Biomedical Engineering Reference
In-Depth Information
•
Collagen III
- reticulate (main component of reticular fibers), commonly found
alongsidetypeI;
•
Collagen IV
- forms bases of cell basement membrane.
Type I collagen is the most abundant protein in human body, and it helps to
maintain the integrity of many tissues
via
its interactions with cell surfaces, other
ECM molecules, and growth and differentiation factors. Nearly 50 molecules have
been found to interact with type I collagen, and for about half of them, binding sites
on this collagen have been elucidated. In addition, over 300 mutations in type I
collagen, associated with human CT disorders, have been described.
The type I collagen is the main component of the bone; it is present in the scar
tissue, the end product when the tissue heals by repair. It is found in tendons, skin,
artery walls, the endomysium of myofibrils, fibrocartilage, and the organic part of
bones and teeth.
The type II collagen is the basis for articular cartilage and hyaline cartilage.
It makes up 50% of all proteins in the cartilage and 85-90% of collagen of
articular cartilage. The fibrillar network of collagen II allows cartilage to entrap the
proteoglycan aggregate as well as provide tensile strength to the tissue. Type II is
present in small amounts, with salts, sugars, and vitrosin, in vitreous humor of the
eye [222-225].
1.3.8
Geometry of Triple Helix
The triple helix is a unique secondary structural motif that is primarily found within
the collagens, and a distinctive feature of collagen is the regular arrangement of
amino acids in each of the three chains of collagen subunits.
Coxeter [231, 232] suggested an extension of the concept of a regular polygon.
A regular polygon as usually defined is a cycle of vertices
...
1, 2, 3,
...
and
edges
which is obtained from a single point by repeated action of a
rotation. Coxeter's extension replaces ''rotation'' by the more general ''isometry''
(distance-preserving transformation). A screw transformation generates a helical
polygon (or polygonal helix), an infinite sequence of vertices
...
12, 23,
...
,and
edges joining consecutive vertices. A Coxeter helix is a polygonal helix such that
every set of four consecutive vertices forms a regular tetrahedron. This produces a
twisted rod of tetrahedra, the Boerdijk-Coxeter (B-C) helix (Figures 1.35 and 1.36).
Helices and dense packing of spherical objects are two closely related problems.
By stacking regular tetrahedral along one direction, one obtains a configuration
called the
Bernal
or
B-C helix
. Also, the name tetrahelix given to the chain of
tetrahedra by Fuller is used, cf. also [20, 236, 237].
The construction of helix is as follows: to one face of the tetrahedron, the next
tetrahedron is glued, and this process of gluing new tetrahedron is continued, with
the condition that at one vertex six triangular faces meet (Figure 1.36). The chain of
tetrahedra built in such a manner is not periodic because of incommensurability
between the distances separating the centers of neighboring tetrahedra and the
pitch of the three helices.
...
−
1, 0, 1, 2,
...
