386
11 Extracellular Structural and Secretory Proteins
[111]. In contrast, the organic matrix of tooth
enamel contains only phosphoserine and small
amounts of acidic amino acids. Enamel, the especially highly mineralized and hard coating of vertebrate teeth, is synthesized and secreted by specialized cells (ameloblasts). The organic matrix
consists of two families of enamel-specific proteins, the amelogenins (22-28 kDa) and enamelins
(40-72 kDa). In reality, the enamelins appear to
be tightly bound blood-plasma proteins with
serum albumin as the main component [122]. The
amelogenins of different mammals are practically
identical in the N-terminal half of their 180amino-acid chain but are very variable in the Cterminal region. Proteins that are immunologically similar to mouse amelogenins have been
detected in the agnathan Eptatretus stoutii [148].
In the mammals, there is only one amelogenin
gene per haploid genome, localized in either the
X or the Y chromosome. Several different
mRNAs are produced from the former by alternative splicing [40]. Enamel contains the largest
hydroxyapatite crystals found in the mammalian
body; at the end of the mineralization process,
the matrix proteins are almost completely reabsorbed. Human and mammalian saliva contains
various proteins which, through calcium binding,
result in the oversaturation of the saliva with
Ca 2 +, the prevention of calcium phosphate precipitation and the stabilization of the hydroxyapatite of the enamel. Amongst them is the
tyrosine-rich statherin [110] and numerous acidic,
proline-rich phosphoglycoproteins (PRPs) which
are encoded by a mUlti-gene family. The sequences of the PRPs show considerable periodicity
and are significantly homologous throughout the
primates and rodents, although they may differ in
length: 100-120 amino acids in humans and
Macaca fascicularis and 200-240 amino acids in
the mouse and rat [5, 61].
1\vo classes of macromolecular components
may be distinguished in the organic matrix of
mollusc shells. About 70-87 % of the soluble proteins are aspartate-rich proteins with a ~-folded
structure and bind Ca 2 + on the charged sidechains. The remainder are serine-rich proteins
which are associated with large quantities of polysaccharide and can also bind calcium [137]. The
calcium metabolism of several mussels (Rangia,
Macrocallista) has a noteworthy feature. The
extrapallial (mantle) fluid has calcium-containing
particles, about 40 nm in diameter, from which
several phosphoproteins of more than 140 kDa
can be isolated. These proteins are rich in aspartate, phosphoserine, phosphothreonine and histidine, and contain up to 10 % of the cross-linking
structure histidinoalanine (Fig. 11.5) [78]. Acidic
matrix proteins have also been found in other
invertebrates, e.g. the foraminifers and echinoderms, but have been little investigated. Surprisingly, antibodies against proteins of the rat incisor
cross-react with the tooth matrix of the sea urchin
Lytechinus variegatus; presumably, common
evolutionally conserved or convergent molecular
structures are present [21, 132].
11.4 Elastic Proteins
All the movements of the animal body rely upon
some sort of bendable or stretchable structure. It
is, therefore, to be expected that all vertebrates
and invertebrates have substances with elastic
properties, i.e. with the capacity to release potential energy conferred during passive stretching
with minimal losses on return to the starting position. Rubber-like materials consist of kinetically
free, randomly folded chains; their elasticity
mainly involves changes in conformation entropy
and not, for example like steel, changes in internal energy resulting from the distortion of atomic
bonds. The chains must be cross-linked to each
other to avoid viscous flowing movements during
deformation. The proteins of animal tissues are
able to meet such physical specifications. So far,
only four types of elastic protein are known from
the whole of the animal kingdom: vertebrate
elastin, insect resilin, abductin from the shellligaments of the mussels, and the elastic proteins
from the arteries of the cephalopod Octopus dofleini. All these proteins characteristically have
small amino acids; one imagines that large,
unwiedly components would only hinder free
movement of the polypeptide chains, and thus
limit the deformability of the protein (Table 11.2). However, this alone does not explain the
elastic properties as there are sequences of small
amino acids in very stable, non-expansive chain
conformations, e.g. those of the collagens and, in
particular, the silk fibroins. Apart from this, the
amino acid composition of the four protein types
mentioned varies so widely that they cannot
really be considered as homologous. Thus, their
similar elastic properties appear to be the result
of convergent evolution [113].
The best-known elastic protein is elastin, which
is found in all vertebrates, except the agnathans,
but not in the invertebrates. Elastins together
with collagens are the most important structural
11 Extracellular Structural and Secretory Proteins
[111]. In contrast, the organic matrix of tooth
enamel contains only phosphoserine and small
amounts of acidic amino acids. Enamel, the especially highly mineralized and hard coating of vertebrate teeth, is synthesized and secreted by specialized cells (ameloblasts). The organic matrix
consists of two families of enamel-specific proteins, the amelogenins (22-28 kDa) and enamelins
(40-72 kDa). In reality, the enamelins appear to
be tightly bound blood-plasma proteins with
serum albumin as the main component [122]. The
amelogenins of different mammals are practically
identical in the N-terminal half of their 180amino-acid chain but are very variable in the Cterminal region. Proteins that are immunologically similar to mouse amelogenins have been
detected in the agnathan Eptatretus stoutii [148].
In the mammals, there is only one amelogenin
gene per haploid genome, localized in either the
X or the Y chromosome. Several different
mRNAs are produced from the former by alternative splicing [40]. Enamel contains the largest
hydroxyapatite crystals found in the mammalian
body; at the end of the mineralization process,
the matrix proteins are almost completely reabsorbed. Human and mammalian saliva contains
various proteins which, through calcium binding,
result in the oversaturation of the saliva with
Ca 2 +, the prevention of calcium phosphate precipitation and the stabilization of the hydroxyapatite of the enamel. Amongst them is the
tyrosine-rich statherin [110] and numerous acidic,
proline-rich phosphoglycoproteins (PRPs) which
are encoded by a mUlti-gene family. The sequences of the PRPs show considerable periodicity
and are significantly homologous throughout the
primates and rodents, although they may differ in
length: 100-120 amino acids in humans and
Macaca fascicularis and 200-240 amino acids in
the mouse and rat [5, 61].
1\vo classes of macromolecular components
may be distinguished in the organic matrix of
mollusc shells. About 70-87 % of the soluble proteins are aspartate-rich proteins with a ~-folded
structure and bind Ca 2 + on the charged sidechains. The remainder are serine-rich proteins
which are associated with large quantities of polysaccharide and can also bind calcium [137]. The
calcium metabolism of several mussels (Rangia,
Macrocallista) has a noteworthy feature. The
extrapallial (mantle) fluid has calcium-containing
particles, about 40 nm in diameter, from which
several phosphoproteins of more than 140 kDa
can be isolated. These proteins are rich in aspartate, phosphoserine, phosphothreonine and histidine, and contain up to 10 % of the cross-linking
structure histidinoalanine (Fig. 11.5) [78]. Acidic
matrix proteins have also been found in other
invertebrates, e.g. the foraminifers and echinoderms, but have been little investigated. Surprisingly, antibodies against proteins of the rat incisor
cross-react with the tooth matrix of the sea urchin
Lytechinus variegatus; presumably, common
evolutionally conserved or convergent molecular
structures are present [21, 132].
11.4 Elastic Proteins
All the movements of the animal body rely upon
some sort of bendable or stretchable structure. It
is, therefore, to be expected that all vertebrates
and invertebrates have substances with elastic
properties, i.e. with the capacity to release potential energy conferred during passive stretching
with minimal losses on return to the starting position. Rubber-like materials consist of kinetically
free, randomly folded chains; their elasticity
mainly involves changes in conformation entropy
and not, for example like steel, changes in internal energy resulting from the distortion of atomic
bonds. The chains must be cross-linked to each
other to avoid viscous flowing movements during
deformation. The proteins of animal tissues are
able to meet such physical specifications. So far,
only four types of elastic protein are known from
the whole of the animal kingdom: vertebrate
elastin, insect resilin, abductin from the shellligaments of the mussels, and the elastic proteins
from the arteries of the cephalopod Octopus dofleini. All these proteins characteristically have
small amino acids; one imagines that large,
unwiedly components would only hinder free
movement of the polypeptide chains, and thus
limit the deformability of the protein (Table 11.2). However, this alone does not explain the
elastic properties as there are sequences of small
amino acids in very stable, non-expansive chain
conformations, e.g. those of the collagens and, in
particular, the silk fibroins. Apart from this, the
amino acid composition of the four protein types
mentioned varies so widely that they cannot
really be considered as homologous. Thus, their
similar elastic properties appear to be the result
of convergent evolution [113].
The best-known elastic protein is elastin, which
is found in all vertebrates, except the agnathans,
but not in the invertebrates. Elastins together
with collagens are the most important structural