quarters of the way along the molecule' the
resulting cleavage products may then be fdrther
degraded by other proteases. The collagenases
from human fibroblasts and neutrophils show
abou~ 57 % sequence agreement [100, 166]. The
gelatmases are not active against the fibrillar collagens but are active against collagens type IV, V
and VII as well as fibronectin; stromelysin predominantly degrades proteoglycans. All these enzymes, however, are homologues and show up to
50 % sequence agreement [39, 240].
The .elastases of the serine enzyme group are
found m all vertebrates above the Agnatha; in
contrast, there is an enzyme, first discovered in
1984, that is restricted to the teleosts, cleaves typical elastase substrates, but belongs to the zinccontaining metalloenzymes [301]. In several vertebrates there are collagenolytic serine proteinases [153]; however, one of the two collagenases of
the starfish Pyenopodia helianthoides is a metal
proteinase that is, in fact, better activated by Ca 2 +
than by Zn 2 + [4]. Four proteinases have been isolated from the digestive fluids of the spider Argiope, these have an alkaline pH optimum and are
all inhibited by EDTA. The inhibition can be
relieved by Zn2+, and they are, therefore, apparently metalloproteinases. They have a particularly low molecular mass of 17 kDa. Two of the
four enzymes cleave the fibroin of the web, and
all the enzymes have elastase activity [137].
An enzyme discovered in 1967 in the hepatopancreas of the crayfish Astacus fluviatilis is unique amongst the zinc proteinases. The molecular
mass of this enzyme is 22.6 kDa and there is one
zinc atom per mole; the previously described
molecules of only 11 kDa were artefacts caused
by ~apid autolysis below pH 4. The polypeptide
cham of 200 amino acids shows no similarity to
other protein sequences; only one short stretch
(amino acids 90-99) is similar to the metalbinding site of thermolysin. The pH optimum is
about 8, and the artificial heptapeptide dansylPro-Lys-Arg-Ala-Pro-Trp-Val, which is cleaved at
the Arg/Ala bond, is processed much better than
~atural protein substrates. Other proteins with
Immunological similarities to the Astacus enzyme
have been detected in other crustaceans. Astacus
f!.uviatilis ~lso ~oss~sses trypsins and carboxypeptIdases WhICh, m thIS case, are very similar to the
mammalian enzymes [260, 278].
3.4.1 Serine-Proteinase Inhibitors
97
3.4 Proteinase Inhibitors
Natural inhibitors are known for many different
!yp~s. of enzyme [286], but only the proteinase
mh~bIt?rs ~ppear to be ubiquitous. Although
theIr bIOlogIcal role can, in general, be defined as
the prevention of undesirable proteolysis, the
detaIls are often not understood. Proteinase inhibitors are usually discovered by observation of
the inhibition of common proteinases like trypsin
or p~pain, but the actual target enzyme may
remam obscure. In only a minority of cases is the
biological function apparent: the trypsin inhibitors prevent the premature activation of trypsinogens and other zymogens in the pancreas; the
proteinase inhibitors of the blood plasma inhibit
the proteolytic cascade of blood clotting and complement activation, and the release of active hormones from precursors. The plant proteinase
i~hibitors serve in the defence against the digestIve sy~tem of herbivorous animals. With a single
exceptIon, the proteinase inhibitors are specific
for one class of proteinases. Only the U2~acroglobulinsl (u2M) inhibit all classes of protemases and they also differ from all other protei~as~ .inhibitors. in their mode of action. Many
mhibitors of ammal serine and cysteine proteinases have already been sequenced but only one
aspartate proteinase inhibitor has been sequenced; this is a pepsin inhibitor from the
roundworm Ascaris suum. In this latter case, the
sequence of 149 amino acids has no homology to
other known proteins [170].
3.4.1 Serine-Proteinase Inhibitors
The presently known inhibitors of serine proteinas~s. belong to several different protein superfamilies (Table 3.7) on the basis of their sequences and spatial structures, but all have the same
mode of action; they thus serve as a good example .of convergent protein evolution. The organIzatIOn of the disulphide bridges suffices to distinguish the individual families, but the absolute
?ef~n~tion ?f ~~e phylogenetic relationships of
mdlVldual mhibitors requires knowledge of the
complete amino acid sequences. In addition to
the examples given in Table 3.7, more than 100
other, less well-known serine-proteinase inhibitors have been described and there may possibly
1 The Greek letters and numbers refer to the localization
?f the inhibitors in electrophoregrams of plasma protems.
resulting cleavage products may then be fdrther
degraded by other proteases. The collagenases
from human fibroblasts and neutrophils show
abou~ 57 % sequence agreement [100, 166]. The
gelatmases are not active against the fibrillar collagens but are active against collagens type IV, V
and VII as well as fibronectin; stromelysin predominantly degrades proteoglycans. All these enzymes, however, are homologues and show up to
50 % sequence agreement [39, 240].
The .elastases of the serine enzyme group are
found m all vertebrates above the Agnatha; in
contrast, there is an enzyme, first discovered in
1984, that is restricted to the teleosts, cleaves typical elastase substrates, but belongs to the zinccontaining metalloenzymes [301]. In several vertebrates there are collagenolytic serine proteinases [153]; however, one of the two collagenases of
the starfish Pyenopodia helianthoides is a metal
proteinase that is, in fact, better activated by Ca 2 +
than by Zn 2 + [4]. Four proteinases have been isolated from the digestive fluids of the spider Argiope, these have an alkaline pH optimum and are
all inhibited by EDTA. The inhibition can be
relieved by Zn2+, and they are, therefore, apparently metalloproteinases. They have a particularly low molecular mass of 17 kDa. Two of the
four enzymes cleave the fibroin of the web, and
all the enzymes have elastase activity [137].
An enzyme discovered in 1967 in the hepatopancreas of the crayfish Astacus fluviatilis is unique amongst the zinc proteinases. The molecular
mass of this enzyme is 22.6 kDa and there is one
zinc atom per mole; the previously described
molecules of only 11 kDa were artefacts caused
by ~apid autolysis below pH 4. The polypeptide
cham of 200 amino acids shows no similarity to
other protein sequences; only one short stretch
(amino acids 90-99) is similar to the metalbinding site of thermolysin. The pH optimum is
about 8, and the artificial heptapeptide dansylPro-Lys-Arg-Ala-Pro-Trp-Val, which is cleaved at
the Arg/Ala bond, is processed much better than
~atural protein substrates. Other proteins with
Immunological similarities to the Astacus enzyme
have been detected in other crustaceans. Astacus
f!.uviatilis ~lso ~oss~sses trypsins and carboxypeptIdases WhICh, m thIS case, are very similar to the
mammalian enzymes [260, 278].
3.4.1 Serine-Proteinase Inhibitors
97
3.4 Proteinase Inhibitors
Natural inhibitors are known for many different
!yp~s. of enzyme [286], but only the proteinase
mh~bIt?rs ~ppear to be ubiquitous. Although
theIr bIOlogIcal role can, in general, be defined as
the prevention of undesirable proteolysis, the
detaIls are often not understood. Proteinase inhibitors are usually discovered by observation of
the inhibition of common proteinases like trypsin
or p~pain, but the actual target enzyme may
remam obscure. In only a minority of cases is the
biological function apparent: the trypsin inhibitors prevent the premature activation of trypsinogens and other zymogens in the pancreas; the
proteinase inhibitors of the blood plasma inhibit
the proteolytic cascade of blood clotting and complement activation, and the release of active hormones from precursors. The plant proteinase
i~hibitors serve in the defence against the digestIve sy~tem of herbivorous animals. With a single
exceptIon, the proteinase inhibitors are specific
for one class of proteinases. Only the U2~acroglobulinsl (u2M) inhibit all classes of protemases and they also differ from all other protei~as~ .inhibitors. in their mode of action. Many
mhibitors of ammal serine and cysteine proteinases have already been sequenced but only one
aspartate proteinase inhibitor has been sequenced; this is a pepsin inhibitor from the
roundworm Ascaris suum. In this latter case, the
sequence of 149 amino acids has no homology to
other known proteins [170].
3.4.1 Serine-Proteinase Inhibitors
The presently known inhibitors of serine proteinas~s. belong to several different protein superfamilies (Table 3.7) on the basis of their sequences and spatial structures, but all have the same
mode of action; they thus serve as a good example .of convergent protein evolution. The organIzatIOn of the disulphide bridges suffices to distinguish the individual families, but the absolute
?ef~n~tion ?f ~~e phylogenetic relationships of
mdlVldual mhibitors requires knowledge of the
complete amino acid sequences. In addition to
the examples given in Table 3.7, more than 100
other, less well-known serine-proteinase inhibitors have been described and there may possibly
1 The Greek letters and numbers refer to the localization
?f the inhibitors in electrophoregrams of plasma protems.