Biology Reference
In-Depth Information
The elastic forces in the fiber are mostly due to entropic forces (Urry and Parker
2002
). As we have stated, elastin is insoluble and is composed of alternating
hydrophobic and hydrophilic sequences. The longest repeating sequence is that of
(GVGVP)
11
although there are other less extensive repeats also present (Sandberg
et al.
1981
; Sandberg et al.
1985
; Urry and Parker
2002
). Despite frequent lysine
cross-links, only about 5% of the total number of residues participate in cross-link
formation. This is very different from rubber, where in the vulcanization process,
all of the monomers are potential cross link targets and thus there is cross link
formation whenever two chains come into contact (Urry and Parker
2002
).
In addition to the large number of hydrophobic residues, there are also very few
charged residues in bovine elastin. The hydrophobic interactions are responsible for
the interactions between chains and that result in parallel-aligned twisted filaments.
The (GVGVP)
n
repeats play an essential role. This structure forms a series of
b
-turns with interactions between the hydrophobic Val
1
to Val
4
. When the temper-
ature is raised, the structure wraps into a
-spiral with hydrophobic spacers between
b
the turns of the
-spirals then further associate hydrophobically to
form filaments (Khaled et al.
1981
; Urry et al.
1989
; Urry and Parker
2002
).
The hydrophobic interaction between turns is what gives the elastic fiber most of
its elastance (Fig.
9.2
). As the fiber is stretched, water molecules intercalate
-spiral. The
b
b
a
b
e
f
c
d
Fig. 9.2
Hydrophobic domains and interactions of tropoelastin
.(a) Structure of GVGVP repeats
showing the series of
b
-turns. (b) Same as a with molecular depiction. (c) When the temperature is
increased, the
-turns form
-spirals which are depicted schematically. (d) Further, more detailed
b
b
-turns represented and the V
1
to V
4
schematic depiction of the
-spirals with the
interactions
b
b
illustrated. (e) Molecular depiction of c and d.(f) Hydrophobic folding of the
-spirals to form
filaments occurs in a cooperative manner (Reproduced with permission from Urry and Parker
2002
.)
b
