic residues also appear in the sequence