664
17 Ester Hydrolases, ATPases and Carboanhydrases
optima and are inhibited by Ca z +; the other cell
organelles contain enzymes with pH optima of
8-9.5 that are stimulated by calcium ions. The
enzymes from the blood platelets of the rat or the
human spleen are atypical in that they are synthesized with a signal sequence but the membranebound, mature polypeptide of 124-125 amino
acids has no transmembrane segment. The
enzyme from human spleen shows only 39 %
agreement with that of the pancreatic secretion
[64, 73]. All phospholipases A2 are monomers or
dimers of small rigid polypeptides with about 125
amino acids and six or seven disulphide bridges.
More than 30 complete and several partial
sequences are known. The enzymes from the
pancreas and snake venom are sequence homologues and have very similar tertiary structures [37],
but, nevertheless, can be subdivided into several
structural classes. The first class includes the
enzymes from the pancreas and the venoms of the
more primitive cobras (Elapidae) and the sea snakes (Hydrophiidae). Characteristic of these enzymes is a disulphide bridge between ll-Cys and 69Cys. The enzymes of the second class, from the
venom of the pit vipers (Crotalidae), have a Cterminal extension of six amino acids, with the
terminal cysteine linked by a disulphide bridge to
the 50-Cys of the active centre. Invariant in both
classes are the 48-His, the 49-Asp of the calciumbinding site, and the loop with 28-Tyr, 30-Gly and
32-Gly; the characteristic residues 48-His and 49Asp are also found in the phospholipase A2 of bee
venom. Thus, it is rather strange to find that the
venoms of the water moccasin Agkistroson piscivorus and the fer-de-lance Bothrops atrox contain
not only typical A2 enzymes of the second class
but also A2 with a lysine residue in position 49. In
contrast to all the others, these enzymes do not
bind calcium in the absence of the substrate; the
order in which the catalytically active ternary
complex is formed is such that calcium is bound
to the enzyme after, rather than before, the phospholipid [94]. Without taking into account the 49Lys, a dendogram has been constructed in which
three groups can be distinguished. The first includes the A2 from the pancreas and the venoms of
the Asiatic Elapidae, the second the Az from the
Hydrophiidae venoms and the Australo-Asiatic
Elapidae, and the third the venoms of the Crotalidae and Viperidae. Surprisingly, all neurotoxic
venoms belong in the second group. The dendogram probably contains paralogous sequences
(p.155) and therefore cannot be accepted as a
reliable description of A2 evolution [37]. The
phospholipase Az from bee venom agrees with
the enzymes from mammalian pancreas and
snake venom in the position of the cysteine residues, but otherwise shows little similarity [80].
The existence of phospholipases B, which can
cleave fatty acids from both phospholipids and
lysophospholipids, was for some time controversial but is now accepted. Phosphatidylinositolspecific phospholipases C (PI-PLC) are components of a widely found signal transduction system
in which they generate the secondary messengers
inositol trisphosphate and diacylglycerol by
hydrolysis of phosphatidylinositol-4,5-bisphosphate. This signal transduction pathway links
very different stimuli with effector processes such
as cell growth and differentiation, contraction,
engram formation and photo transduction in
invertebrates. As in the case of the control of
adenylate cyclases and cGMP phosphodiesterases, activation of PI-PLC probably involves as yet
undefined G proteins. There are at least five
isoenzymes of PI-PLC in mammals; these are
subdivided into the four classes a-o and differ
widely in their molecular size and amino acid
sequence. PLC-a has a mass of 56.6 kDa (cDNA
from rat basophilic leukaemic cells), PLC-~
138.2 kDa, PLC-y 148.4 kDa and PLC-o
85.8 kDa (all cDNAs from rat or bovine brain).
The sequences of PLC-~, -y and -0 are only similar in certain regions, and PLC-a is completely
different from the other three classes. Thus, it is
surprising that these enzymes all have very similar catalytic properties. They hydrolyse phosphatidylinositol (PI), phosphatidylinositol-4-phosphate (PIP) and phosphatidylinositol-4,5-bisphosph ate (PIP2), but not phosphatidylcholine or
phosphatidylethanolamine, and they are Ca 2 +dependent [133]. A cDNA and corresponding
PLC protein isolated from Drosophila melanogaster shows extensive similarity in its sequence
of 1095 amino acids to PLC-o from bovine brain.
This is a product of the gene norpA (no receptor
potential A); mutants with this gene have no PLC
activity in their eyes, and consequently no receptor potential and are blind. Thus, the PLC
encoded by norpA apparently belongs to the phototransduction system of the Drosophila eye
[170]. A further PLC, expressed in Drosophila
brain, has been sequenced via the cDNA and
designated plc-21. Its sequence of 1305 or 1312
amino acids is clearly different from the norpA
sequence [149]. A PLC with a unique specificity is
found in Trypanosoma brucei. This cleaves the
glycosyl phosphatidylinositol, by means of which
the variable surface glycoprotein (VSG) is
anchored in the membrane, but has little activity
17 Ester Hydrolases, ATPases and Carboanhydrases
optima and are inhibited by Ca z +; the other cell
organelles contain enzymes with pH optima of
8-9.5 that are stimulated by calcium ions. The
enzymes from the blood platelets of the rat or the
human spleen are atypical in that they are synthesized with a signal sequence but the membranebound, mature polypeptide of 124-125 amino
acids has no transmembrane segment. The
enzyme from human spleen shows only 39 %
agreement with that of the pancreatic secretion
[64, 73]. All phospholipases A2 are monomers or
dimers of small rigid polypeptides with about 125
amino acids and six or seven disulphide bridges.
More than 30 complete and several partial
sequences are known. The enzymes from the
pancreas and snake venom are sequence homologues and have very similar tertiary structures [37],
but, nevertheless, can be subdivided into several
structural classes. The first class includes the
enzymes from the pancreas and the venoms of the
more primitive cobras (Elapidae) and the sea snakes (Hydrophiidae). Characteristic of these enzymes is a disulphide bridge between ll-Cys and 69Cys. The enzymes of the second class, from the
venom of the pit vipers (Crotalidae), have a Cterminal extension of six amino acids, with the
terminal cysteine linked by a disulphide bridge to
the 50-Cys of the active centre. Invariant in both
classes are the 48-His, the 49-Asp of the calciumbinding site, and the loop with 28-Tyr, 30-Gly and
32-Gly; the characteristic residues 48-His and 49Asp are also found in the phospholipase A2 of bee
venom. Thus, it is rather strange to find that the
venoms of the water moccasin Agkistroson piscivorus and the fer-de-lance Bothrops atrox contain
not only typical A2 enzymes of the second class
but also A2 with a lysine residue in position 49. In
contrast to all the others, these enzymes do not
bind calcium in the absence of the substrate; the
order in which the catalytically active ternary
complex is formed is such that calcium is bound
to the enzyme after, rather than before, the phospholipid [94]. Without taking into account the 49Lys, a dendogram has been constructed in which
three groups can be distinguished. The first includes the A2 from the pancreas and the venoms of
the Asiatic Elapidae, the second the Az from the
Hydrophiidae venoms and the Australo-Asiatic
Elapidae, and the third the venoms of the Crotalidae and Viperidae. Surprisingly, all neurotoxic
venoms belong in the second group. The dendogram probably contains paralogous sequences
(p.155) and therefore cannot be accepted as a
reliable description of A2 evolution [37]. The
phospholipase Az from bee venom agrees with
the enzymes from mammalian pancreas and
snake venom in the position of the cysteine residues, but otherwise shows little similarity [80].
The existence of phospholipases B, which can
cleave fatty acids from both phospholipids and
lysophospholipids, was for some time controversial but is now accepted. Phosphatidylinositolspecific phospholipases C (PI-PLC) are components of a widely found signal transduction system
in which they generate the secondary messengers
inositol trisphosphate and diacylglycerol by
hydrolysis of phosphatidylinositol-4,5-bisphosphate. This signal transduction pathway links
very different stimuli with effector processes such
as cell growth and differentiation, contraction,
engram formation and photo transduction in
invertebrates. As in the case of the control of
adenylate cyclases and cGMP phosphodiesterases, activation of PI-PLC probably involves as yet
undefined G proteins. There are at least five
isoenzymes of PI-PLC in mammals; these are
subdivided into the four classes a-o and differ
widely in their molecular size and amino acid
sequence. PLC-a has a mass of 56.6 kDa (cDNA
from rat basophilic leukaemic cells), PLC-~
138.2 kDa, PLC-y 148.4 kDa and PLC-o
85.8 kDa (all cDNAs from rat or bovine brain).
The sequences of PLC-~, -y and -0 are only similar in certain regions, and PLC-a is completely
different from the other three classes. Thus, it is
surprising that these enzymes all have very similar catalytic properties. They hydrolyse phosphatidylinositol (PI), phosphatidylinositol-4-phosphate (PIP) and phosphatidylinositol-4,5-bisphosph ate (PIP2), but not phosphatidylcholine or
phosphatidylethanolamine, and they are Ca 2 +dependent [133]. A cDNA and corresponding
PLC protein isolated from Drosophila melanogaster shows extensive similarity in its sequence
of 1095 amino acids to PLC-o from bovine brain.
This is a product of the gene norpA (no receptor
potential A); mutants with this gene have no PLC
activity in their eyes, and consequently no receptor potential and are blind. Thus, the PLC
encoded by norpA apparently belongs to the phototransduction system of the Drosophila eye
[170]. A further PLC, expressed in Drosophila
brain, has been sequenced via the cDNA and
designated plc-21. Its sequence of 1305 or 1312
amino acids is clearly different from the norpA
sequence [149]. A PLC with a unique specificity is
found in Trypanosoma brucei. This cleaves the
glycosyl phosphatidylinositol, by means of which
the variable surface glycoprotein (VSG) is
anchored in the membrane, but has little activity