94
3 The Structural Variety and Metabolism of Proteins
include those of the warble fly, the fiddler crab
Uca pugilator, the starfish Pyenopdia helianthoides and the turbellarian Bipallium kewense
[87, 153]. The only vertebrate enzyme that may
be referred to in this connection is a collagenase
from canine pancreas [11]; the other typical vertebrate collagenases all belong to the metalloproteinases (Table 3.3) and have no digestive functions, although they are involved in developmental processes like, for example, degeneration of
the tail in the tadpole. The collagen serine proteinases cleave the collagen triple helix about threequarters of the way along the molecule in the
same region as is cleaved by the metalloproteinases, although other peptide bonds are attacked.
This collagen region apparently has a lower stability. Further degradation of the collagen that has
been destabilized by cleavage is carried out by
other proteinases.
Proteinases that create a way through the egg
membrane for spermatozoa are found in Hydrozoa, various gastropods, the mussel Mytilus, sea
urchins, ascidians, anurans of the Bufo genus,
and various mammals [297]. Despite their similar
biological roles, these enzymes have different
binding specificities and therefore apparently
arose independently during evolution. The acrosin of mammals, which is present as a zymogen
(proacrosin) in the small body at the tip of the
spermatozoon (acrosome), is initially activated by
acrolysin and later activated autocatalytically; it
allows the spermatozoon to find a way through
the zona pellucida of the egg. The enzyme
resembles trypsin in its amino acid sequence and
specificity for -ArglX- and -LyslX-bonds. In the
activation of pro-acrosin, a C-terminal peptide of
14 amino acids is cleaved initially. The cleavage of
the peptide bond at 23-Arg then creates two
different-sized subunits which are linked by a disulphide bridge. For the formation of the active
enzyme, an 18-amino-acid peptide is first cleaved
from the C-terminus and this is followed by a 43amino-acid peptide that contains an unusual
sequence of 23 consecutive proline residues [8].
The proteinase from the sperm of the sea urchin
Hemicentrotus pulcherrimus corresponds rather
more closely to chymotrypsin in its activity with
synthetic substrates and its inhibitor sensitivity,
whereas that of Strongylocentrotus purpuratus
resembles acrosin and trypsin. In the digestion of
the gelationous egg membrane, this proteinase is
aided by an arylsulphatase [297]. In the spermatozoa of the ascidian Halocynthia roretzi there
are, in fact, two very different trypsin-like proteinases; one strongly resembles the acrosin of the
mammals and is so named, whilst the other (spermosin) has a much narrower specificity [241]. In
the snails Haliotis corrugata and H. rufescens, the
acrosome of the spermatozoa contains an amphophilic protein known as "lysin"; this produces a
hole of 3 !-tm diameter in the egg membrane,
although it possesses no enzyme activity. The
lysins of the two species differ in 21 % of their
136-137 amino acids [283].
The egg membrane also presents a problem for
the hatching embryo, and in the insects, echinoderms, teleosts and amphibians enzymes are
involved in the process [157]. The hatching
enzyme of Xenopus laevis, which would normally
be classified as a serine proteinase on the strength
of its inhibition by diisopropylfluorphosphate
(DFP) and phenylmethanesulphonylfluoride
(PMSF) (Table 3.5), is not inhibited by trypsin
inhibitors, however, but by zinc ions and ethylenediaminetetraacetic acid (EDTA); its classification is thus unclear. A similar enzyme is found
in Rana chensinensis; in contrast, the hatching
enzymes of the teleosts are cysteine proteinases
[282]. The best-known hatching enzyme is the
cocoonase of the silkworms (Saturniidae), and
this has been examined in detail in species of the
genera Bombyx and Antheraea. The enzyme solubilizes only the sericin of the cocoon and not the
fibroin, and it attacks other proteins with a
trypsin-like specificity. This enzyme is produced
as an inactive prococoonase of 28 kDa in special
cells of the maxilla (galea) and is stored as a solid.
The zymogen is dissolved and activated by an
aqueous secretion prior to hatching. In some species, the cocoonase contains carbohydrates. The
hatching enzyme of the sea urchin Paracentrotus
lividus is a Ca 2 + -activated protease [157].
3.3.3 Cysteine Proteinases
The best-known cysteine proteinases of vertebrates are the lysosomal cathepsins, which play an
important role in intracellular protein degradation. Cathepsins B, Hand L have been investigated most thoroughly, and cathepsins M, N, Sand
Tto a lesser extent (Table 3.3). The cathepsins B,
Hand L belong to the same super-family as the
plant proteinase papain and agree in up to 48 %
of their sequences [226]. Cathepsin B is also
apparently widely distributed in the invertebrates. Enzymes of this type are detectable, for
example, in the lysosomes of protozoans, and
allow the endozooic amoeba Entamoeba histoly-
3 The Structural Variety and Metabolism of Proteins
include those of the warble fly, the fiddler crab
Uca pugilator, the starfish Pyenopdia helianthoides and the turbellarian Bipallium kewense
[87, 153]. The only vertebrate enzyme that may
be referred to in this connection is a collagenase
from canine pancreas [11]; the other typical vertebrate collagenases all belong to the metalloproteinases (Table 3.3) and have no digestive functions, although they are involved in developmental processes like, for example, degeneration of
the tail in the tadpole. The collagen serine proteinases cleave the collagen triple helix about threequarters of the way along the molecule in the
same region as is cleaved by the metalloproteinases, although other peptide bonds are attacked.
This collagen region apparently has a lower stability. Further degradation of the collagen that has
been destabilized by cleavage is carried out by
other proteinases.
Proteinases that create a way through the egg
membrane for spermatozoa are found in Hydrozoa, various gastropods, the mussel Mytilus, sea
urchins, ascidians, anurans of the Bufo genus,
and various mammals [297]. Despite their similar
biological roles, these enzymes have different
binding specificities and therefore apparently
arose independently during evolution. The acrosin of mammals, which is present as a zymogen
(proacrosin) in the small body at the tip of the
spermatozoon (acrosome), is initially activated by
acrolysin and later activated autocatalytically; it
allows the spermatozoon to find a way through
the zona pellucida of the egg. The enzyme
resembles trypsin in its amino acid sequence and
specificity for -ArglX- and -LyslX-bonds. In the
activation of pro-acrosin, a C-terminal peptide of
14 amino acids is cleaved initially. The cleavage of
the peptide bond at 23-Arg then creates two
different-sized subunits which are linked by a disulphide bridge. For the formation of the active
enzyme, an 18-amino-acid peptide is first cleaved
from the C-terminus and this is followed by a 43amino-acid peptide that contains an unusual
sequence of 23 consecutive proline residues [8].
The proteinase from the sperm of the sea urchin
Hemicentrotus pulcherrimus corresponds rather
more closely to chymotrypsin in its activity with
synthetic substrates and its inhibitor sensitivity,
whereas that of Strongylocentrotus purpuratus
resembles acrosin and trypsin. In the digestion of
the gelationous egg membrane, this proteinase is
aided by an arylsulphatase [297]. In the spermatozoa of the ascidian Halocynthia roretzi there
are, in fact, two very different trypsin-like proteinases; one strongly resembles the acrosin of the
mammals and is so named, whilst the other (spermosin) has a much narrower specificity [241]. In
the snails Haliotis corrugata and H. rufescens, the
acrosome of the spermatozoa contains an amphophilic protein known as "lysin"; this produces a
hole of 3 !-tm diameter in the egg membrane,
although it possesses no enzyme activity. The
lysins of the two species differ in 21 % of their
136-137 amino acids [283].
The egg membrane also presents a problem for
the hatching embryo, and in the insects, echinoderms, teleosts and amphibians enzymes are
involved in the process [157]. The hatching
enzyme of Xenopus laevis, which would normally
be classified as a serine proteinase on the strength
of its inhibition by diisopropylfluorphosphate
(DFP) and phenylmethanesulphonylfluoride
(PMSF) (Table 3.5), is not inhibited by trypsin
inhibitors, however, but by zinc ions and ethylenediaminetetraacetic acid (EDTA); its classification is thus unclear. A similar enzyme is found
in Rana chensinensis; in contrast, the hatching
enzymes of the teleosts are cysteine proteinases
[282]. The best-known hatching enzyme is the
cocoonase of the silkworms (Saturniidae), and
this has been examined in detail in species of the
genera Bombyx and Antheraea. The enzyme solubilizes only the sericin of the cocoon and not the
fibroin, and it attacks other proteins with a
trypsin-like specificity. This enzyme is produced
as an inactive prococoonase of 28 kDa in special
cells of the maxilla (galea) and is stored as a solid.
The zymogen is dissolved and activated by an
aqueous secretion prior to hatching. In some species, the cocoonase contains carbohydrates. The
hatching enzyme of the sea urchin Paracentrotus
lividus is a Ca 2 + -activated protease [157].
3.3.3 Cysteine Proteinases
The best-known cysteine proteinases of vertebrates are the lysosomal cathepsins, which play an
important role in intracellular protein degradation. Cathepsins B, Hand L have been investigated most thoroughly, and cathepsins M, N, Sand
Tto a lesser extent (Table 3.3). The cathepsins B,
Hand L belong to the same super-family as the
plant proteinase papain and agree in up to 48 %
of their sequences [226]. Cathepsin B is also
apparently widely distributed in the invertebrates. Enzymes of this type are detectable, for
example, in the lysosomes of protozoans, and
allow the endozooic amoeba Entamoeba histoly-