Biomineralization: Apatite Protein Interaction - Bioinspiration: From Nano to Micro Scales - page 139

[ 33 ]. It is unclear how this structural change in statherin induced by HAP binding
can influence affects crystal growth. Schwartz reported that the C-terminal domain
of statherin contributes to the inhibition of HAP precipitation [ 34 ].

4.3 Controlling Calcium Phosphate Precipitation

Mahamid et al. demonstrated the existence of a large amount of ACP in the early
stage of bone formation by analyzing the mineral phase of fin bone formation in
zebrafish [ 10 ]. Beniash et al. demonstrated the existence of ACP in the early stage of
mouse incisor enamel formation [ 11 ]. These findings indicate that the first calcium
phosphate that appears in bone and enamel is ACP. The hierarchical complex
structures seen in bone and enamel are thus initially formed by ACP as a building
block, and are organized in a spatiotemporal manner by the aid of proteins in
extracellular matrices produced by osteoblasts or ameloblasts. Interaction between
the ACP and biomolecules is discussed in this chapter.
As described above, since extracellular fluid is supersaturated with respect to
HAP, it is not surprising that deposition takes place ubiquitously in the human
body. However deposition in unsuitable areas is strictly controlled so that hard
tissues are formed properly. For example, acidic biomolecules inhibit deposition of
calcium phosphate in vitro. Acidic functional groups such as phosphoryl, carboxyl,
and sulfate groups bind and chelate calcium ions, which reduce the concentration
of calcium ions and the degree of supersaturation in body fluids. For example,
the C-terminus of amelogenin, which is expressed in the early stage of enamel
development, is proteolytically cleaved resulting in a P148 protein consisting of
148 amino acid residues [ 35 , 36 ]. P148 is a major component of the organic matrix
in enamel; it inhibits the deposition of ACP when it is phosphorylated [ 37 ].
One detailed mechanism of the inhibitory effects of the deposition of cal-
cium phosphate has been proposed. DMP1, an acidic protein with a molecular
weight of 50,000, is a SIBLING protein. Like many other proteins involved
in biomineralization, DMP1 does not have an ordered backbone conformation
[ 38 ]. However, in the presence of Ca 2 C , a secondary structure suitable for self-
assembly is formed. In this formation, the proteins aggregate into microfibrils [ 39 ].
Atomic force microscope (AFM) and transmission electron microscope (TEM)
measurements revealed that complexes were formed by the combining of DMP1
oligomers and calcium phosphate particles. DMP1 oligomers were shown to inhibit
mineral deposition, apparently by binding to the mineral particles and sequestering
them from the external solution [ 40 ]. DMP1 has the ability to bind collagen, and,
in the presence of collagen, accelerates HAP crystal formation [ 41 ]. On the basis
of several observations, He et al. proposed a model in which DMP1 oligomers bind
to calcium phosphate particles, thereby preventing nonspecific deposition. DMP1
binds to suitable areas of the collagen fiber in the extracellular matrix and induces
calcification at specific sites (Fig. 4.1 ,[ 40 ]).