86
3 The Structural Variety and Metabolism of Proteins
sion and an additional 13 positive charges [20].
The glycosomal phosphoglycerate kinase of Crithidia Jasciculata also has a C-terminal topographical signal [264]. Compared with the homologous cytoplasmic enzyme of other eukaryotes,
the glyceraldehyde-3-phosphate dehydrogenase
in the glycosomes of T. brucei is five amino acids
longer, but this does not apply to the aldolase or
the triosephosphate isomerase; all three enzymes, however, have a particularly large number
of positive charges, which apparently represent
the translocation signal [20]. The alanine:glyoxylate aminotransferase AGTI is located in the peroxisomes of the primates and hare-like species
(lagomorphs), in the mitochondria of the carnivores, and in both organelles of the rodents. The
mitochondrial target signal of the primates has
been lost during evolution by mutation of the
initiation codon ATG to ATA; the corresponding
reverse mutation in humans leads to the lethal
heritable condition of primary hyperoxaluria,
because the location of the AGTI in the mitochondria reduces its efficiency [268].
The enzyme proteins of the Iysosomes either
originate directly from the ER, from whence the
lysosomes themselves come, or they are later
taken up via "coated vesicles" from the cytoplasm. The markers for inclusion in lysosomes are
apparently oligosaccharides with a terminal
mannose-6-phosphate that is created by attachment of I-phospho-N-acetylglucosamine to a terminal mannose residue, followed by cleavage of
the N-acetylglucosamine. There are two types of
mannose-6-phosphate receptors in the Golgi
membrane, of which the larger (CI-MPR) is
cation independent and the smaller (CD-MPR)
requires divalent cations. The CI-MPR begins
with an extracytoplasmic domain of 2269 amino
acids made up of 15 repeats of a 147-amino-acid
sequence and carrying 19 glycosylation sites. This
is followed by a transmembrane segment of 23
amino acids and the C-terminal cytoplasmic
domain of 163 amino acids. In the subunits of the
homodimeric CD-MPR, the extracytoplasmic
domain includes only 159 amino acids with five
glycosylation sites, but shows significant
sequence agreement (14-28 %) with the repeats
of the CI-MPR. Its transmembrane segment has
25 amino acids, and the cytoplasmic domain 67
amino acids. Surprisingly, it has been found that
the CI-MPR serves at the same time as a receptor
for the insulin-like growth factor IGF-II [45, 144,
146, 163]. The proteins of the ceO nucleus must
possess internal non-cleavable signals as the nuclear membrane is dispersed during cell division.
The signals consistently have one proline and severallysine residues; for example, in the antigen to
the SV40 virus one finds the sequence -Pro-LysLys-Lys-Arg-Lys-Val [2, 191, 251, 288].
3.3 Proteolysis
Proteolytic enzymes (proteases) have a variety of
biological functions. In the digestion of food and
the autolysis of dead cells, the amino acid constituents of all the proteins involved are similarly
and completely released and thus become available again for cell metabolism. The complete
cleavage of a polypeptide into its individual components requires the successive action of several
proteases with varying specificities (Table 3.3).
Initially, endopeptidases (proteinases) break the
chain into smaller and smaller fragments by cleavage of internal bonds; several proteinases with
different bond specificities are often involved.
The resulting cleavage peptides are further stepTable 3.3. Classification of the proteases [11, 189]. Only a
few examples are given in each case; enzymes not of animal origin are shown within brackets. The characteristic
amino acids of the active centres are given in parentheses:
the position numbers refer to the first named enzyme
Exopeptidases: cleave bonds at the end of the peptide
chain
Aminopeptidases: release the N -terminal amino acid
Carboxypeptidases: release the C-terminal amino acid
Carboxypeptidase A (Zn 2 +, 255-Ile)
Carboxypeptidase B (Zn2+, 255-Asp)
Dipeptidyl aminopeptidases: release the N-terminal
dipeptide
Dipeptidases: cleave dipeptides
Eodopeptidases or proteinases: cleave internal bonds of
peptide chains
Serine proteinases I (102-Asp, 195-Ser, 57-Ser)
Chymotrypsin, trypsin, elastase, cathepsin G,
acrosin, enteropeptidase, plasmin, kallikrein
Thrombin and other blood-clotting factors
Complement components
Serine proteinases II (32-Asp, 221-Ser, 64-His)
[Subtilisin]
Cysteine proteinases (25-Cys, 159-His, 158-Asp)
[Papain]
Calpains (calcium-dependent, papain-like
proteinases)
Aspartate proteinases (33-Asp, 213-Asp)
[Penicillopepsin]
Pepsin, chymosin, cathepsins D and E, renin
Metalloproteinases I (ZN2+)
Collagenases
Metalloproteinases II (Zn2+, 243-Glu, 231-His)
[Thermolysin]
3 The Structural Variety and Metabolism of Proteins
sion and an additional 13 positive charges [20].
The glycosomal phosphoglycerate kinase of Crithidia Jasciculata also has a C-terminal topographical signal [264]. Compared with the homologous cytoplasmic enzyme of other eukaryotes,
the glyceraldehyde-3-phosphate dehydrogenase
in the glycosomes of T. brucei is five amino acids
longer, but this does not apply to the aldolase or
the triosephosphate isomerase; all three enzymes, however, have a particularly large number
of positive charges, which apparently represent
the translocation signal [20]. The alanine:glyoxylate aminotransferase AGTI is located in the peroxisomes of the primates and hare-like species
(lagomorphs), in the mitochondria of the carnivores, and in both organelles of the rodents. The
mitochondrial target signal of the primates has
been lost during evolution by mutation of the
initiation codon ATG to ATA; the corresponding
reverse mutation in humans leads to the lethal
heritable condition of primary hyperoxaluria,
because the location of the AGTI in the mitochondria reduces its efficiency [268].
The enzyme proteins of the Iysosomes either
originate directly from the ER, from whence the
lysosomes themselves come, or they are later
taken up via "coated vesicles" from the cytoplasm. The markers for inclusion in lysosomes are
apparently oligosaccharides with a terminal
mannose-6-phosphate that is created by attachment of I-phospho-N-acetylglucosamine to a terminal mannose residue, followed by cleavage of
the N-acetylglucosamine. There are two types of
mannose-6-phosphate receptors in the Golgi
membrane, of which the larger (CI-MPR) is
cation independent and the smaller (CD-MPR)
requires divalent cations. The CI-MPR begins
with an extracytoplasmic domain of 2269 amino
acids made up of 15 repeats of a 147-amino-acid
sequence and carrying 19 glycosylation sites. This
is followed by a transmembrane segment of 23
amino acids and the C-terminal cytoplasmic
domain of 163 amino acids. In the subunits of the
homodimeric CD-MPR, the extracytoplasmic
domain includes only 159 amino acids with five
glycosylation sites, but shows significant
sequence agreement (14-28 %) with the repeats
of the CI-MPR. Its transmembrane segment has
25 amino acids, and the cytoplasmic domain 67
amino acids. Surprisingly, it has been found that
the CI-MPR serves at the same time as a receptor
for the insulin-like growth factor IGF-II [45, 144,
146, 163]. The proteins of the ceO nucleus must
possess internal non-cleavable signals as the nuclear membrane is dispersed during cell division.
The signals consistently have one proline and severallysine residues; for example, in the antigen to
the SV40 virus one finds the sequence -Pro-LysLys-Lys-Arg-Lys-Val [2, 191, 251, 288].
3.3 Proteolysis
Proteolytic enzymes (proteases) have a variety of
biological functions. In the digestion of food and
the autolysis of dead cells, the amino acid constituents of all the proteins involved are similarly
and completely released and thus become available again for cell metabolism. The complete
cleavage of a polypeptide into its individual components requires the successive action of several
proteases with varying specificities (Table 3.3).
Initially, endopeptidases (proteinases) break the
chain into smaller and smaller fragments by cleavage of internal bonds; several proteinases with
different bond specificities are often involved.
The resulting cleavage peptides are further stepTable 3.3. Classification of the proteases [11, 189]. Only a
few examples are given in each case; enzymes not of animal origin are shown within brackets. The characteristic
amino acids of the active centres are given in parentheses:
the position numbers refer to the first named enzyme
Exopeptidases: cleave bonds at the end of the peptide
chain
Aminopeptidases: release the N -terminal amino acid
Carboxypeptidases: release the C-terminal amino acid
Carboxypeptidase A (Zn 2 +, 255-Ile)
Carboxypeptidase B (Zn2+, 255-Asp)
Dipeptidyl aminopeptidases: release the N-terminal
dipeptide
Dipeptidases: cleave dipeptides
Eodopeptidases or proteinases: cleave internal bonds of
peptide chains
Serine proteinases I (102-Asp, 195-Ser, 57-Ser)
Chymotrypsin, trypsin, elastase, cathepsin G,
acrosin, enteropeptidase, plasmin, kallikrein
Thrombin and other blood-clotting factors
Complement components
Serine proteinases II (32-Asp, 221-Ser, 64-His)
[Subtilisin]
Cysteine proteinases (25-Cys, 159-His, 158-Asp)
[Papain]
Calpains (calcium-dependent, papain-like
proteinases)
Aspartate proteinases (33-Asp, 213-Asp)
[Penicillopepsin]
Pepsin, chymosin, cathepsins D and E, renin
Metalloproteinases I (ZN2+)
Collagenases
Metalloproteinases II (Zn2+, 243-Glu, 231-His)
[Thermolysin]