Transformation of AA to PGH2

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The COX enzyme possesses two isoforms, COX-1 and COX-2, of which both produce bioactive eicosanoids. The two isoforms have nearly identical active site residues (Simmons et al. 2004), but COX-1 is a constitutive enzyme, whereas COX-2 is an inducible enzyme. Both enzymes add molecular oxygen to arachidonic acid, generating an endoperoxide species, PGH2, by catalysing the two-step reaction of cycloooxygenation and oxygenation, followed by a hydroperoxide reduction.

PGH2 possesses a short life time due to the formation of a reactive functional group. The endoperoxide undergoes rapid isomerisation reactions, catalysed by a series of synthase enzymes (PGES, PGDS, PGIS, PGFS and TXAS). The resultant species of the isomerisation reactions are PGs (PGE2, PGD2, PGJ2, deoxy-PGJ2 and PGF2α), TXs (TXA2 and TXB2) and prostacyclin (PGI2), all of which have varying effects on the immune system.

Reaction

Chemical equation

$AA \rightleftharpoons PGH2$

Rate equation

COX-1/-2 Enzyme Parameters

Gibbs Free Energy Change

Value	Units	Species	Notes	Reference
(-30)	kcal/mol	Unspecified	Calculations with a Gaussian98 suite of programs Enzyme: COX (Unspecific) Substrate: Arachidonate Temperature: 298.15 K Pressure: 1 bar

COX-1 Enzyme Parameters

COX-1 Michaelis-Menten Constants

Value	Units	Species	Notes	Reference
0.0088 ± 0.0022	$mM$	Human	Expression Vector: Human Enzyme: Cyclooxygenase-1 pH: 7.6 - 8.4 Temperature: 37 °C
0.0019 ± 0.0002	$mM$	Ram	Expression Vector: Ram Enzyme: Cyclooxygenase-1 pH: 8 Temperature: 30 ± 0.2 °C

COX-1 Turnover Number

Value	Units	Species	Notes	Reference
8820 ± 360	$per minute$	Ram	Expression Vector: Ram Enzyme: Cyclooxygenase-1 pH: 8 Temperature: 30 ± 0.2 °C

COX-1 Abundance

Value	Units	Species	Notes	Reference
1176	$ppm$	Human	Expression Vector: Urinary Bladder Enzyme: Cyclooxygenase-1 (PGTS1) pH: 7.5 Temperature: 37 °C
849	$ppm$	Human	Expression Vector: Platlet Enzyme: Cyclooxygenase-1 (PGTS1) pH: 7.5 Temperature: 37 °C
208	$ppm$	Human	Expression Vector: Stomach Enzyme: Cyclooxygenase-1 (PGTS1) pH: 7.5 Temperature: 37 °C
130	$ppm$	Human	Expression Vector: Esophagus Enzyme: Cyclooxygenase-1 (PGTS1) pH: 7.5 Temperature: 37 °C
70.8	$ppm$	Human	Expression Vector: Oral Cavity Enzyme: Cyclooxygenase-1 (PGTS1) pH: 7.5 Temperature: 37 °C
19.1	$ppm$	Human	Expression Vector: Lung Enzyme: Cyclooxygenase-1 (PGTS1) pH: 7.5 Temperature: 37 °C

COX-2 Enzyme Parameters

K_ms

Literature COX-2 Michaelis-Menten Constant (When AA is the substrate)

Value	Units	Species	Notes	Reference
1.62E-02 ± 0.22E-02	$mM$	Human	Expression Vector: Embryonic kidney cells Enzyme: Cyclooxygenase-2 pH: Not specified Temperature: Not specified
5.14E-03 ± 2.90E-04	mM	Mouse	Expression Vector: Baculovirus (Insect Cell) Enzyme: COX-2 pH: 8.0 Temperature: 37 °C 1–200 µM of substrate.
2.1E-03 ± 4.00E-04	mM	Human	Expression Vector: E. Coli Enzyme: Wild Type Cyclooxygenase-2 Enzyme pH: 8.5 Temperature:30 °C
2.1E-03 ± 4.00E-04	mM	Human	Expression Vector: Baculuvirus Enzyme: Wild Type Cycloxygenase-2 pH: 7.2 Temperature: 30 °C 100 uM arachidonate substrate,

Summary of the COX-2 Kms distribution

Mode (mM)	Confidence Interval	Location parameter (μ)	Scale parameter (σ)
4.90E-03	5.70E+00	-4.72E+00	7.77E-01

K_mp (Dependent parameter)

Summary of the COX-2 Kmp distribution

Mode	Location parameter (µ)	Scale parameter (σ)
4.70E-03	-4.75E+00	7.87E-01

k_cat

|+ style="text-align: left;" | Literature COX-2 k_cat

Value	Units	Species	Notes	Reference
1620 ± 24	per minute	Mouse	Expression Vector: Baculovirus (Insect Cell) Enzyme: COX-2 pH: 8.0 Temperature: 37 °C 1–200 µM of substrate.

|+ style="text-align: left;" | Summary of the COX-2 k_cat distribution

Mode (min-1)	Confidence Interval	Location parameter (µ)	Scale parameter (σ)
1.62E+03	1.01E+00	7.39E+00	1.48E-02

Enzyme concentration

|+ style="text-align: left;" | Literature COX-2 concentration

Value	Units	Species	Notes	Reference
13.7	$ppm$	Human	Expression Vector: Platlet Enzyme: Cyclooxygenase-2 (PGTS2) pH: 7.5 Temperature: 37 °C
4.11	$ppm$	Human	Expression Vector: Stomach Enzyme: Cyclooxygenase-2 (PGTS2) pH: 7.5 Temperature: 37 °C
1.49	$ppm$	Human	Expression Vector: Oral Cavity Enzyme: Cyclooxygenase-2 (PGTS2) pH: 7.5 Temperature: 37 °C

The abundance of COX-2 (ppm) was converted to COX-2 (mM). As a result, the concentration of COX-2 in unstimulated tissue was estimated as 2.27 x10-5 mM. The upregulation of COX-2 in HaCaT keratinocytes was estimated using western blotting in (Kiezel-Tsugunova, 2017), figure * shows an example. Using this information, in silico experiments which included an upregulation of COX-2, included the concentration of COX-2 reaching 100 times higher concentration than the estimated unstimulated concentration. Therefore, the COX-2 induction event includes the concentration of COX-2 eventually reaching a concentration of 2.27 x10-3 mM after 6h post irradiation.

COX Enzyme Parameters with other substrates

Michaelis-Menten Constants (When EPA is the substrate)

Value	Units	Species	Notes	Reference
9.45E-03 ± 7.30E-04	mM	Mouse	Expression Vector: Baculovirus (Insect Cell) Enzyme: COX-2 pH: 8.0 Temperature: 37 °C 1–200 µM of substrate.

Enzyme Turnover Numbers (When EPA is the substrate)

Value	Units	Species	Notes	Reference
522 ± 12.6	per minute	Mouse	Expression Vector: Baculovirus (Insect Cell) Enzyme: COX-2 pH: 8.0 Temperature: 37 °C 1–200 µM of substrate.

Michaelis-Menten Constants (When DHA is the substrate)

Value	Units	Species	Notes	Reference
3.68E-02 ± 4.25E-03	mM	Mouse	Expression Vector: Baculovirus (Insect Cell) Enzyme: COX-2 pH: 8.0 Temperature: 37 °C 1–200 µM of substrate.

Enzyme Turnover Numbers (When DHA is the substrate)

Value	Units	Species	Notes	Reference
207 ± 9	per minute	Mouse	Expression Vector: Baculovirus (Insect Cell) Enzyme: COX-2 pH: 8.0 Temperature: 37 °C 1–200 µM of substrate.

Related Reactions

• Transformation of PL to AA
• Transformation of PGH2 to PGF2α
• Transformation of PGH2 to TXA2
• Transformation of PGH2 to PGI2
• Transformation of PGH2 to PGD2
• Transformation of PGH2 to PGE2