210
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
because the number and order of the cysteines in
such metal-binding proteins are largely determined by functional considerations, convergent
evolution cannot be excluded, and the apparent
homology between the proteins of Drosophila
and Neurospora, for example, and those of the
vertebrates is difficult to confirm. The attempt to
find a common central region in all these proteins
has also failed to demonstrate unambiguous relationships [188]. The first invertebrate metallothioneins to be characterized closely were the isoforms MT-l and MT-2 from the hepato-pancreas
of the crab Scylla serrata. The chains of 58 and
57 amino acids contain 18 cysteines in two clusters, each of which binds three metal ions; the
two forms agree 83 % overall, and MT-2 of Scylla
is 47 % similar to human MTII (Fig.5.4a). In
contrast to the mammals, the Scylla MTs do not
have an N-acetyl group blocking the N-terminus
[142]. Drosophila melanogaster has two MT genes
which code for very different sequences. The MT
encoded by the gene Mtn includes 10 cysteine
residues amongst its 40 amino acids, and the Mto
product has 12 cysteines among 43 amino acids.
The genes have only 11 amino acids in common,
of which 8 are cysteines (Fig. 5.4a). The Mto gene
is expressed from early in embryogenesis until the
third larval stage, and Mtn is expressed from late
embryogenesis (12-15 hours) until the adult
stage [235]. Individuals with MT gene duplications have been found in natural Drosophila
populations from several continents; these flies
produce more MT mRNA and have increased
cadmium and copper tolerance [138, 156].
During adaptation to high cadmium concentrations, the nematode Caenorhabditis elegans produces two cysteine-rich, MT-like proteins of 62
and 74 amino acids; however, their sequences
have no significant homology to the mammalian
MTs [113, 237]. A cysteine-rich protein with just
as little homology to mammalian MTs has also
been sequenced via the cDNA from the sea
urchin Strongylocentrotus purpuratus (Fig. 5.4)
[188, 189]. Finally, there are small, cadmiumbinding glycoproteins in the hepato-pancreas of
the sea-snail Buccinum tenuissimum and the terrestrial snails Helix pomatia, Arianta arbustorium
and Cepaea hortensis [54, 212].
5.11 Ferritins
Under the conditions predominating inside organisms, iron exists almost entirely in the trivalent
form; Fe H ions, however, have such strong
hydrolytic properties that they precipitate as insoluble iron (III) hydroxide at concentrations above
10- 17 molll. To retain essential iron in a soluble
form it is bound to specific proteins. Iron transport in vertebrates is carried out by transferrins
found in blood serum (serotransferrin), in milk
and other secretions (Iactotransferrin), and in egg
albumen (ovotransferrin or conalbumin). These
proteins were referred to in Section 5.3.1. The
storage forms of iron are the ferritins and
the structurally less well-defined haemosiderin.
About 0.54 mmol (30 mg) iron is set free daily in
human macrophages following the degradation of
erythrocytes, and it is stored initially as ferritin.
Iron which is slowly remobilized from the ferritin
is bound to the apotransferrin of blood plasma,
transported to the erythroblasts, and the cycle
repeated. Some of the ferritins found in vertebrate cells have special functions, e.g. in the
degradation of erythrocytes in the macrophages
or as iron reserves in the liver, whereas others are
simply involved in iron metabolism in the cell.
The ferritin concentration can correspondingly
vary by orders of magnitude. The ferritins are
apparently ubiquitous, having been detected in
many invertebrates, in higher plants and fungi
(phyto- and mycoferritins), and in bacteria
[52, 253].
In mammals, ferritins are found at especially
high concentrations in the spleen, liver and bone
marrow; the first to be investigated in detail was
that from horse spleen. The iron-free apoferritins
are hollow spheres of about 480 kDa, with an
external diameter of 12 nm and walls 2 nm thick;
they are made up of 24 subunits of about 20 kDa.
The inner space consists of eight pockets for iron
uptake. Loading of the apoferritins occurs when
Fe2+ ions pass through channels in the protein
membrane, are oxidized to Fe 3 + and form the
crystalline nucleus of iron (III) hydroxide; the
simultaneous storage of phosphate ions leads to
disturbance of the crystal structure. Each ferritin
molecule can incorporate up to 4500 iron atoms,
corresponding to an iron content of 30 %; normally, the iron content lies between 10 and 20 % ,
according to the tissue, with higher values, for
example, in the spleen and liver, and lower values
in heart muscle. The remobilization of the ferritin
iron is the result of reduction or chelate formation
[52,253].
The ferritin subunit is a polypeptide of 172183 amino acids, the chain of which is subdivided
into two bundles of a-helices with a long connecting segment. Human mRNAs for two types of
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
because the number and order of the cysteines in
such metal-binding proteins are largely determined by functional considerations, convergent
evolution cannot be excluded, and the apparent
homology between the proteins of Drosophila
and Neurospora, for example, and those of the
vertebrates is difficult to confirm. The attempt to
find a common central region in all these proteins
has also failed to demonstrate unambiguous relationships [188]. The first invertebrate metallothioneins to be characterized closely were the isoforms MT-l and MT-2 from the hepato-pancreas
of the crab Scylla serrata. The chains of 58 and
57 amino acids contain 18 cysteines in two clusters, each of which binds three metal ions; the
two forms agree 83 % overall, and MT-2 of Scylla
is 47 % similar to human MTII (Fig.5.4a). In
contrast to the mammals, the Scylla MTs do not
have an N-acetyl group blocking the N-terminus
[142]. Drosophila melanogaster has two MT genes
which code for very different sequences. The MT
encoded by the gene Mtn includes 10 cysteine
residues amongst its 40 amino acids, and the Mto
product has 12 cysteines among 43 amino acids.
The genes have only 11 amino acids in common,
of which 8 are cysteines (Fig. 5.4a). The Mto gene
is expressed from early in embryogenesis until the
third larval stage, and Mtn is expressed from late
embryogenesis (12-15 hours) until the adult
stage [235]. Individuals with MT gene duplications have been found in natural Drosophila
populations from several continents; these flies
produce more MT mRNA and have increased
cadmium and copper tolerance [138, 156].
During adaptation to high cadmium concentrations, the nematode Caenorhabditis elegans produces two cysteine-rich, MT-like proteins of 62
and 74 amino acids; however, their sequences
have no significant homology to the mammalian
MTs [113, 237]. A cysteine-rich protein with just
as little homology to mammalian MTs has also
been sequenced via the cDNA from the sea
urchin Strongylocentrotus purpuratus (Fig. 5.4)
[188, 189]. Finally, there are small, cadmiumbinding glycoproteins in the hepato-pancreas of
the sea-snail Buccinum tenuissimum and the terrestrial snails Helix pomatia, Arianta arbustorium
and Cepaea hortensis [54, 212].
5.11 Ferritins
Under the conditions predominating inside organisms, iron exists almost entirely in the trivalent
form; Fe H ions, however, have such strong
hydrolytic properties that they precipitate as insoluble iron (III) hydroxide at concentrations above
10- 17 molll. To retain essential iron in a soluble
form it is bound to specific proteins. Iron transport in vertebrates is carried out by transferrins
found in blood serum (serotransferrin), in milk
and other secretions (Iactotransferrin), and in egg
albumen (ovotransferrin or conalbumin). These
proteins were referred to in Section 5.3.1. The
storage forms of iron are the ferritins and
the structurally less well-defined haemosiderin.
About 0.54 mmol (30 mg) iron is set free daily in
human macrophages following the degradation of
erythrocytes, and it is stored initially as ferritin.
Iron which is slowly remobilized from the ferritin
is bound to the apotransferrin of blood plasma,
transported to the erythroblasts, and the cycle
repeated. Some of the ferritins found in vertebrate cells have special functions, e.g. in the
degradation of erythrocytes in the macrophages
or as iron reserves in the liver, whereas others are
simply involved in iron metabolism in the cell.
The ferritin concentration can correspondingly
vary by orders of magnitude. The ferritins are
apparently ubiquitous, having been detected in
many invertebrates, in higher plants and fungi
(phyto- and mycoferritins), and in bacteria
[52, 253].
In mammals, ferritins are found at especially
high concentrations in the spleen, liver and bone
marrow; the first to be investigated in detail was
that from horse spleen. The iron-free apoferritins
are hollow spheres of about 480 kDa, with an
external diameter of 12 nm and walls 2 nm thick;
they are made up of 24 subunits of about 20 kDa.
The inner space consists of eight pockets for iron
uptake. Loading of the apoferritins occurs when
Fe2+ ions pass through channels in the protein
membrane, are oxidized to Fe 3 + and form the
crystalline nucleus of iron (III) hydroxide; the
simultaneous storage of phosphate ions leads to
disturbance of the crystal structure. Each ferritin
molecule can incorporate up to 4500 iron atoms,
corresponding to an iron content of 30 %; normally, the iron content lies between 10 and 20 % ,
according to the tissue, with higher values, for
example, in the spleen and liver, and lower values
in heart muscle. The remobilization of the ferritin
iron is the result of reduction or chelate formation
[52,253].
The ferritin subunit is a polypeptide of 172183 amino acids, the chain of which is subdivided
into two bundles of a-helices with a long connecting segment. Human mRNAs for two types of