Biomedical Engineering Reference
In-Depth Information
FIGURE 6.3
A space-filling model of the collagen triple helix, showing all the atoms in a 10-residue segment of
repeating triplet sequence (Gly-Pro-Hyp)
n
. The arrow shows an interchain hydrogen bond. The arrowheads iden-
tify the hydroxy groups of hydroxyproline in one chain. The circle shows a hydrogen-bonded water molecule. The
short white lines identify the ridge of amino acid chains. The short black lines indicate the supercoil of one chain.
(Reprinted from
Extracellular Matrix Biochemistry
, In K.A. Piez and A.H. Reddi (Eds.), New York, Piez, K.A.,
Molecular and aggregate structures of the collagens. p. 5, Copyright 1984, with permission from Elsevier.)
The telopeptides are regions where
intermolecular crosslinks
are formed
in vivo
. A common inter-
molecular crosslink is formed between an
allysine
(t he ε-amino group of
lysine
or hydroxy-lysine has
been converted to an aldehyde) of one telopeptide of one molecule and an ε-amino group of a lysine
or
hydroxylysine
in the triple helix or a second molecule (Equation 6.2). Thus, the method commonly
used to solubilize the collagen molecules from crosslinked
fibrils
with
proteolytic enzymes
such as
pep-
sin
removes the telopeptides (cleaves the intermolecular crosslinks) from the collagen molecule. The
pepsin-solubilized collagen is occasionally referred to as
atelocollagen
(Stenzl et al., 1974).
OH
|
Pr
−
CH
−
CH
−
CH
−
CHOHNCH HCHCH r
+
−
−
−
−
−
2
2
2
2
2
2
2
Allysine
Hydroxy
lysine
(6.2)
OH
|
→ −
Pr
CH
−
CH
−
CH
− =− −
CH
NCH HCHCH r
−
−
−
2
2
2
2
2
2
Dehydrohyd
roxylysinonorleucine
Since the presence of hydroxyproline is unique in collagen (
elastin
contains a small amount), the
determination of collagen content in a collagen-rich tissue is readily done by assaying the hydroxypro-
line content.
Collagen does not appear to exist as isolated molecules in the extracellular space in the body. Instead,
collagen molecules aggregate into
fibrils
. Depending on the tissue and age, a collagen fibril varies from
about 50 to 300 nm in diameter with indeterminate length and can easily be seen under electron
microscopy (Figure 6.4). The fibrils are important structural building units for large
fibers
(Figure 6.5).
Collagen molecules are arranged in specific orders both longitudinally and in cross-sectionally, and the
