266
7 Respiratory Pigments
ceans is composed of 10-24 such subunits, and
that from the body cavity fluid of the roundworm
Ascaris and its relatives has 8 subunits. The polypeptide chains of the third group each carry 8-20
haem groups; the haemoglobins of certain freshwater snails (Planorbidae), marine mussels
(Astartidae and Carditidae) and the brine shrimp
(Artemia salina) are made up of two or more such
subunits. The fourth type includes the haemoglobins of the annelids and Pogonophora as well as
the polychaete chlorocruorins, which contain a
somewhat different prosthetic group but are
otherwise extraordinarily similar to the haemoglobins. The pigments of this fourth group are all
molecules of several thousand kilodaltons, and in
the electron microscope have the characteristic
appearance of two superimposed hexagons. The
molecular architecture of these giant molecules
has not yet been defined; they often consist of
subunits of about 16 kDa with one haem group
which are partly linked by disulphide bridges, but
they may also have subunits of twice, or sometimes three or four times, the size and haem-free
polypeptides.
The haem-bearing subunits or domains consistently have molecular masses of 16 kDa and
lengths of between 132 and 157 amino acids [9].
The amino acid sequences of haemoglobins from
very different invertebrate groups have recently
been determined; these include the ciliates, annelids, pogonophorans, echiurids, crustaceans,
insects, molluscs and holothurians. In contrast,
the genes are known in only two cases: a globin
gene from the earthworm Lumbricus has a structure similar to that of the vertebrates with two
introns, whereas the globin genes of the nonbiting midges (Chironomidae) have no introns.
Pairwise comparison of the amino acid sequences
of distantly related animal species usually shows
only a few identical positions. The phylogenetic
history of these haemoglobins apparently goes far
back in time. Even different globin chains of the
same species can vary greatly. For example, the
chains I and II of the haemoglobin from the
earthworm Lumbricus terrestris have only 59
(42 %) identical amino acids [148]; pairwise comparisons of the 12 haemoglobins in Chironomus
thummi thummi consistently show only 50 %
agreement [128]. Only when a large number of
sequences are available will it be possible to derive a molecular genealogy which reflects the relationships between the various invertebrate haemoglobins. Despite the large sequence differences, there is still no doubt that all haemoglobins are homologous. For example, the sequence
of the very unusual globin F-I of the echiurid
Urechis caupo in fact agrees in only 11-20 % of
positions with those of man, the agnathan Petromyzon, the holothurian Molpadia, the insect Chironomus, the annelids Glycera and Lumbricus,
the molluscs Anadara and Aplysia, and the soybean, but 51 % of the amino acids of Urechis are
also present in at least one of the other chains
[46]. The spatial structure of invertebrate haemoglobin has, up to now, been examined only in
the case of CIT-III from Chironomus thummi
thummi, where there is extensive similarity to vertebrate haemoglobin (Fig.7.4). Extrapolating from
the amino acid sequences, it would appear that all
known haemoglobin chains have similar secondary and tertiary structures, and this is not analogous to that of any other protein family [9]. Thus,
despite fundamental changes in sequence in the
course of evolution, structural parameters essential for the function have been retained [174].
Only two amino acids are invariant in all haemoglobins: CD1-Phe and the haem-binding
"proximal" F8-His. The haem iron is coordinately
bound, on the one hand, to the four pyrrole-N
residues and, on the other hand, to the imidazole
ring of this histidine; the haem itself is anchored
in the haem pocket by non-polar interactions with
about 15 other amino acids. The region of the
haem pocket has been especially conserved during evolution. Position E7 is also occupied by histidine, the "distal" histidine, in almost all haemoglobin chains. On oxygenation, this amino
acid swings away from the entrance to the haem
pocket, thereby providing access for O2, Apart
from histidine, sterically the most suitable amino
acid for this function is glutamine, which in fact
replaces E7-His in some globins, e.g. the myoglobin of the African and Indian elephants, the
myoglobin of the shark Mustelus antarcticus, the
a-chain of the opossum, the ~-chain of the snake
Liophis miliaris, the globin of the agnathan Myxine, the larval globin CT-IIIA of the non-biting
midge Chironomus thummi thummi, the globin of
the echiurid Urechis caupo, and haemoglobin I of
the mussel Calyptogena soyoae [46, 108,118, 159,
174]. One finds E7-Leu in the monomeric globin
of the polychaete Glycera dibranchiata, but in the
monomeric myoglobins of the snails Aplysia limadna and Dolabella auricularia E7-Val occurs
[156, 189]. The haemoglobin of the small liverfluke Dicrocoelium dendriticum has tyrosine at
position E7, and not glycine as was thought earlier [97].
Cooperativity and heterotropic modulations in
the tetrameric vertebrate haemoglobins involve
7 Respiratory Pigments
ceans is composed of 10-24 such subunits, and
that from the body cavity fluid of the roundworm
Ascaris and its relatives has 8 subunits. The polypeptide chains of the third group each carry 8-20
haem groups; the haemoglobins of certain freshwater snails (Planorbidae), marine mussels
(Astartidae and Carditidae) and the brine shrimp
(Artemia salina) are made up of two or more such
subunits. The fourth type includes the haemoglobins of the annelids and Pogonophora as well as
the polychaete chlorocruorins, which contain a
somewhat different prosthetic group but are
otherwise extraordinarily similar to the haemoglobins. The pigments of this fourth group are all
molecules of several thousand kilodaltons, and in
the electron microscope have the characteristic
appearance of two superimposed hexagons. The
molecular architecture of these giant molecules
has not yet been defined; they often consist of
subunits of about 16 kDa with one haem group
which are partly linked by disulphide bridges, but
they may also have subunits of twice, or sometimes three or four times, the size and haem-free
polypeptides.
The haem-bearing subunits or domains consistently have molecular masses of 16 kDa and
lengths of between 132 and 157 amino acids [9].
The amino acid sequences of haemoglobins from
very different invertebrate groups have recently
been determined; these include the ciliates, annelids, pogonophorans, echiurids, crustaceans,
insects, molluscs and holothurians. In contrast,
the genes are known in only two cases: a globin
gene from the earthworm Lumbricus has a structure similar to that of the vertebrates with two
introns, whereas the globin genes of the nonbiting midges (Chironomidae) have no introns.
Pairwise comparison of the amino acid sequences
of distantly related animal species usually shows
only a few identical positions. The phylogenetic
history of these haemoglobins apparently goes far
back in time. Even different globin chains of the
same species can vary greatly. For example, the
chains I and II of the haemoglobin from the
earthworm Lumbricus terrestris have only 59
(42 %) identical amino acids [148]; pairwise comparisons of the 12 haemoglobins in Chironomus
thummi thummi consistently show only 50 %
agreement [128]. Only when a large number of
sequences are available will it be possible to derive a molecular genealogy which reflects the relationships between the various invertebrate haemoglobins. Despite the large sequence differences, there is still no doubt that all haemoglobins are homologous. For example, the sequence
of the very unusual globin F-I of the echiurid
Urechis caupo in fact agrees in only 11-20 % of
positions with those of man, the agnathan Petromyzon, the holothurian Molpadia, the insect Chironomus, the annelids Glycera and Lumbricus,
the molluscs Anadara and Aplysia, and the soybean, but 51 % of the amino acids of Urechis are
also present in at least one of the other chains
[46]. The spatial structure of invertebrate haemoglobin has, up to now, been examined only in
the case of CIT-III from Chironomus thummi
thummi, where there is extensive similarity to vertebrate haemoglobin (Fig.7.4). Extrapolating from
the amino acid sequences, it would appear that all
known haemoglobin chains have similar secondary and tertiary structures, and this is not analogous to that of any other protein family [9]. Thus,
despite fundamental changes in sequence in the
course of evolution, structural parameters essential for the function have been retained [174].
Only two amino acids are invariant in all haemoglobins: CD1-Phe and the haem-binding
"proximal" F8-His. The haem iron is coordinately
bound, on the one hand, to the four pyrrole-N
residues and, on the other hand, to the imidazole
ring of this histidine; the haem itself is anchored
in the haem pocket by non-polar interactions with
about 15 other amino acids. The region of the
haem pocket has been especially conserved during evolution. Position E7 is also occupied by histidine, the "distal" histidine, in almost all haemoglobin chains. On oxygenation, this amino
acid swings away from the entrance to the haem
pocket, thereby providing access for O2, Apart
from histidine, sterically the most suitable amino
acid for this function is glutamine, which in fact
replaces E7-His in some globins, e.g. the myoglobin of the African and Indian elephants, the
myoglobin of the shark Mustelus antarcticus, the
a-chain of the opossum, the ~-chain of the snake
Liophis miliaris, the globin of the agnathan Myxine, the larval globin CT-IIIA of the non-biting
midge Chironomus thummi thummi, the globin of
the echiurid Urechis caupo, and haemoglobin I of
the mussel Calyptogena soyoae [46, 108,118, 159,
174]. One finds E7-Leu in the monomeric globin
of the polychaete Glycera dibranchiata, but in the
monomeric myoglobins of the snails Aplysia limadna and Dolabella auricularia E7-Val occurs
[156, 189]. The haemoglobin of the small liverfluke Dicrocoelium dendriticum has tyrosine at
position E7, and not glycine as was thought earlier [97].
Cooperativity and heterotropic modulations in
the tetrameric vertebrate haemoglobins involve