Page 46 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: ZBU Final Pages
Encyclopedia of Physical Science and Technology EN002F-55 May 22, 2001 21:6

134 Bioinorganic Chemistry

ferritin subunits assemble to form a large, hollow, spheri- 4. Cellular processing is the large-scale cellular activ-
cal ball where iron can be stored in the form of iron oxide. ity following the initial mineralization. This process can
2. Interfacial molecular recognition involves the con- involve the proper positioning/placement of the mineral
trolled nucleation of inorganic structures from aqueous so- or assembly of several minerals to give the final higher
lution confined within the organic structure made in step 1. ordered architecture with elaborate properties. As an ex-
The general view of this step is that it occurs at functional- ample, caccolith scales are a result of the transport of indi-
ized surfaces within the structure. These surfaces serve to vidual calcite crystals across the cell membrane of marine
initiate and accelerate the growth of inorganic clusters by algae and their extracellular assembly.
lowering the activation barrier in much in the same way
that an enzyme accelerates a reaction.
C. Ferritin
3. Vectorial regulation is the assembly of the mineral
phase through control of direction and size of crystal Perhaps the best-understood process of biomineralization
growth and termination. This regulation can be brought is the storage of iron within ferritin. Mammalian ferritins
about by rigid formation of the mineralization structure consist of a protein shell made up of 24 individual pep-
mentioned in step 1. It also can be brought about by dy- tide subunits (Fig. 10). The resulting structure is roughly
namic forces during the mineralization process. a hollow sphere where iron can be stored as ferric oxide.

FIGURE 10 The iron storage protein ferritin, a subunit, and the fourfold and threefold channels. Reproduced by
permission from Berg, J., and Lippard, S. J. (1994) “Principles of Bioinorganic Chemistry,” University Science Books,
Mill Valley, CA.