Syngamus trachea, possesses a haemoglobin of
38 kDa which probably carries two haem groups
[174]. In the marine gastrotrich species Neodysis
sp. (Nemathelminthes), haemoglobins are found
in special cells associated with nerve and muscle
cells. These animals have such a small body size
(less than 1 mm) that the function of these respiratory pigments is something of a mystery [35].
The phoronidans possess red blood cells from
which four different globin chains of 16 kDa have
been isolated in both Phoronopsis viridis and
P. architecta. Two of these are always monomeric
and the other two aggregate to form dimers on
deoxygenation (P. viridis) or CO binding (P. architecta) [170]. Haemoglobins in the cells of the
coelom fluid are widely distributed in the holothurians and may also occur in the opbiurids. Several Cucumaria species have been closely examined; their haemoglobins are dimers of about
35 kDa when oxygenated and aggregate to give
tetramers and higher polymers on deoxygenation.
They show weak cooperativity (n = 1.4), which
may either involve homotropic interaction within
the dimers or be the result of Oz-dependent dissociation. The coelom cells of the holothurian
Paracaudina chilensis contain three globin chains.
The sequence of 157 amino acids of globin I
agrees 25 % with the human ~-chain, but has an
N-terminal extension of 9-10 amino acids. The Nterminus is also acetylated. Furthermore, globin I
has 59 % similarity to globin D from Molpadia
arenicola. In the latter, there are four different
chain types, all of which are acetylated. In contrast, M. oolitica and other holothurian species
have only one chain type, also with a blocked Nterminus [157].
7.3 Haemocyanins
Haemocyanins are found only in the arthropods
and the molluscs (Table 7.4). Amongst the arthropods, haemocyanin is generally distributed in
the higher crab genera (Malacostraca), the horseshoe crabs (Xiphosura) and various groups of spiders (Arachnida), but it is also present in the chilopod Scutigera coleoptrata. In the molluscs, it is
found in the polyplacophores, gastropods and
cephalopods and has just recently been detected
in several mussel species. There have been only a
few investigations of the changes in haemocyanins during development. For example, in the
edible crabs of the genus Cancer, the haemocyanins of the embryonal and larval stages have dif7.3 Haemocyanins
275
ferent subunits and a lower O2 affinity [179]. The
polypeptide chains of the haemocyanins have a
"binuclear" Oz-binding site with two copper
atoms, Cu(A) and Cu(B) , which in the deoxygenated state are present as Cu(l) and in the oxygenated state as Cu(II) (Fig. 7.1). Although both
copper atoms are apparently bound to a histidine
residue, the region around them may vary greatly.
Sequence comparisons between haemocyanin
subunits from the spiny lobster Panulirus interruptus (Pint-a), the spider Eurypelma califomicum (Eury-e) and the edible snail Helix pomatia
(~c-Hcd) and tyrosinases from Neurospora crassa,
Streptomyces glaucescens and the mouse give only
one region with significant similarity, viz., the
42-60 amino acids around the Cu(B). Thus,
whilst Cu(B) is apparently phylogenetically very
old, Cu(A) appeared on several independent
occasions. In contrast, the Cu-binding sites (one
dinuclear and two mononuclear) of a third type of
copper protein, caeruloplasmin, show no similarity at all to haemocyanins or tyrosinases
[100, 108, 109, 178].
The quaternary structures of arthropod and
mollusc haemocyanins are very different but, in
both cases, extraordinarily complex. Mollusc
haemocyanins are cylindrical molecules with a
diameter of about 35 nm, a length of 38 nm and
a molecular mass of up to 10 MDa; they consist
of 10 or 20 subunits, each with seven or eight Ozbinding domains of about 55 kDa. In contrast,
the arthropod haemocyanins are composed of
one, two, four or eight hexameric basic units
which are themselves made up of 75-kDa subunits, each with one Oz-binding site. Each molecule may consist of up to eight different types of
subunit. Some haemocyanins contain carbohydrate, and the structures of the oligosaccharides
differ significantly between arthropods and molluscs (see Fig. 7.10) [91, 92]. The scientific interest in the haemocyanins extends far beyond their
role in the respiratory physiology of arthropods
and molluscs; they not only have very welldefined and measurable functions but also
exhibit molecular structures of great complexity.
It is difficult to envisage a better protein for
studying the way in which interaction between
different subunits and domains can lead to the
structural and functional properties of a very
large molecule. Particularly intensive research in
recent years has contributed significantly to our
knowledge of the haemocyanins [108, 174]. The
abbreviation "Hc" should be used for haemocyanin to avoid confusion with homocysteine "Hcy"
[108].
38 kDa which probably carries two haem groups
[174]. In the marine gastrotrich species Neodysis
sp. (Nemathelminthes), haemoglobins are found
in special cells associated with nerve and muscle
cells. These animals have such a small body size
(less than 1 mm) that the function of these respiratory pigments is something of a mystery [35].
The phoronidans possess red blood cells from
which four different globin chains of 16 kDa have
been isolated in both Phoronopsis viridis and
P. architecta. Two of these are always monomeric
and the other two aggregate to form dimers on
deoxygenation (P. viridis) or CO binding (P. architecta) [170]. Haemoglobins in the cells of the
coelom fluid are widely distributed in the holothurians and may also occur in the opbiurids. Several Cucumaria species have been closely examined; their haemoglobins are dimers of about
35 kDa when oxygenated and aggregate to give
tetramers and higher polymers on deoxygenation.
They show weak cooperativity (n = 1.4), which
may either involve homotropic interaction within
the dimers or be the result of Oz-dependent dissociation. The coelom cells of the holothurian
Paracaudina chilensis contain three globin chains.
The sequence of 157 amino acids of globin I
agrees 25 % with the human ~-chain, but has an
N-terminal extension of 9-10 amino acids. The Nterminus is also acetylated. Furthermore, globin I
has 59 % similarity to globin D from Molpadia
arenicola. In the latter, there are four different
chain types, all of which are acetylated. In contrast, M. oolitica and other holothurian species
have only one chain type, also with a blocked Nterminus [157].
7.3 Haemocyanins
Haemocyanins are found only in the arthropods
and the molluscs (Table 7.4). Amongst the arthropods, haemocyanin is generally distributed in
the higher crab genera (Malacostraca), the horseshoe crabs (Xiphosura) and various groups of spiders (Arachnida), but it is also present in the chilopod Scutigera coleoptrata. In the molluscs, it is
found in the polyplacophores, gastropods and
cephalopods and has just recently been detected
in several mussel species. There have been only a
few investigations of the changes in haemocyanins during development. For example, in the
edible crabs of the genus Cancer, the haemocyanins of the embryonal and larval stages have dif7.3 Haemocyanins
275
ferent subunits and a lower O2 affinity [179]. The
polypeptide chains of the haemocyanins have a
"binuclear" Oz-binding site with two copper
atoms, Cu(A) and Cu(B) , which in the deoxygenated state are present as Cu(l) and in the oxygenated state as Cu(II) (Fig. 7.1). Although both
copper atoms are apparently bound to a histidine
residue, the region around them may vary greatly.
Sequence comparisons between haemocyanin
subunits from the spiny lobster Panulirus interruptus (Pint-a), the spider Eurypelma califomicum (Eury-e) and the edible snail Helix pomatia
(~c-Hcd) and tyrosinases from Neurospora crassa,
Streptomyces glaucescens and the mouse give only
one region with significant similarity, viz., the
42-60 amino acids around the Cu(B). Thus,
whilst Cu(B) is apparently phylogenetically very
old, Cu(A) appeared on several independent
occasions. In contrast, the Cu-binding sites (one
dinuclear and two mononuclear) of a third type of
copper protein, caeruloplasmin, show no similarity at all to haemocyanins or tyrosinases
[100, 108, 109, 178].
The quaternary structures of arthropod and
mollusc haemocyanins are very different but, in
both cases, extraordinarily complex. Mollusc
haemocyanins are cylindrical molecules with a
diameter of about 35 nm, a length of 38 nm and
a molecular mass of up to 10 MDa; they consist
of 10 or 20 subunits, each with seven or eight Ozbinding domains of about 55 kDa. In contrast,
the arthropod haemocyanins are composed of
one, two, four or eight hexameric basic units
which are themselves made up of 75-kDa subunits, each with one Oz-binding site. Each molecule may consist of up to eight different types of
subunit. Some haemocyanins contain carbohydrate, and the structures of the oligosaccharides
differ significantly between arthropods and molluscs (see Fig. 7.10) [91, 92]. The scientific interest in the haemocyanins extends far beyond their
role in the respiratory physiology of arthropods
and molluscs; they not only have very welldefined and measurable functions but also
exhibit molecular structures of great complexity.
It is difficult to envisage a better protein for
studying the way in which interaction between
different subunits and domains can lead to the
structural and functional properties of a very
large molecule. Particularly intensive research in
recent years has contributed significantly to our
knowledge of the haemocyanins [108, 174]. The
abbreviation "Hc" should be used for haemocyanin to avoid confusion with homocysteine "Hcy"
[108].