96
3 The Structural Variety and Metabolism of Proteins
marked individual differences in the relative proportions of the isoforms of pepsinogen A, and
these are the result of differences in the number
of genes [266].
The polypeptide chains in molecules of the
pepsin-like proteinases form two lobes that are
bound together by two antiparallel ~-strands; the
active aspartate residues sit opposite each other
in the groove between the two lobes [121, 129].
Similar to the three-dimensional structures, the
internal symmetries of the nucleotide and amino
acid sequences also support the conclusion that
gene duplication and fusion occurred during the
evolution of these proteinases [108]. An Nterminal pro-sequence is cleaved off during the
activation of the zymogens that form in the stomach. The pro-sequence of the pepsinogen in mammals is 48 amino acids long and in the chicken has
44 amino acids; it is normally sequentially
removed in several short fragments, except in
Macaca fuscata, where it is removed as a whole by
one cleavage reaction [129]. 1\vo activation peptides are removed from the progastricsin of M. fuscata,' one is 25 and the other 18 amino acids long
[130].
The further away from the mammals in the vertebrate hierarchy, the less comparable are the
stomach proteinases with those of the mammals.
The chicken pepsinogen is still quite similar to pig
pepsin A, with 62 % identical amino acids. This is
a glycoprotein that is electrophoretically heterogeneous due to a variable number of sulphate
residues in the carbohydrate fraction [13]. In the
frog Rana catesbeiana, the same four pepsinogens
are produced in the oesophagal glands and in the
stomach wall. Their molecular masses of
31-34 kDa are significantly lower than that of the
pig pepsinogen A (43 kDa), although this omission of about 100 amino acids does not reduce
either the catalytic activity or the immunological
cross-reactivity to the human pepsinogen [250].
The two "gastricsinogenS" of the marine pike
Merluccius have a similar reduced size. The stomach proteinases of the cartilaginous and bony fish
also differ from those of the mammals in other
properties, e.g. in the presence of a chymosin-like
activity, in the reduced activity with certain synthetic substrates, and in the greater resistance to
alkali [80, 91].
Renin is an aspartate proteinase of extremely
narrow specificity whose pH optimum, in contrast
to all other enzymes of this class, extends over the
neutral range (pH 5-8). The enzyme is produced
in juxtaglomerular cells in the kidney and
released into the blood plasma in response to certain signals. There it cleaves off angiotensin I
from the dodecapeptide angiotensinogen; by the
action of a "converting enzyme", the former is
then converted to angiotensin II, the most effective of all blood-pressure increasing substances
(see Fig. 8.5, p. 302). Whilst the renin activity of
the kidneys is actually quite low, high activities
are found in the submaxillary glands of males of
several mouse strains. However, the glycosylated
enzyme of the kidney and the carbohydrate-free
enzyme of the salivary gland are encoded by different genes; the sequences of the two enzymes
differ by only 3 %, with the glycosylation site, as
expected, being amongst the 21 exchanged amino
acids. In man there is only one renin gene, the
product of which shows 69 % agreement with the
kidney enzyme of the mouse. Thus, it would
appear that the gene duplication which led to the
formation of distinct kidney and salivary gland
renins in the mouse occurred about 12 million
years ago, assuming that the evolutionary lines of
man and mouse separated about 80 million years
ago [214].
The catbepsins D and E are closely related to
the other aspartate proteinases (Table 3.6), but
are the only members of this family to be found in
the lysosomes [18]. They are involved, for example, in tail shortening during the metamorphosis
of the tadpole to the frog [186]. Similar intracellular enzymes have been detected in a range of very
different invertebrates, from the Protozoa
upwards [138, 196, 236]. In insects, they are
apparently involved in the lysis of the larval tissues during metamorphosis [138]. In the shelled
molluscs they are particularly active in mantle tissues and may therefore have something to do
with shell formation [196]. In the Hemiptera, on
the other hand, cathepsin D appears to function
extracellularly as a digestive enzyme, helped in
the case of the blood-sucking forms by cathepsin
C, which is otherwise normally intracellular and
belongs to the cysteine proteinases [112].
3.3.5 Metalloproteinases
The best-known metalloproteinases are the vertebrate collagenases, which are inhibited by
EDTA and reactivated by Zn 2 + [71]. There are
several types of such enzymes in the intercellular
matrix and they have varying specificities for different matrix components. The collagenases secreted by fibroblasts and white blood cells cleave a
particular bond in the triple helix of collagens
type I, II, III and VII at a position about three-
3 The Structural Variety and Metabolism of Proteins
marked individual differences in the relative proportions of the isoforms of pepsinogen A, and
these are the result of differences in the number
of genes [266].
The polypeptide chains in molecules of the
pepsin-like proteinases form two lobes that are
bound together by two antiparallel ~-strands; the
active aspartate residues sit opposite each other
in the groove between the two lobes [121, 129].
Similar to the three-dimensional structures, the
internal symmetries of the nucleotide and amino
acid sequences also support the conclusion that
gene duplication and fusion occurred during the
evolution of these proteinases [108]. An Nterminal pro-sequence is cleaved off during the
activation of the zymogens that form in the stomach. The pro-sequence of the pepsinogen in mammals is 48 amino acids long and in the chicken has
44 amino acids; it is normally sequentially
removed in several short fragments, except in
Macaca fuscata, where it is removed as a whole by
one cleavage reaction [129]. 1\vo activation peptides are removed from the progastricsin of M. fuscata,' one is 25 and the other 18 amino acids long
[130].
The further away from the mammals in the vertebrate hierarchy, the less comparable are the
stomach proteinases with those of the mammals.
The chicken pepsinogen is still quite similar to pig
pepsin A, with 62 % identical amino acids. This is
a glycoprotein that is electrophoretically heterogeneous due to a variable number of sulphate
residues in the carbohydrate fraction [13]. In the
frog Rana catesbeiana, the same four pepsinogens
are produced in the oesophagal glands and in the
stomach wall. Their molecular masses of
31-34 kDa are significantly lower than that of the
pig pepsinogen A (43 kDa), although this omission of about 100 amino acids does not reduce
either the catalytic activity or the immunological
cross-reactivity to the human pepsinogen [250].
The two "gastricsinogenS" of the marine pike
Merluccius have a similar reduced size. The stomach proteinases of the cartilaginous and bony fish
also differ from those of the mammals in other
properties, e.g. in the presence of a chymosin-like
activity, in the reduced activity with certain synthetic substrates, and in the greater resistance to
alkali [80, 91].
Renin is an aspartate proteinase of extremely
narrow specificity whose pH optimum, in contrast
to all other enzymes of this class, extends over the
neutral range (pH 5-8). The enzyme is produced
in juxtaglomerular cells in the kidney and
released into the blood plasma in response to certain signals. There it cleaves off angiotensin I
from the dodecapeptide angiotensinogen; by the
action of a "converting enzyme", the former is
then converted to angiotensin II, the most effective of all blood-pressure increasing substances
(see Fig. 8.5, p. 302). Whilst the renin activity of
the kidneys is actually quite low, high activities
are found in the submaxillary glands of males of
several mouse strains. However, the glycosylated
enzyme of the kidney and the carbohydrate-free
enzyme of the salivary gland are encoded by different genes; the sequences of the two enzymes
differ by only 3 %, with the glycosylation site, as
expected, being amongst the 21 exchanged amino
acids. In man there is only one renin gene, the
product of which shows 69 % agreement with the
kidney enzyme of the mouse. Thus, it would
appear that the gene duplication which led to the
formation of distinct kidney and salivary gland
renins in the mouse occurred about 12 million
years ago, assuming that the evolutionary lines of
man and mouse separated about 80 million years
ago [214].
The catbepsins D and E are closely related to
the other aspartate proteinases (Table 3.6), but
are the only members of this family to be found in
the lysosomes [18]. They are involved, for example, in tail shortening during the metamorphosis
of the tadpole to the frog [186]. Similar intracellular enzymes have been detected in a range of very
different invertebrates, from the Protozoa
upwards [138, 196, 236]. In insects, they are
apparently involved in the lysis of the larval tissues during metamorphosis [138]. In the shelled
molluscs they are particularly active in mantle tissues and may therefore have something to do
with shell formation [196]. In the Hemiptera, on
the other hand, cathepsin D appears to function
extracellularly as a digestive enzyme, helped in
the case of the blood-sucking forms by cathepsin
C, which is otherwise normally intracellular and
belongs to the cysteine proteinases [112].
3.3.5 Metalloproteinases
The best-known metalloproteinases are the vertebrate collagenases, which are inhibited by
EDTA and reactivated by Zn 2 + [71]. There are
several types of such enzymes in the intercellular
matrix and they have varying specificities for different matrix components. The collagenases secreted by fibroblasts and white blood cells cleave a
particular bond in the triple helix of collagens
type I, II, III and VII at a position about three-