Biomedical Engineering Reference
In-Depth Information
5.2.2
Components and Characteristics of Enamel Matrix
Proteins
The enamel matrix proteins consist of amelogenin, enamelin, sheathlin, and pro-
teinases. Amelogenins account for about 90% of the enamel protein in the early
stage of enamel formation. The 25 kDa amelogenins in porcine enamel matrix
are cleaved almost immediately after being secreted and split into two fragments,
a hydrophilic C-terminal region and a hydrophobic 20 kDa fragment, which is
further cleaved into 13, 11, and 6 kDa fragments. The amino acid composition
was initially characterized by enrichment in proline, glutamic acid, leucine, and
histidine [
14
]. Since amelogenin has a highly aggregative property and multiple
amelogenin genes, its characterization was a complicated puzzle [
15
]. The amino
acid sequences of amelogenin were determined directly [
16
] or derived from the
sequence of amelogenin cDNA [
17
]. It was a decade after the name “amelogenin”
was bestowed by Eastoe [
14
]. After amelogenin was successfully expressed in an
Escherichia coli
system [
18
], investigation of amelogenin proceeded. The physico-
chemical properties are described in detail elsewhere, for example, solubility [
19
],
aggregation [
20
-
22
], and interaction with HAP [
23
-
25
]. Proteomic and genetic of
amelogenon are also studied [
26
-
28
]. The biology and physicochemical properties
of amelogenin are reviewed by several authors [
29
,
30
].
About 2% of the enamel protein is enamelin, which has a strong affinity for
enamel crystal [
31
,
32
]. It is also processed immediately following secretion produc-
ing intermediate fragments. Enamelin was isolated, purified, and characterized as an
acidic glycoprotein enriched with aspartic acid, glutamic acid, serine, and glycine
[
33
]. More detailed characterization then followed. Enamelin, like amelogenin, is
processed immediately after secretion. Porcine intact enamelin (186 kDa) produces
intermediate products (155, 145, 89, and 32 kDa) that are not stable, except the 32
kDa fragment [
34
-
38
].
The third minor component of the enamel matrix proteins is sheathlin, which
has a distinctive amino acid composition, different from that of amelogenin and
enamelin [
39
,
40
]. Sheathlin distributes in the sheath (or interprismatic space) that
partially separates the rod and inter-rod enamel [
41
].
The enamel matrix is characterized by two specific properties, which are mainly
due to amelogenin: (1) the component changes during enamel formation as both
amelogenin and enamelin degrade into several smaller fragments that are removed
from the matrix; (2) the solubility, aggregation, and interaction with the mineral
phase depend on the molecular weight, pH, temperature, and amelogenin and
inorganic ion concentrations. Amelogenin and enamelin are now known to regulate
enamel crystal formation.