Chemistry Reference
In-Depth Information
the Golgi, the individual molecules appear to be aligned in loose bundles with the same
axial orientation. The procollagen molecules within the secretory vesicles retain the bulky
propeptide extensions at both ends and these inhibit fibrillar packing within the vesicle. The
parallel, aligned collagen aggregates are similar to, but less densely packed than the
segment-long-spacing (SLS) form (Bruns et al
.
1979; Hulmes et al. 1983). The secretory
process has been described at the electron microscopic level by several beautiful studies
(Weinstock and Leblond 1974; Birk and Linsenmeyer 1994; Prockop and Hulmes 1994)
.
The mineralized tissues appear to handle the exocytosis of the procollagen from the
odontoblasts and osteoblasts quite differently from the soft tissue fibroblasts. According
to Birk and Linsenmeyer (1994), in tendon the contents of the vesicles are exocytosed
into confined spaces in extracytoplasmic channels where they fuse or merge into
filaments within the restricted channel space. Osteoblasts and odontoblasts do not show
the kind of channels demonstrated in tendon, rather the secretory vesicles appear to
release their contents directly into the open pericellular space (Weinstock and Leblond
1974; Rabie and Veis 1995). There, the procollagen molecules are trimmed and assemble
into fibrils. It is not clear whether the aligned packets of parallel procollagen molecules
aggregate to form fibrils directly or disaggregate and then reassemble into fibrils.
Fibrillogenesis
The procollagen molecules, which can be readily isolated from fibroblast tissue culture
supernatant, are very soluble so long as they retain both N- and C-propeptide domains.
In
vivo
, different procollagen peptidases specific for the N- and C- propeptides are present and
cleave the propeptides from the main triple helical section. The order of cleavage is
important, it is not possible to form filaments with collagens containing the C-propeptide
(Kadler et al. 1987, 1990), and thus, the C-propeptide must be excised first. Molecules with
the N-propeptide attached still can form axially extended structures (Lenaers et al. 1971;
Pierard et al. 1987), as seen in Dermatosparaxis (Levene 1966; Lenaers et al. 1971)
,
but the
aggregation is generally restricted to thin sheets in staggered arrays, with the bulky N-
propeptides on either side of the collagen sheets (Hulmes et al. 1989)
.
When both propeptides have been cleaved one is left with rod-like collagen I
monomers with an uninterrupted triple helical segment that consists of 1014 residues, 338
GXY triplets in each polypeptide chain, trimmed by the short telopeptide regions at each
end (see Fig. 7). The molecules are about 296 nm in length and 1.5 nm in diameter. The
type I monomers are semi-flexible as clearly demonstrated by electron micrographs of
rotary shadowed collagen monomers (Fig. 9) (Veis 1982; Hofmann et al. 1984), by
viscoelasticity measurements (Nestler et al.1983; Amis et al.1985), and by electric
birefringence and dynamic light scattering solution measurements (Bernengo et al. 1983).
The electron microscopy and viscoelasticity studies, in particular, demonstrated that there
were several points of higher flexibility. The major triple helix stability arises from GPP
and GPO rich regions (Kuznetsova and Leikin 1999), while sequence regions with a
lower content of P and O are more flexible (Malone and Veis 2003). These regions of
lesser stability are of crucial importance for the collagen structure. They represent the
sites of helix region-telopeptide interactions that are most important for establishing the
axial repeat order in the fibril and the regions where cross-linkages can form. When both
propeptides are properly excised, the collagen forms fibrillar structures in which all of the
molecules are essentially axially oriented. The
in vitro
assembly of collagen monomers is
a path dependent process. While the detailed mechanism of assembly is debatable (Veis
and George 1994; Kuznetsova and Leikin 1999) the result is the formation of fibrils
indistinguishable in packing arrangement from those formed
in vivo
.
In vitro
the kinetics
of assembly and fidelity of the packing order are crucially dependent upon the intactness
of the telopeptide regions.
