274
7 Respiratory Pigments
The juvenile hormone switches on the transcription of haemoglobin genes and ecdysterone switches them off [125, 128]. In C. thummi thummi,
these genes lie close together on the third of the
four chromosomes, as expected for genes that
have arisen by duplication. Several copies exist of
CTT-I1I, -IV and VIIB [82]. More recently,
investigations of the haemoglobins and haemoglobin genes of the subspecies C. thummi piger
have been initiated [82, 142]. In contrast to all
other globin genes from plants or animals, the
sequences available so far for this family are unique in having no introns.
All the Chironomus haemoglobins have a high
O2 affinity (p50 = 40-170 Pa), and most have a
considerable alkaline Bohr effect (cp = -0.30 to
-0.94; but cp(CTT-I) = 0) and a high oxygenation
enthalpy (~H = -40 to -81 kJ/mol) [182]. The
dimeric haemoglobins also show no cooperativity
(n = 0). The Bohr effect, which is also observed
with the monomeric haemoglobins, results from a
change in conformation which can be described in
the case of CTT-III as a switch between two
states: t ~ r + H+. In the t state, the distance
between the haem iron and the proximal F8-His is
reduced and the O2 affinity decreases; this state is
stabilized by a salt brigde between G2-His and
H22-Met. This bridge, which is ultimately responsible for the Bohr effect, does not occur in CTT-I
due to the exchange G2-His ~ Pro [31, 128].
The biological importance of the Chironomus
haemoglobins lies, above all, in the improvement
of O2 diffusion in low p02 environments and in O2
storage. Because, like all haemoglobins, they also
have pseudoperoxidase and mono-oxygenase
activity, they may also contribute to resistance
against toxic substances [128]. Finally, where they
constitute as much as 27 % of the dry weight of
larvae, they may also have a nutritional role. It
has been shown that haemoglobins are taken up
by maturing oocytes, processed and stored; these
altered forms disappear during embryo development [166]. The biological significance of the marked heterogeneity of Chironomus haemoglobins
is still a puzzle.
7.2.5 Haemoglobins of Other Invertebrates
There is really very little biochemical information
available about the haemoglobins in representatives of other phyla (Table 7.2). Ciliates of the
genera Paramecium and Tetrahymena contain low
concentrations (0.1-1.0 mglg fro wt.) of haemoglobins with molecular masses of 13-15 kDa. The
globin chains of Paramecium caudatum and Tetrahymena pyriformis agree with each other in 34 %
of their 116-119 amino acids but are so different
from other globins that sequence alignment and
comparison are very difficult. Analysis, centred
upon the invariant residues CD1-Phe and F8-His,
suggests the absence of the D helix and shortening of the A helix. The distal E7-His is apparently
replaced by glutamine. The biological role of this
pigment is unclear [73]. Haemoglobins have been
detected in the parenchyme cells and the pharynx
musculature of many turbeUarians and trematodes. The only pigment to have been examined in
detail, and almost completely sequenced, is that
from the small liver-fluke Dicrocoelium lanceolatum; this is a monomeric non-cooperative haemoglobin of about 16 kDa. Its O2 affinity is one
of the highest recorded (p50 = 2-20 Pa); the
explanation lies possibly in the unusual structure
of the haem pocket. The distal E7-His is replaced
by tyrosine and not, as initially assumed, by glycine. This haemoglobin is the only one known
with an acidic but no alkaline Bohr effect
(cp = +0.96) [97, 151].
Haemoglobins are widely found in both parasitic and free-living nematodes, either in the body
cavity fluid or in various tissues. The best
described are those of the round-worms (Ascaridae). The body cavity fluids of Ascaris suum (in
the pig), A.lumbricoides (in humans) and Parascaris equorum (in the horse) contain haemoglobins of about 330 kDa, which are composed of
eight subunits of 42 kDa with two haems. The
haem content of up to 2.86 % corresponds to
21.6 kDaimol haem, but is sometimes lower due
to the reduced occupancy of some haem-binding
sites [39]. This haemoglobin shows no cooperativity but an extremely high O2 affinity (p50
= 0.2 Pa); this is in accordance with an O2 dissociation constant of 0.004 s-l, which is lOOO-fold
lower than that of mammalian haemoglobin
(Table 7.5). In contrast, the dissociation constants for CO and ethylisocyanide are not significantly different from those of other haemoglobins
[36]. Nematode haemoglobin is one of the few
with a lower affinity for CO than for O2, As it
requires about 10 min to unload 50 % of the haemoglobin, a transport function would appear to
be unlikely; however, a role as an O2 store is not
excluded. Although the roundworms have an
anaerobic metabolism to supply energy, they do
require oxygen for the hydroxylation of the proline in collagen. The haemoglobin in the body
wall of Ascaris is about 40 kDa and has a low O2
affinity. The parasite of fowl respiratory tracts,
7 Respiratory Pigments
The juvenile hormone switches on the transcription of haemoglobin genes and ecdysterone switches them off [125, 128]. In C. thummi thummi,
these genes lie close together on the third of the
four chromosomes, as expected for genes that
have arisen by duplication. Several copies exist of
CTT-I1I, -IV and VIIB [82]. More recently,
investigations of the haemoglobins and haemoglobin genes of the subspecies C. thummi piger
have been initiated [82, 142]. In contrast to all
other globin genes from plants or animals, the
sequences available so far for this family are unique in having no introns.
All the Chironomus haemoglobins have a high
O2 affinity (p50 = 40-170 Pa), and most have a
considerable alkaline Bohr effect (cp = -0.30 to
-0.94; but cp(CTT-I) = 0) and a high oxygenation
enthalpy (~H = -40 to -81 kJ/mol) [182]. The
dimeric haemoglobins also show no cooperativity
(n = 0). The Bohr effect, which is also observed
with the monomeric haemoglobins, results from a
change in conformation which can be described in
the case of CTT-III as a switch between two
states: t ~ r + H+. In the t state, the distance
between the haem iron and the proximal F8-His is
reduced and the O2 affinity decreases; this state is
stabilized by a salt brigde between G2-His and
H22-Met. This bridge, which is ultimately responsible for the Bohr effect, does not occur in CTT-I
due to the exchange G2-His ~ Pro [31, 128].
The biological importance of the Chironomus
haemoglobins lies, above all, in the improvement
of O2 diffusion in low p02 environments and in O2
storage. Because, like all haemoglobins, they also
have pseudoperoxidase and mono-oxygenase
activity, they may also contribute to resistance
against toxic substances [128]. Finally, where they
constitute as much as 27 % of the dry weight of
larvae, they may also have a nutritional role. It
has been shown that haemoglobins are taken up
by maturing oocytes, processed and stored; these
altered forms disappear during embryo development [166]. The biological significance of the marked heterogeneity of Chironomus haemoglobins
is still a puzzle.
7.2.5 Haemoglobins of Other Invertebrates
There is really very little biochemical information
available about the haemoglobins in representatives of other phyla (Table 7.2). Ciliates of the
genera Paramecium and Tetrahymena contain low
concentrations (0.1-1.0 mglg fro wt.) of haemoglobins with molecular masses of 13-15 kDa. The
globin chains of Paramecium caudatum and Tetrahymena pyriformis agree with each other in 34 %
of their 116-119 amino acids but are so different
from other globins that sequence alignment and
comparison are very difficult. Analysis, centred
upon the invariant residues CD1-Phe and F8-His,
suggests the absence of the D helix and shortening of the A helix. The distal E7-His is apparently
replaced by glutamine. The biological role of this
pigment is unclear [73]. Haemoglobins have been
detected in the parenchyme cells and the pharynx
musculature of many turbeUarians and trematodes. The only pigment to have been examined in
detail, and almost completely sequenced, is that
from the small liver-fluke Dicrocoelium lanceolatum; this is a monomeric non-cooperative haemoglobin of about 16 kDa. Its O2 affinity is one
of the highest recorded (p50 = 2-20 Pa); the
explanation lies possibly in the unusual structure
of the haem pocket. The distal E7-His is replaced
by tyrosine and not, as initially assumed, by glycine. This haemoglobin is the only one known
with an acidic but no alkaline Bohr effect
(cp = +0.96) [97, 151].
Haemoglobins are widely found in both parasitic and free-living nematodes, either in the body
cavity fluid or in various tissues. The best
described are those of the round-worms (Ascaridae). The body cavity fluids of Ascaris suum (in
the pig), A.lumbricoides (in humans) and Parascaris equorum (in the horse) contain haemoglobins of about 330 kDa, which are composed of
eight subunits of 42 kDa with two haems. The
haem content of up to 2.86 % corresponds to
21.6 kDaimol haem, but is sometimes lower due
to the reduced occupancy of some haem-binding
sites [39]. This haemoglobin shows no cooperativity but an extremely high O2 affinity (p50
= 0.2 Pa); this is in accordance with an O2 dissociation constant of 0.004 s-l, which is lOOO-fold
lower than that of mammalian haemoglobin
(Table 7.5). In contrast, the dissociation constants for CO and ethylisocyanide are not significantly different from those of other haemoglobins
[36]. Nematode haemoglobin is one of the few
with a lower affinity for CO than for O2, As it
requires about 10 min to unload 50 % of the haemoglobin, a transport function would appear to
be unlikely; however, a role as an O2 store is not
excluded. Although the roundworms have an
anaerobic metabolism to supply energy, they do
require oxygen for the hydroxylation of the proline in collagen. The haemoglobin in the body
wall of Ascaris is about 40 kDa and has a low O2
affinity. The parasite of fowl respiratory tracts,