90
3 The Structural Variety and Metabolism of Proteins
substrates. They have also been recorded in birds,
amphibians, fish, insects and molluscs, but have
been little investigated [95, 194]. Associated with
this group of enzymes are the angiotensinconverting enzymes that release the octapeptide
angiotensin II (causing an increase in blood pressure) from angiotensin I, and inactivate the nonapeptide bradykinin (which lowers the blood
pressure). One form of 160-170 kDa is found in
the lungs and various other tissues, and one form
of 100-110 kDa, which is identical at the Cterminus with the lung enzyme, is found in the
testis [64]. Various mammalian organs contain tripeptidyl hydrolases; for example, there is an
enzyme in the hypophysis and ovary that
sequentially cleaves tripeptides from growth hormone [139].
The carboxypeptidases of mammals are zinc
enzymes of 34 kDa and are secreted from the
pancreas as inactive precursors (procarboxypeptidases), although they also occur in the kidney and
other organs. The activation reactions of the procarboxypeptidases have been recently described.
Trypsin cleaves off a C-terminal activation peptide of 94 amino acids, from which a C-terminal
arginine residue is then cleaved by the now active
carboxypeptidase [284]. Two types of carboxypeptidase (CP), A and B, are distinguished and
these differ in sequence by 53 % and have different specificities: CP-A cleaves off aromatic amino
acids, whereas CP-B shows a preference for basic
amino acids. The rat and other mammals possess
two isoenzymes of type A (AI and A2). Sequence comparisons show that the sole bovine
carboxypeptidase A corresponds to type AI, and
A2 is missing. In addition to types A and B, there
are other carboxypeptidases that have special
functions relating to the release of active hormones from their precursor forms; these enzymes
are all homologous but show only 15-49 %
sequence agreement [77, 271]. Carboxypeptidase
are also widely found amongst the invertebrates
as digestive enzymes and, like the vertebrate
enzymes, are typical metalloenzymes [32, 113,
203]. The enzyme from the mid-gut gland of the
crayfish Astacus fluviatilis is the only one of this
type to have been sequenced so far. It has a
length of 303 amino acids and a mass (without the
zinc atom) of 33899 Da; the sequence shows
44-45 % agreement with the bovine carboxypeptidases A and B. Like the bovine carboxypeptidase B, there is an aspartate residue at the primary substrate-binding site, whilst the rat and
bovine carboxypeptidase A molecules have an
isoleucine residue [277].
3.3.2 Serine Proteinases
Apart from in the serine proteinases, bloodclotting factors and complement components,
serine is also found in the active centres of the
alkaline phosphatases, esterases, lipases and haptoglobins. Just within the serine proteinases, one
finds TCN (e.g. in trypsinogen) or AGY (e.g. in
prothrombin) as the codon for the active serine.
This leads to the hypothesis that the co dons for
the active serines have emerged by convergent
evolution from various amino acid codons of
evolutionary ancestors: e.g. from the threonine
codon ACN, or from the cysteine codon TGY
[22].
The mammalian pancreas secretes the zymogens of several serine proteinases with differing
specificities, the chymotrypsins, trypsins and elastases (Table 3.5). Although the amino acid
sequences of these enzymes differ by up to 60 % ,
their spatial structures agree to within 0.1 nm. The
genes of these proteinases have, without doubt,
all arisen from the same ancestral gene. The differences in specificity are due to only one or two
amino acid exchanges in the region of the binding
site: 189-Serl Asp in chymotrypsinltrypsin; and
216-GlyNal and 226-GlylThr in chymotrypsinl
elastase. Similar enzymes are apparently present
in all classes of vertebrates. Thus, the chymotrypsin, trypsin and elastase of the carp are all very
similar to the mammalian enzymes in terms of
amino acid composition, the partially known
sequences and the number of disulphide bridges.
The differences between the three types are notably greater than the differences found between
fish and humans for just one type, suggesting that
these enzyme types are already very old.
About 60 functionally different types of serine
proteinases have so far been described in mammals and in several cases show very narrow substrate and binding specificities [11]: e.g. enteropeptidase (previously enterokinase), which converts trypsinogen into active trypsin by cleavage
of a Lys/Ile bond; kallikrein (previously kininogenase), which produces the hormonally active bradykinin from kininogen by cleavage of -Lys/Argand -Arg/Ser- bonds (see Fig.8.5b, p. 302);
thrombin, which produces the fibrinopeptides A
and B by cleavage of -Arg/Gly- bonds in the A(a)
and B(B) chains of fibrinogen, and other components of the blood-clotting cascade; plasmin, which
solubilizes fibrin by cleavage of -LyslX- and -Arg/
X- bonds and has a specificity, though it is somewhat narrower, for other proteins similar to trypsin; and, finally, many components of the comple-
3 The Structural Variety and Metabolism of Proteins
substrates. They have also been recorded in birds,
amphibians, fish, insects and molluscs, but have
been little investigated [95, 194]. Associated with
this group of enzymes are the angiotensinconverting enzymes that release the octapeptide
angiotensin II (causing an increase in blood pressure) from angiotensin I, and inactivate the nonapeptide bradykinin (which lowers the blood
pressure). One form of 160-170 kDa is found in
the lungs and various other tissues, and one form
of 100-110 kDa, which is identical at the Cterminus with the lung enzyme, is found in the
testis [64]. Various mammalian organs contain tripeptidyl hydrolases; for example, there is an
enzyme in the hypophysis and ovary that
sequentially cleaves tripeptides from growth hormone [139].
The carboxypeptidases of mammals are zinc
enzymes of 34 kDa and are secreted from the
pancreas as inactive precursors (procarboxypeptidases), although they also occur in the kidney and
other organs. The activation reactions of the procarboxypeptidases have been recently described.
Trypsin cleaves off a C-terminal activation peptide of 94 amino acids, from which a C-terminal
arginine residue is then cleaved by the now active
carboxypeptidase [284]. Two types of carboxypeptidase (CP), A and B, are distinguished and
these differ in sequence by 53 % and have different specificities: CP-A cleaves off aromatic amino
acids, whereas CP-B shows a preference for basic
amino acids. The rat and other mammals possess
two isoenzymes of type A (AI and A2). Sequence comparisons show that the sole bovine
carboxypeptidase A corresponds to type AI, and
A2 is missing. In addition to types A and B, there
are other carboxypeptidases that have special
functions relating to the release of active hormones from their precursor forms; these enzymes
are all homologous but show only 15-49 %
sequence agreement [77, 271]. Carboxypeptidase
are also widely found amongst the invertebrates
as digestive enzymes and, like the vertebrate
enzymes, are typical metalloenzymes [32, 113,
203]. The enzyme from the mid-gut gland of the
crayfish Astacus fluviatilis is the only one of this
type to have been sequenced so far. It has a
length of 303 amino acids and a mass (without the
zinc atom) of 33899 Da; the sequence shows
44-45 % agreement with the bovine carboxypeptidases A and B. Like the bovine carboxypeptidase B, there is an aspartate residue at the primary substrate-binding site, whilst the rat and
bovine carboxypeptidase A molecules have an
isoleucine residue [277].
3.3.2 Serine Proteinases
Apart from in the serine proteinases, bloodclotting factors and complement components,
serine is also found in the active centres of the
alkaline phosphatases, esterases, lipases and haptoglobins. Just within the serine proteinases, one
finds TCN (e.g. in trypsinogen) or AGY (e.g. in
prothrombin) as the codon for the active serine.
This leads to the hypothesis that the co dons for
the active serines have emerged by convergent
evolution from various amino acid codons of
evolutionary ancestors: e.g. from the threonine
codon ACN, or from the cysteine codon TGY
[22].
The mammalian pancreas secretes the zymogens of several serine proteinases with differing
specificities, the chymotrypsins, trypsins and elastases (Table 3.5). Although the amino acid
sequences of these enzymes differ by up to 60 % ,
their spatial structures agree to within 0.1 nm. The
genes of these proteinases have, without doubt,
all arisen from the same ancestral gene. The differences in specificity are due to only one or two
amino acid exchanges in the region of the binding
site: 189-Serl Asp in chymotrypsinltrypsin; and
216-GlyNal and 226-GlylThr in chymotrypsinl
elastase. Similar enzymes are apparently present
in all classes of vertebrates. Thus, the chymotrypsin, trypsin and elastase of the carp are all very
similar to the mammalian enzymes in terms of
amino acid composition, the partially known
sequences and the number of disulphide bridges.
The differences between the three types are notably greater than the differences found between
fish and humans for just one type, suggesting that
these enzyme types are already very old.
About 60 functionally different types of serine
proteinases have so far been described in mammals and in several cases show very narrow substrate and binding specificities [11]: e.g. enteropeptidase (previously enterokinase), which converts trypsinogen into active trypsin by cleavage
of a Lys/Ile bond; kallikrein (previously kininogenase), which produces the hormonally active bradykinin from kininogen by cleavage of -Lys/Argand -Arg/Ser- bonds (see Fig.8.5b, p. 302);
thrombin, which produces the fibrinopeptides A
and B by cleavage of -Arg/Gly- bonds in the A(a)
and B(B) chains of fibrinogen, and other components of the blood-clotting cascade; plasmin, which
solubilizes fibrin by cleavage of -LyslX- and -Arg/
X- bonds and has a specificity, though it is somewhat narrower, for other proteins similar to trypsin; and, finally, many components of the comple-