Chemistry Reference
In-Depth Information
FIGURE 19.12
Model for magnetosome formation.
within the vesicles by transmembrane iron transporters and the invaginations/vesicles are assembled into a chain
with the help of MamJ and MamK proteins. In the third and final step, iron is transformed into highly ordered
magnetite crystals within the magnetosome membrane with the triggering of magnetite crystal nucleation by
a number of tightly bound magnetosome proteins. This requires strict control of iron supersaturation, pH, and
redox potentials.
It is striking to see the vast difference in the degree of complexity required to carry out the biomineralisation of
magnetite in magnetosomes when we compare it to the relative simplicity of that of ferritin.
Calcium-Based Biominerals e Calcium Carbonates in Ascidians and Molluscs
Biominerals based on calcium carbonate are found in a very large number of organisms. The shells of molluscs are
among the most abundant biogenic minerals, and are composed of 95%
99% calcium carbonate crystal and less
than 5% organic matrix. Shell layers are formed from calcium carbonate as aragonite or calcite crystals corre-
sponding to different mineral textures (also called microstructures). The organic matrix of the shell is composed of
proteins and polysaccharides, which are thought to direct the formation of the calcium carbonate crystal and thus
are responsible for the extraordinary properties of the shell. For example, nacre, the aragonite layer of the shell,
exhibits a fracture resistance 3000 times higher than that of abiotic aragonite. The different crystal polymorphisms
and microstructures of the layers are controlled by proteins secreted from outer epithelial cells in different regions
of the mantle.
Figure 19.13
shows the molecular correspondence at the inorganic
e
e
organic interface in the nacre
shell layer of Nautilus repertus. There is a close geometric match between the periodicity of the protein
-sheet
and the lattice spacings of the aragonite, and it has been suggested that there is molecular complementarity
between Ca atoms in the aragonite and aspartic acid residues organised in the sequence Asp-X-Asp (where X
b
¼
a
neutral residue) along the
-sheet.
Ascidians (Tunicates) are a class of marine organisms (including sea squirts) which have a characteristic tough
covering, called a tunic. The tunic is composed of cellulose and contains small calcium-containing 'spicules' in
a wide variety of shapes (
Figure 19.14
): they are thought to contribute to the stiffness of the tissues. They are
formed in close association with cells called sclerocytes, which secrete a very tough enveloping organic layer as
well as in forming the mineralised spicule. The two spicules shown in
Figure 19.7
from the tunicate P. pachy-
dermatina both contain an unusual and unstable form of calcium carbonate called amorphous calcium carbonate
(ACC) together with the organic envelope. The knobbed or “dogbone” tunic spicules also contain an overlay of
calcite. Addition of macromolecules extracted from the calcite layer of these spicules speeds up formation of
b
