Biomedical Engineering Reference
In-Depth Information
Because glycine is the smallest amino acid with no side chain, it plays a unique
role in fibrous structural proteins. In collagen,
Gly
is required at every third
position, because the assembly of the triple helix puts this residue at the interior
(axis) of the helix, where there is no space for a larger side group than glycine's
single hydrogen atom. For the same reason, the rings of
Pro
and
Hyp
must point
outward. These two amino acids,
Pro
and
Hyp
, help stabilize the triple helix. A lower
concentration of them is required in animals such as fish, whose body temperatures
are lower than those of warm-blooded animals. Proline or hydroxyproline constitute
about one-sixth of the total sequence. With glycine accounting for one-third of the
sequence, this means that approximately half of the collagen sequence is not
glycine, proline, or hydroxyproline.
The element phosphorus is not present in any of the 20 amino acids from which
proteins are made (but is present in DNA, cf. the Hershey-Chase experiments
[202]).
1.3.7
Collagen and its Properties
Collagen belongs to the long fibrous structural proteins. These are main compo-
nents of the ECM that supports most tissues and assures cells structure from the
outside. Collagen is the main protein of CT in animals and the most abundant
protein in mammals, making up about 25% of the total protein content. Thus,
collagen is found in large quantities in tendon, bone, skin, cornea, and cartilage.
Collagen is also found inside certain cells, cf. [127, 128].
The TC or ''collagen macromolecule'' is a subunit of larger collagen aggregates
such as fibrils. It is approximately 300 nm long and 1.5 nm in diameter, made up
of three polypeptide strands (called
α
-chains
), each possessing the conformation
of a left-handed helix. These three left-handed helices are twisted together into a
right-handed triple helix or ''super helix'' [11]. The triple helix is composed of three
polypeptide chains, each with the repeating triplet Gly-X-Y, where X and Y are
frequently proline and hydroxyproline, respectively.
The TC macromolecules are synthesized within fibroblast cells, pass into the
intercellular tissue spaces, and in particular, aggregate at the appropriate places
and time to form fibers. Possibly in all fibrillar collagens if not in all collagens, each
TC triple helix associates into a right-handed super-super-coil, which is referred to
as the
collagen microfibril
.
The molecular conformation of collagen has been determined primarily from an
interpretation of its high-angle X-ray diffraction pattern. Following the pioneering
work [203] of Herzog and Jancke, a number of investigators have attempted to find
the structure of collagen (and of gelatin, which gives similar X-ray photographs).
Astbury showed that there were drastic changes in the diffraction of moist wool
or hair fibers as they were stretched significantly (100%). The data suggested that
the unstretched fibers had a coiled molecular structure with a characteristic repeat
of 5
1
˚
A(
51 nm). Astbury proposed that (i) the unstretched protein molecules
formed a helix (which he called the
.
=
0
.
α
-form
) and (ii) the stretching caused the helix to
