272
7 Respiratory Pigments
with subunits of other Arcidae haemoglobins.
Although the polymeric Barbatia haemoglobins
are the largest known intracellular haemoglobins,
they have a relatively low Hill coefficient of
n = 1.8 [108, 158]. Of the erythrocyte haemoglobins from the other mussel families, only those of
the genus Calyptogena (Vesicomyidae) have been
investigated. The famous C. magnifica of the hot
hydrothermal vents possesses one tetrameric haemoglobin, whereas C. soyoae from cooler deepsea regions has two homodimeric haemoglobins.
The subunits of C. soyoae show 42 % sequence
agreement with each other but only 12-20 % with
other mollusc haemoglobins; they therefore
represent a very primitive type of globin. The distal E7-His of Hb I has been replaced by glutamine [159].
The molluscs are the only animal phylum in
which are present not only monomeric myoglobins (of about 16 kDa) but also dimers of twice
the size. The myoglobins of the shell-closing
muscle of the mussels Mercenaria mercenaria and
Saxidomus nuttali are monomeric. Monomeric
myoglobins are also found in the radula muscles
of opisthobranchs (e.g. Aplysia) , pulmonates
(e.g. Siphonaria, Helisoma) and certain pro sobranchs (Patella, Nerita, Lunatia, Polinices); other
prosobranchs have dimeric myoglobins (Buccinum, Busycon, Littorina, Nassa). Dimeric and
monomeric myoglobins exist side by side in all
examined Polyplacophora, and the amino acid
composition is much more similar within each of
the two classes than between the monomers and
dimers of the same species. The amino acid
sequences are known for the monomeric myoglobins of Aplysia kurodai, A. limacina and Dolabella auricularia and for the dimers of Busycon
canaliculatum and Cerithidea rhizophorarum. All
monomeric myoglobins lack the distal E7-His but
it is present in the dimeric forms [156, 163]. Several of the dimeric myoglobins (in Amaurochiton,
Nassa, Buccinum) show weak cooperativity
(n = 1.2-1.5); however, this is more the result of
a dissociation equilibrium between monomers
and dimers than of real homotropic interactions;
the dimeric myoglobin of Busycon is not cooperative [22, 163].
7.2.3 Haemoglobins of the Crustaceans
The typical respiratory pigment of the higher
crabs (Malacostraca) is haemocyanin; haemoglobins in the crustaceans are restricted to the Copepoda, Cirripedia, Ostracoda, Phyllopoda and
Anostraca. Detailed investigations have been carried out on just a few species of the latter two
groups; here, haemoglobins are always found dissolved in the haemolymph and their structure in
the Anostraca is completely different to those of
the three subgroups of the Phyllopoda, viz.
Notostraca, Conchostraca and Cladocera. In the
cirripede Briarosaccus callosus, a parasite on
brachyurans, there is a heterogenous mixture
of haemoglobins with molecular masses of
250-4000 kDa, which appear on SDS gels under
reducing conditions as subunits of 17-19 kDa;
they have no similarity to any other arthropod
haemoglobin [108]. Intracellular haemoglobins
are also found in the crustaceans, e.g. in muscle,
nerve and fat cells and in the eggs of the water
flea Daphnia [174]. Three haemoglobins of about
260 kDa are present in the brine shrimp Artemia
salina (Anostraca) and these differ in some functional characteristics; two are homodimers and
the third is heterodimeric. The 130-kDa subunits
consist of eight domains (E1 to E8) that are all
different (the sequence agreement between pairs
is 17-38 %) but display about 20 % similarity with
vertebrate haemoglobins. The residues CD1-Phe
and F8-His are present in all nine domains that
have been sequenced so far [167]. Their secondary and tertiary structures are clearly similar to
those of vertebrate globins. The molecules appear
in electron micrographs as two superimposed
disks, and in bisexual populations these haemoglobins are highly polymorphic. The related species Parartemia zietziana and Streptocephalus torvicornis also have haemoglobins of this type.
The haemoglobins of the Phyllopoda have very
variable molecular masses. The largest are found
in the Notostraca: for example, about 800 kDa in
Lepidurus apus and about 600 kDa in Triops longicaudatus. Values between 420 and 700 kDa are
reported for the haemoglobins of various Daphnia species (Cladocera); that of D. magna is
490 kDa and appears in electron micrographs as
two superimposed octagons with a diameter of
14 nm. The haemoglobin of Cyzicus hierosolymitanus (Conchostraca) is a double pentagon with a
diameter of 13 nm and a mass of 280 kDa. Denaturation of all these haemoglobins produces subunits of 30-34 kDa. The iron and haem contents
correspond to the usual values of 15-17 kDa per
haem, i.e. the subunits apparently have two
haem-bearing domains. The native haemoglobin
molecules are composed of 10-24 such subunits
which, according to electron microscope studies,
are organized in two superimposed rings. SDS
electrophoresis of the haemoglobins of Daphnia
7 Respiratory Pigments
with subunits of other Arcidae haemoglobins.
Although the polymeric Barbatia haemoglobins
are the largest known intracellular haemoglobins,
they have a relatively low Hill coefficient of
n = 1.8 [108, 158]. Of the erythrocyte haemoglobins from the other mussel families, only those of
the genus Calyptogena (Vesicomyidae) have been
investigated. The famous C. magnifica of the hot
hydrothermal vents possesses one tetrameric haemoglobin, whereas C. soyoae from cooler deepsea regions has two homodimeric haemoglobins.
The subunits of C. soyoae show 42 % sequence
agreement with each other but only 12-20 % with
other mollusc haemoglobins; they therefore
represent a very primitive type of globin. The distal E7-His of Hb I has been replaced by glutamine [159].
The molluscs are the only animal phylum in
which are present not only monomeric myoglobins (of about 16 kDa) but also dimers of twice
the size. The myoglobins of the shell-closing
muscle of the mussels Mercenaria mercenaria and
Saxidomus nuttali are monomeric. Monomeric
myoglobins are also found in the radula muscles
of opisthobranchs (e.g. Aplysia) , pulmonates
(e.g. Siphonaria, Helisoma) and certain pro sobranchs (Patella, Nerita, Lunatia, Polinices); other
prosobranchs have dimeric myoglobins (Buccinum, Busycon, Littorina, Nassa). Dimeric and
monomeric myoglobins exist side by side in all
examined Polyplacophora, and the amino acid
composition is much more similar within each of
the two classes than between the monomers and
dimers of the same species. The amino acid
sequences are known for the monomeric myoglobins of Aplysia kurodai, A. limacina and Dolabella auricularia and for the dimers of Busycon
canaliculatum and Cerithidea rhizophorarum. All
monomeric myoglobins lack the distal E7-His but
it is present in the dimeric forms [156, 163]. Several of the dimeric myoglobins (in Amaurochiton,
Nassa, Buccinum) show weak cooperativity
(n = 1.2-1.5); however, this is more the result of
a dissociation equilibrium between monomers
and dimers than of real homotropic interactions;
the dimeric myoglobin of Busycon is not cooperative [22, 163].
7.2.3 Haemoglobins of the Crustaceans
The typical respiratory pigment of the higher
crabs (Malacostraca) is haemocyanin; haemoglobins in the crustaceans are restricted to the Copepoda, Cirripedia, Ostracoda, Phyllopoda and
Anostraca. Detailed investigations have been carried out on just a few species of the latter two
groups; here, haemoglobins are always found dissolved in the haemolymph and their structure in
the Anostraca is completely different to those of
the three subgroups of the Phyllopoda, viz.
Notostraca, Conchostraca and Cladocera. In the
cirripede Briarosaccus callosus, a parasite on
brachyurans, there is a heterogenous mixture
of haemoglobins with molecular masses of
250-4000 kDa, which appear on SDS gels under
reducing conditions as subunits of 17-19 kDa;
they have no similarity to any other arthropod
haemoglobin [108]. Intracellular haemoglobins
are also found in the crustaceans, e.g. in muscle,
nerve and fat cells and in the eggs of the water
flea Daphnia [174]. Three haemoglobins of about
260 kDa are present in the brine shrimp Artemia
salina (Anostraca) and these differ in some functional characteristics; two are homodimers and
the third is heterodimeric. The 130-kDa subunits
consist of eight domains (E1 to E8) that are all
different (the sequence agreement between pairs
is 17-38 %) but display about 20 % similarity with
vertebrate haemoglobins. The residues CD1-Phe
and F8-His are present in all nine domains that
have been sequenced so far [167]. Their secondary and tertiary structures are clearly similar to
those of vertebrate globins. The molecules appear
in electron micrographs as two superimposed
disks, and in bisexual populations these haemoglobins are highly polymorphic. The related species Parartemia zietziana and Streptocephalus torvicornis also have haemoglobins of this type.
The haemoglobins of the Phyllopoda have very
variable molecular masses. The largest are found
in the Notostraca: for example, about 800 kDa in
Lepidurus apus and about 600 kDa in Triops longicaudatus. Values between 420 and 700 kDa are
reported for the haemoglobins of various Daphnia species (Cladocera); that of D. magna is
490 kDa and appears in electron micrographs as
two superimposed octagons with a diameter of
14 nm. The haemoglobin of Cyzicus hierosolymitanus (Conchostraca) is a double pentagon with a
diameter of 13 nm and a mass of 280 kDa. Denaturation of all these haemoglobins produces subunits of 30-34 kDa. The iron and haem contents
correspond to the usual values of 15-17 kDa per
haem, i.e. the subunits apparently have two
haem-bearing domains. The native haemoglobin
molecules are composed of 10-24 such subunits
which, according to electron microscope studies,
are organized in two superimposed rings. SDS
electrophoresis of the haemoglobins of Daphnia