212
Dimerization Domain Hydrophobic interactions, salt bridges and hydrogen bondings hold the dimers of both COX 1 and COX 2 together. Heterodimerization of
these subunits does not occur. Approximately 50 amino acids are encoded by the
dimerization domain near the amino terminus of the functional protein. Three disulphide bonds hold this domain together. A fourth disulphide bond links the dimerization domain with the catalytic domain which are required for maintaining oxidizing
environment. Membrane Binding Domain creates a hydrophobic surface through
sequence of four amphipathic helices that spear into the upper portion of the luminal
side of lipid bilayer. Helices are encoded by about 50 amino acids which are found
close, after carboxy-terminal to the dimerization domain. About 80% of protein
comes under the Catalytic Domain and have two distinct active sites. Carboxyterminal of membrane binding domain constitutes the catalytic domain. The two
distinct active sites include Peroxidase active site and Cyclooxygenase active site
(Simmons et al. 2004).
Peroxidase active site has two separate interlinking lobes which create a shallow
cleft on upper surface of enzyme where heme is bound and an iron-histidine
(His388) coordination is involved. Other important interactions identified include
those between the protoporphyrin and coordination of PGG 2 with specific amino
acids. The geometry of heme binding leaves a large portion of one side of the heme
exposed in the open cleft of the peroxidase active site. This facilitates its interaction
with PGG 2 and other lipid peroxides.
Active site of cyclooxygenase is a long, narrow, dead-end channel of largely
hydrophobic character whose entrance is bordered by the four amphipathic helices
of the membrane-binding domain. The globular catalytic domain has an extent of 25
Å with an average width of about 8 Å. However, the channel narrowing is observed
where Arg120, one of the two ionic residues, found in the COX active site, protrudes into the channel to form a hydrogen-bonded network with Glu524 and
Tyr355. Arg120 is essential for binding substrates and carboxylate-containing
NSAIDs in COX 1. Major difference in the structure of COX 1 and COX 2 is a
substitution of Ile523 in COX 1 for Val523 in COX 2 (Fig.9.12). This structural difference makes COX 2 drugs accessible for the active site. Other differences in the
structure make changes which result in slightly enlarged active site of COX 2 than
COX 1 (Simmons et al. 2004).
COX 3, the variant form of cyclooxygenase, is present in brain, and it is having
a structure similar to COX 1 and COX 2 but have an array of additional 30 amino
acids. COX 3 shows a property of unusual drug sensitivity. Simmons et al. (2004)
experiments revealed that COX3 was more sensitive to paracetamol, diclofenac,
ibuprofen and aspirin than COX1 or COX2. COX 3 was also demonstrated to be
sensitive to drugs like antipyrine, aminopyrine, dipyrone and phenacetin, which are
not inhibitors to other COX preparations.
9.2.2.2 Mechanism of the Cyclooxygenase Reaction
The conversion of PGG 2 from arachidonic acid includes the abstraction of pro-S
hydrogen from carbon-13. For the catalytic action to occur cyclooxygenase must
activate the process dependent on peroxidase activity. These reactions (electron
C.S. Sharanya and M. Haridas
Dimerization Domain Hydrophobic interactions, salt bridges and hydrogen bondings hold the dimers of both COX 1 and COX 2 together. Heterodimerization of
these subunits does not occur. Approximately 50 amino acids are encoded by the
dimerization domain near the amino terminus of the functional protein. Three disulphide bonds hold this domain together. A fourth disulphide bond links the dimerization domain with the catalytic domain which are required for maintaining oxidizing
environment. Membrane Binding Domain creates a hydrophobic surface through
sequence of four amphipathic helices that spear into the upper portion of the luminal
side of lipid bilayer. Helices are encoded by about 50 amino acids which are found
close, after carboxy-terminal to the dimerization domain. About 80% of protein
comes under the Catalytic Domain and have two distinct active sites. Carboxyterminal of membrane binding domain constitutes the catalytic domain. The two
distinct active sites include Peroxidase active site and Cyclooxygenase active site
(Simmons et al. 2004).
Peroxidase active site has two separate interlinking lobes which create a shallow
cleft on upper surface of enzyme where heme is bound and an iron-histidine
(His388) coordination is involved. Other important interactions identified include
those between the protoporphyrin and coordination of PGG 2 with specific amino
acids. The geometry of heme binding leaves a large portion of one side of the heme
exposed in the open cleft of the peroxidase active site. This facilitates its interaction
with PGG 2 and other lipid peroxides.
Active site of cyclooxygenase is a long, narrow, dead-end channel of largely
hydrophobic character whose entrance is bordered by the four amphipathic helices
of the membrane-binding domain. The globular catalytic domain has an extent of 25
Å with an average width of about 8 Å. However, the channel narrowing is observed
where Arg120, one of the two ionic residues, found in the COX active site, protrudes into the channel to form a hydrogen-bonded network with Glu524 and
Tyr355. Arg120 is essential for binding substrates and carboxylate-containing
NSAIDs in COX 1. Major difference in the structure of COX 1 and COX 2 is a
substitution of Ile523 in COX 1 for Val523 in COX 2 (Fig.9.12). This structural difference makes COX 2 drugs accessible for the active site. Other differences in the
structure make changes which result in slightly enlarged active site of COX 2 than
COX 1 (Simmons et al. 2004).
COX 3, the variant form of cyclooxygenase, is present in brain, and it is having
a structure similar to COX 1 and COX 2 but have an array of additional 30 amino
acids. COX 3 shows a property of unusual drug sensitivity. Simmons et al. (2004)
experiments revealed that COX3 was more sensitive to paracetamol, diclofenac,
ibuprofen and aspirin than COX1 or COX2. COX 3 was also demonstrated to be
sensitive to drugs like antipyrine, aminopyrine, dipyrone and phenacetin, which are
not inhibitors to other COX preparations.
9.2.2.2 Mechanism of the Cyclooxygenase Reaction
The conversion of PGG 2 from arachidonic acid includes the abstraction of pro-S
hydrogen from carbon-13. For the catalytic action to occur cyclooxygenase must
activate the process dependent on peroxidase activity. These reactions (electron
C.S. Sharanya and M. Haridas