Biomedical Engineering Reference
In-Depth Information
tendon. Due to the abundance and ready accessibility of these tissues, they have been frequently used
as a source for the preparation of collagen. This chapter will not review the details of the structure
of the different collagens. The readers are referred to recent reviews for a more in-depth discussion
of this subject (Nimni, 1988; van der Rest et al., 1990; Fukai et al., 1994; Brodsky and Ramshaw,
1997). It is, however, of particular relevance to review some salient structural features of the type I
collagen in order to facilitate the subsequent discussions of properties and its relation to biomedical
applications.
A type I collagen molecule (also referred to as
tropocollagen
) isolated from various tissues has a molec-
ular weight of about 283,000 Da. It is comprised of three left-handed helical polypeptide chains (Figure
6.2a) which are intertwined forming a right-handed helix around a central molecular axis (Figure 6.2b).
Two of the polypeptide chains are identical (α
1
) having 1056 amino acid residues, and the third poly-
peptide chain (α
2
) has 1029 amino acid residues (Miller, 1984). The triple-helical structure has a rise per
residue of 0.286 nm and a unit twist of 108°, with 10 residues in three turns and a
helical pitch
(repeat-
ing distance within a single chain) of 30 residues or 8.68 nm (Fraser et al., 1983). More than 95% of the
amino acids have the sequence Gly-
X
-
Y
. The remaining 5% of the molecules do not have the sequence
Gly-
X
-
Y
and are, therefore, not triple helical. These nonhelical portions of the molecules are located at
the N- and C-terminal ends and are referred to as
telopeptides
(9-26 residues) (Miller, 1984). The whole
molecule has a length of about 280 nm and a diameter of about 1.5 nm and has a conformation similar
to a rigid rod (Figure 6.2c).
The triple-helical structure of a collagen molecule is stabilized by several factors (Figure 6.3): (1) a
tight fit of the amino acids within the triple helix—this geometrical stabilization factor can be appreci-
ated from a space-filling model constructed from a triple helix with Gly-Pro-Hyp sequence (Figure
6.3); (2) the interchain hydrogen bond formation between the backbone carbonyl and amino hydrogen
interactions; and (3) the contribution of water molecules to the interchain hydrogen bond formation.
(a)
0.286 nm
2.86 nm
(b)
α
1
α
1
α
2
(c)
1.5 nm
280 nm
(d)
280 nm
0.4
D
0.6
D
4.4
D
64 nm
FIGURE 6.2
Formation of collagen, which can be visualized as taking place in several steps: (a) single-
chain left-handed helix; (b) three single chains intertwined into a triple stranded helix; (c) a collagen (tropocol-
lagen) molecule; (d) collagen molecules aligned in
D
staggered fashion in a fibril producing overlap and hole
regions.
