Biomedical Engineering Reference
In-Depth Information
4.2.1
Role of Collagen in Forming Insoluble Matrix
Type 1 collagen determines the shape of bone, dentin, and cementum. Collagen
consists of polypeptides known as
-chain consists of approximately
1,000 amino acids, each with a molecular weight (M
w
) of about 100,000. Collagen
has a high glycine content, which makes up approximately one third of the total
amino acid content. Following glycine is proline, which comprises one fifth of the
total amino acid content. Some proline and lysine residues are hydroxylated after
translation and exist as hydroxyl-proline and hydroxyl-lysine, which are specific
for collagen. Each
'
-chains. One
'
-chain forms a left handed polyproline II type (PPII) helical
coil, and further associates to form right handed triple helix. Each
'
-chain has a
repetitive motif consisting of (Gly-X-Y)
n
in which glycine repeats at every third
amino acid. The structure of the triple helix is maintained by glycine, which has
a small side chain, placed inside the helix. The linkage of adjacent
'
-chains is
stabilized through hydrogen bonds linking the amino and carboxyl groups of the
backbone of the polypeptides [
12
,
13
].
The collagen is secreted into the extracellular matrix as a protocollagen strand
containing propeptides at its N- and C-termini. The strand associates to form procol-
lagen triple helix after post-translation modification of lysine and proline residues.
The propeptide domains are cleaved by proteases, producing tropocollagen triple
helix, which self-assembles into collagen microfibiril. The microfibiril assembles to
be an insoluble collagen fiber. Examination of collagen fibers under an electron
microscope reveals striped patterns at 67 nm intervals. Each interval is called a
“D-period” and arises from the staggered arrangement of the triple helices. Each
space created by this arrangement is called a “gap zone,” and this is the area in
which mineral deposition occurs. It is believed that extracellular matrix proteins
bind to this gap zone and induce mineral deposition [
2
].
'
4.2.2
Role of Amelogenin in Forming Insoluble Matrix
Although the amino acid sequence of human amelogenin is known, amelogenin
from bovine, porcine, and mouse are primarily used for research purposes. Among
these, bovine amelogenin is the longest, with 197 amino acid residues, while porcine
amelogenin is the shortest, with 173 amino acid residues [
14
]. The molecular
weight of amelogenin estimated from the amino acid sequence is 20-22 kDa, with
an isoelectric point of 6.5. Amelogenin can be divided into three domains on
the basis of the properties of the amino acid sequence. The N-terminal domain
consists of 45 amino acid residues, and is known as TRAP (tyrosine-rich amelogenin
peptide). Serine residues that eventually become phosphorylated are also located
in this domain. The proline-rich and hydrophobic middle-core domain has been
reported to be responsible for the formation of the PPII structure [
15
]. The PPII
structure is a mild helix structure that can also be seen in collagen, and is often
