Biomineral formation

Bres EF, Steuer P, Voegel J-C, Frank RM, Cuisinier FJG (1993b) Observation of the loss of the hydroxyapatite six-fold symmetry in a human fetal tooth enamel crystal. J Micros 170 147-154 Brown WE (1966) Crystal growth of bone mineral. Clin Orthopaedics 44 205-220 Brown WE, Eidelman N, Tomazic B (1987) Octacalcium phosphate as a prectrrsor in biomineral formation. Adv Dent Res 1 306-313... 

A range of oxides, phosphates, carbonates and elemental nanoparticles can be produced. Of special interest to nanotechnologists are the enzymatically controlled redox changes that result in biomineral formation which are linked to microbial respiration such as dissimilatory metal reduction. This latter process has been shown, for instance, to produce Ag(0) nanoparticles 5-40 pm in size ° (Fig. 2), selenium/selenide/telluride nanospheres and rods (Figs. 3 and 4), Au(0), Pd(0) " as well as Tc(IV) and U(IV) (see also for recent reviews). 

Since some structural and dynamic features of w/o microemulsions are similar to those of cellular membranes, such as dominance of interfacial effects and coexistence of spatially separated hydrophilic and hydrophobic nanoscopic domains, the formation of nanoparticles of some inorganic salts in microemulsions could be a very simple and realistic way to model or to mimic some aspects of biomineralization processes [216,217]. 

The initial stages of iron incorporation requires the ferroxidase sites of the protein. Thereafter the inner surface of the protein shell provides a surface which supplies ligands that can partially coordinate iron but which leave some coordination spheres available for mineral phase anions, thereby enabling the biomineralization process to proceed, with formation of one or more polynuclear ferrihydrite crystallites. Iron is transferred from the ferroxidase sites to the core nucleation sites by the net reaction (Yang et ah, 1998) ... 

As mentioned earlier, biological systems have developed optimized strategies to design materials with elaborate nanostructures [6]. A straightforward approach to obtaining nanoparticles with controlled size and organization should therefore rely on so-called biomimetic syntheses where one aims to reproduce in vitro the natural processes of biomineralization. In this context, a first possibility is to extract and analyze the biological (macro)-molecules that are involved in these processes and to use them as templates for the formation of the same materials. Such an approach has been widely developed for calcium carbonate biomimetic synthesis [13]. In the case of oxide nanomaterials, the most studied system so far is the silica shell formed by diatoms [14]. 

Watanabe, J. and Akashi, M. (2006) Novel biomineralization for hydrogels electrophoresis approach accelerates hydroxyapatite formation in hydrogels. Biomacromolecules, 7, 3008-3011. 

About a quarter of the total body iron is stored in macrophages and hepatocytes as a reserve, which can be readily mobilized for red blood cell formation (erythropoiesis). This storage iron is mostly in the form of ferritin, like bacterioferritin a 24-subunit protein in the form of a spherical protein shell enclosing a cavity within which up to 4500 atoms of iron can be stored, essentially as the mineral ferrihydrite. Despite the water insolubility of ferrihydrite, it is kept in a solution within the protein shell, such that one can easily prepare mammalian ferritin solutions that contain 1 M ferric iron (i.e. 56 mg/ml). Mammalian ferritins, unlike most bacterial and plant ferritins, have the particularity that they are heteropolymers, made up of two subunit types, H and L. Whereas H-subunits have a ferroxidase activity, catalysing the oxidation of two Fe2+ atoms to Fe3+, L-subunits appear to be involved in the nucleation of the mineral iron core once this has formed an initial critical mass, further iron oxidation and deposition in the biomineral takes place on the surface of the ferrihydrite crystallite itself (see a further discussion in Chapter 19). 

Bone and teeth in mammals and bony fishes all rely on calcium phosphates in the form of hydroxyapatite [Ca5(P04)30H]2, usually associated with around 5% carbonate (and referred to as carbonated apatite). The bones of the endoskeleton and the dentin and enamel of teeth have a high mineral content of carbonated apatite, and represent an extraordinary variety of structures with physical and mechanical properties exquisitely adapted to their particular function in the tissue where they are produced. We begin by discussing the formation of bone and then examine the biomineralization process leading to the hardest mineralized tissue known, the enamel of mammalian teeth. 

Finally, it is intriguing that in terms of biomineralization, invertebrates have based their reliance on calcium carbonates, while vertebrates appear to have used almost exclusively calcium phosphate. We say almost, because, while the use of calcium phosphates for biomineralization is an invention of some vertebrates, they still use calcium carbonate for the formation of otoliths4 of the inner ear. It remains to be established if the equivalent of the gene starmaker required for otolith formation in zebrafish has homologues among invertebrates. 

The formation of the solid phase (nucleation, precipitation, crystal growth, biomineralization) ... 

The most important organic components of bone are collagens (mainly type 1 see p.344) and proteoglycans (see p. 346). These form the extracellular matrix into which the apatite crystals are deposited (biomineralization). Various proteins are involved in this not yet fully understood process of bone formation, including collagens and phosphatases. Alkaline phosphatase is found in osteoblasts and add phosphatase in osteoclasts. Both of these enzymes serve as marker enzymes for bone cells. 

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