section
4.3
Secondary Structure
55
-O
-Hv
J
V H-
/
\
/ S x
\
c—R
/
H-
. o A
N-
A
A
A
.o A
N-
R
— < /
A
N"
R - ç 7
,c—R
•H
•O
'
O. .
H-
\
/ ^ a .
N
\
C— R
/
' ^ V
h"
A
A
A
A
•
R— C
7
R—C
/H '
•O-
/
•o A
N
y
v
•H
/
H \ N
\
C— R
■
A ' «
A
A
'
A
' ^
V
h-
/
R— C
O. .
C— R
*
0
—
N— H- ■
/
RA
C— O- ■
• H—
c—R
A
N— H- ■
A
o
/c
°
■ -H— N
\
_
/ C
R
• °
c x
N— H- •
R
- /
R— Q
C— R
N— H- • 0 = C
•o=c
/
•H— N
>
•0 = C
•H— N
R— C
\
N— H-
/
:0* • -H— N
C— O-
/
\
N— H- ■ 0 = C
/
C
R
R— C
C— R
/
\
N— H-
/
\
c=o-
C = 0 • -H— N
R— C
/
•o=c
\
\
.
N— H- • 0 = C .
y
\
N— H- •
C— R
R— C
/
Parallel R-pleated sheet structure
(a)
Antiparallel R-pleated sheet structure
(b)
F IG U R E 4 -6
Hydrogen-bonding pattern of parallel (a) and antiparallel (b) /S-pleated sheet structures.
hair, skin, and nails. These proteins are rich in amino acid
residues that favor the formation of an a-helix. In addi-
tion, consistent with their properties of water insolubility
and cohesive strength, «-keratins are rich in hydropho-
bic amino acid residues and disulfide cross-links. The
a-helices are arranged parallel to their length with all
the N-terminal residues present at the same end. Three
«-helical polypeptides are intertwined to form a left-
handed supercoil, called a
protofibril
(the «-helix itself
is right handed). Eleven protofibrils form a microfibril.
The polypeptides within the supercoil are held together by
disulfide linkages and are also stabilized by van der Waals
interactions between the nonpolar side chains. The num-
ber of disulfide cross-linkages in a-keratins varies from
one source to another. Skin is stretchable because of fewer
cross-links, whereas nails are inflexible and tough because
of many more cross-links.
/
3
-PIeated Sheet
The /1-structure has the amino acids in an extended
confirmation with a distance between adjacent residues
of 0.35 nm (in the a-helix, the distance along the axis is
0.15 nm). The structure is stabilized by intermolecular
hydrogen bonds between the -NH and -CO groups of
adjacent
polypeptide chains. The ^-structure can occur
between separate peptide chains (e.g., silk fibroin) or be-
tween segments of the same peptide chain, where it folds
back upon itself (e.g., lysozyme). Two types of /1-pleated
sheets exist:
parallel
and
antiparallel.
In the parallel sheet
structure, adjacent chains are aligned in the same direc-
tion with respect to N-terminal and C-terminal residues,
whereas in the antiparallel sheet structure, the alignments
are in the opposite directions (Figure 4-6). Some amino
acid residues promote the formation of /
1
-pleated sheets.
For examples, in silk fibroin, which consists almost
entirely of antiparallel /
1
-structures, every other amino
acid is glycine and alanine predominates in the remaining
positions. Thus, one side of the peptide has only H in
the R-position, whereas the other side has predominantly
methyl groups. These small R-groups allow the formation
of stacked /1-pleated sheet structures. The methyls of one
sheet fit into the pleat between the hydrogens of the over-
lying sheet. Thus, layers of silk proteins are associated
but not covalently bonded. Bulky or similarly charged
R-groups that cannot form pleated sheets create regions
of flexibility in the otherwise uniform silk structure.
Disrupting the hydrogen bonds of silk with heat does not
appreciably change the length of the fiber because the
/J-structure is fully extended, but it does break interchain
associations, whereas heat disruption of the intrachain
hydrogen bonds in «-keratin facilitates conversion of
the fiber from the helical to the extended /
1
-structure
(nonsheet).
