264
7 Respiratory Pigments
bins in the chick, Jt2Q2, Jt' 2Q2, a\£2 and aD 2£2, and
two adult forms, a\f32 and aD 2132, which altogether contain four a-like chains (Jt, Jt', a A and aD)
and three f3-like chains (Q, £ and (3). All these
chains have been sequenced. A particular 13 type
(f3H) is produced at the time of hatching. Mature
embryos produce three so-called Koelliker haemoglobins with a A chains lacking the terminal
141-Arg. Unfortunately, these are found in quantities sufficient for analysis only in the breeding
strain "HYPECO" [33]. The chicken also contains two globin clusters. In the a cluster, as in the
mammals, the genes are so arranged that the
early expressed genes (Jt, aD, a A ) are 5'; in contrast, the gene arrangement of the 13 cluster
(probably Q, f3H, 13, £) does not appear to follow
this rule [140]. The switch between embryonal
and adult haemoglobins in the chicken is peculiar
in that embryonal and adult genes are transcribed
in haematopoietic cells towards the end of the
switching period, but only adult globin is pro~
duced; this suggests the existence of post·
transcriptional control.
Unlike in the mammals and birds, the a- and 13gene clusters of the clawed frog Xenopus Zaevis
are close. The tetraploid species X. Zaevis has two
af3 clusters and the diploid species X.tropicaZis
has only one. The two gene clusters of X.Zaevis
each contain two larval and one adult gene (Table 7.4). The adult globin chains differ between
the primitive X. tropicalis and the tetraploid species X. Zaevis and X. borealis in 12 % of positions;
this is a larger difference than that found between
mammalian species of different orders. The difference between homologous chains from the two
clusters of the tetraploid species is 3-4 %. It can
be estimated from these data that the genome
doubling which separated X. tropicalis from the
other species occurred about 110-120 million
years ago [84, 152]. The larval globins differ from
the adult forms in 40-50 % of their amino acids,
and the different larval genes are expressed with
different time schedules [6]. The larval a chains
are more than 90 % similar; however, in the tadpole there is a further a chain (aT5) of unknown
gene localization which has only 72-75 % similarity to the other larval a chains. Finally, in the earlier stages of embryo development, two other 13
chains are produced (f3E1 and f3E2) but these disappear in the feeding stage [7].
Thus, in the amphibians there is also at least
one switch in globin gene expression which is
related to metamorphosis, the timing of which,
however, is very species specific. In many urodelans, the switch begins long before metamorphosis but extends over a long period. In the anurans,
the switch usually occurs rapidly, mostly at the
time of metamorphosis, although in HyZa it
occurs 10 days later. Only in the case of Bufo bufo
does the switch over begin months after metamorphosis and lasts for several months [30]. Experimental anaemic stress in Rana and Xenopus
results in the renewed production of larval haemoglobins. Most of the investigated fish species
showed alterations in haemoglobins during postembryonal development. In many amphibians
and fish, the availability of a whole spectrum of
structurally and functionally different haemoglobins facilitates adaptation to changing environmental conditions (p. 257). The heterogeneity of
haemoglobins in fish is very great; 71 out of 77
Amazonian fish genera examined possessed several Hb bands (2-12, with a mean of 3.9). As
many as 18 different haemoglobins appear in the
course of the life cycle of Oncorhynchus and
other salmonids. The coexistence of numerous
haeruoglobins is made possible by the fact that
the dissociation of fish haemoglobins to dimers is
restrained compared with the situation in other
vertebrates, and therefore heterotetramers, like
aa'f3f3', are stable.
7.1.6 Myoglobins
Myoglobins are present in the heart and skeletal
muscle of most vertebrates and in the muscle of
many invertebrates. They promote O2 supply to
the sarcosomes, their most important property in
this connection being their high rate of O2 dissociation. Even Antarctic fish which lack haemoglobin-containing erythrocytes have myoglobins
in their heart muscle [41]. The concentration of
myoglobin is particularly high in the muscles of
aquatic mammals, where is also serves as an O2
store. Vertebrate myoglobins are all monomers
with 153-154 amino acids and a molecular mass
of 17.8 kDa. The sequences of more than 60 myoglobins are now known, mainly from the mammals [110]. One myoglobin gene has been
described for each of man, the seal and the
mouse, species in which there are probably multiple Mb genes. Myoglobin genes, like those for all
globins, have two introns, but they are unusually
long: intron 1 in the species mentioned is about
4.4 kb; the human intron 2 is 3.6 kb, that of the
seal is 3.2 kb, and in mouse it is only 1.5 kb [14].
Myoglobin exists in solution in two conformational isomers in which the haem is rotated 180 a
around the axis a-yo One of the forms generally
7 Respiratory Pigments
bins in the chick, Jt2Q2, Jt' 2Q2, a\£2 and aD 2£2, and
two adult forms, a\f32 and aD 2132, which altogether contain four a-like chains (Jt, Jt', a A and aD)
and three f3-like chains (Q, £ and (3). All these
chains have been sequenced. A particular 13 type
(f3H) is produced at the time of hatching. Mature
embryos produce three so-called Koelliker haemoglobins with a A chains lacking the terminal
141-Arg. Unfortunately, these are found in quantities sufficient for analysis only in the breeding
strain "HYPECO" [33]. The chicken also contains two globin clusters. In the a cluster, as in the
mammals, the genes are so arranged that the
early expressed genes (Jt, aD, a A ) are 5'; in contrast, the gene arrangement of the 13 cluster
(probably Q, f3H, 13, £) does not appear to follow
this rule [140]. The switch between embryonal
and adult haemoglobins in the chicken is peculiar
in that embryonal and adult genes are transcribed
in haematopoietic cells towards the end of the
switching period, but only adult globin is pro~
duced; this suggests the existence of post·
transcriptional control.
Unlike in the mammals and birds, the a- and 13gene clusters of the clawed frog Xenopus Zaevis
are close. The tetraploid species X. Zaevis has two
af3 clusters and the diploid species X.tropicaZis
has only one. The two gene clusters of X.Zaevis
each contain two larval and one adult gene (Table 7.4). The adult globin chains differ between
the primitive X. tropicalis and the tetraploid species X. Zaevis and X. borealis in 12 % of positions;
this is a larger difference than that found between
mammalian species of different orders. The difference between homologous chains from the two
clusters of the tetraploid species is 3-4 %. It can
be estimated from these data that the genome
doubling which separated X. tropicalis from the
other species occurred about 110-120 million
years ago [84, 152]. The larval globins differ from
the adult forms in 40-50 % of their amino acids,
and the different larval genes are expressed with
different time schedules [6]. The larval a chains
are more than 90 % similar; however, in the tadpole there is a further a chain (aT5) of unknown
gene localization which has only 72-75 % similarity to the other larval a chains. Finally, in the earlier stages of embryo development, two other 13
chains are produced (f3E1 and f3E2) but these disappear in the feeding stage [7].
Thus, in the amphibians there is also at least
one switch in globin gene expression which is
related to metamorphosis, the timing of which,
however, is very species specific. In many urodelans, the switch begins long before metamorphosis but extends over a long period. In the anurans,
the switch usually occurs rapidly, mostly at the
time of metamorphosis, although in HyZa it
occurs 10 days later. Only in the case of Bufo bufo
does the switch over begin months after metamorphosis and lasts for several months [30]. Experimental anaemic stress in Rana and Xenopus
results in the renewed production of larval haemoglobins. Most of the investigated fish species
showed alterations in haemoglobins during postembryonal development. In many amphibians
and fish, the availability of a whole spectrum of
structurally and functionally different haemoglobins facilitates adaptation to changing environmental conditions (p. 257). The heterogeneity of
haemoglobins in fish is very great; 71 out of 77
Amazonian fish genera examined possessed several Hb bands (2-12, with a mean of 3.9). As
many as 18 different haemoglobins appear in the
course of the life cycle of Oncorhynchus and
other salmonids. The coexistence of numerous
haeruoglobins is made possible by the fact that
the dissociation of fish haemoglobins to dimers is
restrained compared with the situation in other
vertebrates, and therefore heterotetramers, like
aa'f3f3', are stable.
7.1.6 Myoglobins
Myoglobins are present in the heart and skeletal
muscle of most vertebrates and in the muscle of
many invertebrates. They promote O2 supply to
the sarcosomes, their most important property in
this connection being their high rate of O2 dissociation. Even Antarctic fish which lack haemoglobin-containing erythrocytes have myoglobins
in their heart muscle [41]. The concentration of
myoglobin is particularly high in the muscles of
aquatic mammals, where is also serves as an O2
store. Vertebrate myoglobins are all monomers
with 153-154 amino acids and a molecular mass
of 17.8 kDa. The sequences of more than 60 myoglobins are now known, mainly from the mammals [110]. One myoglobin gene has been
described for each of man, the seal and the
mouse, species in which there are probably multiple Mb genes. Myoglobin genes, like those for all
globins, have two introns, but they are unusually
long: intron 1 in the species mentioned is about
4.4 kb; the human intron 2 is 3.6 kb, that of the
seal is 3.2 kb, and in mouse it is only 1.5 kb [14].
Myoglobin exists in solution in two conformational isomers in which the haem is rotated 180 a
around the axis a-yo One of the forms generally