Biomineralisation - Biological Inorganic Chemistry - page 374

FIGURE 19.16 Hierarchies of structure in the femur. (From Nyman, Reyes, & Wang, 2005 .)

biomineralisation process. Collagen is secreted by bone-forming cells, osteoblasts, as a precursor, procollagen,
a helical rod made up of three intertwining polypeptide chains ( Figure 19.17 ) . It has a typical amino acid

FIGURE 19.17 After cleavage of the terminal ends, the collagen molecule begins to assemble into a fibril via enzymatic cross-links. (From
Nyman et al., 2005 . )

sequence, with glycine residues in almost every third position, and frequent occupancy of the second and third
positions by proline and hydroxyproline, respectively (the latter introduced by the action of the Fe 3 þ - and
ascorbate-dependent prolyl hydroxylase). After the amino- and carboxyl-terminal ends have undergone proteo-
lytic cleavage, collagen begins to assemble into a collagen fibril, some 300 nm long and 1.2 nm in diameter,
stabilised by cross-links between hydroxylysine residues (introduced by the copper-dependent enzyme lysyl
hydroxylase). Other types of cross-link can also be formed.
The mineral phase constitutes about 43% of the volume of bone and mostly contains calcium and phosphate,
with small, but highly significant amounts of carbonate (and a few other impurities). The bone mineral is not
hydroxyapatite, but rather can be classified as carbonated apatite [Ca 5 (PO 4 )CO 3 ] 3 . Within the mammalian
skeleton, bone mineralisation depends on the organisation of the cross-linked collagen network. Initially, water
fills the void space within the organic framework of the collagen matrix of the osteoid. Crystal nucleation occurs