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Chapter 11
The Regulatory Role of Matrix
Proteins in Mineralization of Bone
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FIGURE 11.1 Maturational stage and bone matrix gene expression. Osteoblast cells pass through a series of
maturational stages, each of which can be partially characterized by the bone matrix proteins that they produce.
In addition, osteoclasts also secrete proteins that become incorporated into mineralized matrix.
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FIGURE 11.2 Crystal lattice structure. A portion of the apatite structure is depicted as it would be viewed along
the length (c axis) of the hydroxyapatite crystal, showing the hexagonal arrangement of the Ca 2+ and PO43– ions
about the OH– position.
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FIGURE 11.3 Disaccharide composition of glycosaminoglycans (GAGs). The GAGside chains that are covalently
attached to proteoglycan core proteins are composed of repeating disaccharide units. The composition of the
disaccharides, along with modifications by acetylation, results in the formation of chondroitin sulfate, which is
epimerized to form dermatan sulfate, heparin sulfate, and keratan sulfate. Hyaluronan is the sole GAG the
remains unsulfated and is not covalently lined to core proteins.
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FIGURE 11.4 A representation of the chemical features of the large hyaluronic acid-binding proteoglycan,
aggrecan. CRP, C-reactive protein; CS, chondroitin sulfate; EGF, epidermal growth factor; G1, G2, G3, globular
domains; GAG, glycosaminoglycan: KS, keratan sulfate.
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FIGURE 11.5 The two most abundant proteoglycans present in bone matrix are the small chondroitin
sulfate/dermatan sulfate proteoglycans, decorin, and biglycan. The core protein of each is highly homologous to
a number of proteins due to the presence of a leucine-rich repeat sequence, as shown for both decorin and
biglycan.
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FIGURE 11.6 Alkaline phosphatase in developing bone. By histochemical staining for alkaline phosphatase
activity during development, areas that are destined to become bone, as shown here in developing human
subperiosteal bone, can be clearly illustrated. The fibrous layer (F) of the periosteum is negative, whereas
preosteoblasts (POb) and osteoblasts (Ob) produce high levels of activity. Although a glycoprotein with alkaline
phosphatase activity has been isolated from the bone matrix, it is not easily detected in mineralized matrix (MM)
by this histochemical assay. Source: courtesy of Dr. Paolo Bianco.
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FIGURE 11.7 The chemical characteristics of osteonectin indicate the presence of two α-helical regions at the
amino terminus, along with an ovomucoid like sequence with extensive disulfide bonding, and two EF hand
structures that bind to calcium.
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FIGURE 11.8 Thrombospondin is a disulfide-linked trimer that has globular domains at the amino and carboxy
termini, interconnected by a stalk region. Each of these domains has a number of binding sites for other proteins,
suggesting numerous potential functions in cell-matrix interactions. The cell attachment consensus sequence,
RGD, is in the C-terminal domain; however, its availability depends on the calcium ion concentration, which is
known to affect the conformation of this region. EGF: epidermal growth factor.
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FIGURE 11.9 Exon structure defines the SIBLINGfamily. The exon structures of the six candidate genes for the
SIBLINGfamily are illustrated. Exons are drawn as boxes and introns as connecting lines. Exon 1 is noncoding.
For all but ENAM, exon 2 encodes for the leader sequence plus the first two amino acids of the mature protein.
Exon 3 often contains the consensus sequence of casein kinase II-mediated phosphorylation (SSEE), as does
exon 5. Exon 4 is usually relatively proline rich (PPPP). The last one or two exons encode the vast majority of
the protein (figure not drawn to scale), and always contain the integrin-binding tripeptide, Arg-Gly-Asp (RGD).
The shadowing of exons illustrates those exons known to be involved in splice variants. ENAM is a more
distantly related gene that has two noncoding 5’ regions and is also likely to contain disulfide bonds that other
SIBLINGs do not. BSP: bone sialoprotein; DMP: dentin matrix protein; DSPP: dentin sialophosphoprotein;
MEPE: matrix extracellular phosphoglycoprotein; OPN: osteopontin; ENAM, enamelin. Source: courtesy of Dr.
Larry W. Fisher.
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FIGURE 11.10 The osteopontin molecule is composed of numerous stretches of α helix (depicted as cylinders)
interconnected in several cases by α-pleated sheets, one of which contains the cell attachment consensus
sequence (RGD). A stretch of polyaspartic acid (Poly Asp), along with phosphorylated residues (PO 4), make
osteopontin a highly acidic molecule. Source: adapted from Denhardt and Guo (1993) [167].
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FIGURE 11.11 Sequence analysis of bone sialoprotein predicts the presence of multiple stretches of
polyglutamic acid (Poly-Glu) in the first half of the molecule and tyrosine-rich regions in the amino- and carboxyterminal domains. In the carboxy-terminal region, many of these tyrosines are sulfated. The cell attachment
consensus sequence (RGD) is flanked by such regions at the carboxy terminus of the molecule. The molecule is
composed of ~50% carbohydrate, including a high concentration of sialic acid residues. Glycosylation is
somewhat restricted to the amino terminal 50% of the molecule. Source: adapted from Fisher et al. (1990) [168].
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FIGURE 11.12 Osteocalcin contains two stretches of α helix (depicted as cylinders) and two regions of α-pleated
sheet (arrows). The α-carboxylated residues of glutamic acid in the amino-terminal helix orient the carboxyl
groups to the exterior, thereby conferring calcium ion binding with relatively high affinity. There is one
intramolecular disulfide bridge (C–C) in the middle region of the molecule. Source: adapted from Hauschka and
Carr (1982) [169].
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FIGURE 11.13 Cell-mediated matrix mineralization in developing chicken bone. Electron micrograph showing a
17-day-old embryonic tibia, stained with uranyl acetate and lead citrate. Mineral clusters (C) outside of the
osteoblast (OB) are associated with collagen (thin arrows) and extracellular matrix vesicles (inset). Empty
vesicles (thin arrows) as well as vesicles with mineral are seen. Source: courtesy of Dr. Steven B. Doty.
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