Prostacyclin is a potent mediator of vasodilation and anti-platelet aggregation. It is synthesized from prostaglandin H(2) by prostacyclin synthase (PGIS), a member of Family 8 in the cytochrome P450 superfamily. Unlike most P450s, which require exogenous reducing equivalents and an oxygen molecule for mono-oxygenation, PGIS catalyzes an isomerization with an initial step of endoperoxide bond cleavage of prostaglandin H(2) (PGH(2)). The low abundance of PGIS in natural tissues necessitates heterologous expression for studies of structure/function relationships and reaction mechanism. We report here a high-yield prokaryotic system for expression of enzymatically active human PGIS. The PGIS cDNA is modified by replacing the hydrophobic amino-terminal sequence with the more hydrophilic amino-terminal sequence from P450 2C5 and by adding a four-histidine tag at the carboxyl terminus. The resulting recombinant PGIS associates with host cell membranes and was purified to electrophoretic homogeneity by nickel affinity, hydroxyapatite and CM Sepharose column chromatography. The recombinant PGIS, with a heme:protein ratio of 0.9:1, catalyzes prostacyclin formation at a K(m) of 13.3 muM PGH(2) and a V(max) of 980 per min. The dithionite-reduced PGIS binds CO with an on-rate of 5.6 x 10(5) M(-1) s(-1) and an off-rate of 15 s(-1). The ferrous-CO complex of PGIS is very short-lived and decays at a rate of 0.7 s(-1). Spectral binding assays showed that imidazole binds weakly to PGIS (K(d) approximately 0.5 mM,) but clotrimazole, a bulky and rigid imidazole derivative, binds strongly (K(d) approximately 1 microM). The transient nature of the CO complex and the weak imidazole binding seem to support an earlier proposal that PGIS active site has a limited space, but the tight binding of clotrimazole argues against this view. It appears that the heme distal pocket of PGIS is fairly adaptable to ligands of various structures. UV-visible absorption, magnetic circular dichroism and electron paramagnetic resonance spectra indicate that PGIS has a typical low-spin heme with a hydrophobic active site. PGIS catalyzes homolytic scission of the peroxide bond of a test substrate, 10-hydroperoxyoctadeca-8,12-dienoic acid, accompanied by formation of a heme intermediate with a Compound II-like optical spectrum.

Thromboxane A 2 (TXA 2 ) is a potent inducer of vasoconstriction and platelet aggregation. Large scale expression of TXA 2 synthase (TXAS) is very useful for studies of the reaction mechanism, structural/functional relationships, and drug interactions. We report here a heterologous system for overexpression of human TXAS. The TXAS cDNA was modified by replacing the sequence encoding the first 28 amino acid residues with a CYP17 amino-terminal sequence and by adding a polyhistidine tag sequence prior to the stop codon; the cDNA was inserted into the pCW vector and co-expressed with chaperonins groES and groEL in Escherichia coli. The resulting recombinant protein was purified to electrophoretic homogeneity by affinity, ion exchange, and hydrophobic chromatography. UV-visible absorbance (UVVis), magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) spectra indicate that TXAS has a typical low spin cytochrome P450 heme with an oxygen-based distal ligand. The UV-Vis and EPR spectra of recombinant TXAS were essentially identical to those of TXAS isolated from human platelets, except that a more homogenous EPR spectrum was observed for the recombinant TXAS. The recombinant protein had a heme:protein molar ratio of 0.7:1 and a specific activity of 12 mol of TXA 2 /min/mg of protein at 23°C. Furthermore, it catalyzed formation of TXA 2 , 12-hydroxy-5,8,10-heptadecatrienoic acid, and malondialdehyde in a molar ratio of 0.94:1.0:0.93. Spectral binding titrations showed that bulky heme ligands such as clotrimazole bound strongly to TXAS (K d ϳ0.5 M), indicating ample space at the distal face of the heme iron. Analysis of MCD and EPR spectra showed that TXAS was a typical low spin hemoprotein with a proximal thiolate ligand and had a very hydrophobic distal ligand binding domain.

Thromboxane synthase (TXAS) is a "non-classical" cytochrome P450. Without any need for an external electron donor, or for a reductase or molecular oxygen, it uses prostaglandin H 2 (PGH 2 ) to catalyze either an isomerization reaction to form thromboxane A 2 (TXA 2 ) or a fragmentation reaction to form 12-L-hydroxy-5,8,10-heptadecatrienoic acid and malondialdehyde (MDA) at a ratio of 1:1:1 (TXA 2 :heptadecatrienoic acid:MDA). We report here kinetics of TXAS with heme ligands in binding study and with PGH 2 in enzymatic study. We determined that 1) binding of U44069, an oxygen-based ligand, is a two-step process; U44069 first binds TXAS, then ligates the hemeiron with a maximal rate constant of 105-130 s ; the rate of catalytic conversion of PGH 2 to TXA 2 or MDA was at least 15,000 s ؊1 and the lower limit of the rates for products release was 4,000 -6,000 s ؊1. Given that the cellular PGH 2 concentration is quite low, we concluded that under physiological conditions, the substrate-binding step is the rate-limiting step of the TXAS-catalyzed reaction, in sharp contrast with "classical" P450 enzymes.

Human thromboxane A 2 synthase (TXAS) exhibits spectral characteristics of cytochrome P450 but lacks monooxygenase activity. Its distinctive amino acid sequence makes TXAS the sole member of family 5 in the P450 superfamily. To better understand the structurefunction relationship of this unusual P450, we have recently constructed a three-dimensional model for TXAS using P450 BM-3 as the template (Ruan, K.-H., Milfeld, K., Kulmacz, R. J., and Wu, K. K. (1994) Protein Eng. 7, 1345-1551) and have identified a potential active site region. The catalytic roles of several putative active site residues were evaluated using selectively mutated recombinant TXAS expressed in COS-1 cells. Mutation of Ala-408 to Glu or Arg-413 to Gly led to a complete loss of enzyme activity despite expression of mutant protein levels equivalent to that of the wild-type TXAS. Mutation of Ala-408 to Gly or Leu retained the enzyme activity at levels of 30 or 40%, respectively. This suggests that Ala-408 provides a hydrophobic environment for substrate binding. Mutation of Arg-413 to Lys or Gln completely abolished the enzyme activity, indicating that this residue is essential to catalytic activity and supports its identification as an active site residue. Mutation of Arg-410 to Gly or Glu-433 to Ala resulted in >50% reduction in the enzyme activity without appreciably altering mutant protein expression, consistent with a more subtle effect of these residues on TXAS catalytic efficiency. Mutation of residues predicted to be involved in binding the heme prosthetic group, including the heme thiolate ligand Cys-480, Arg-478, Phe-127, and Asn-110, each markedly reduced the expressed protein level and abolished enzyme activity. This suggests that proper heme binding is important to synthesis or stability of recombinant TXAS. Mutation of Ile-346, which corresponds to P450 cam -Thr-252, an essential amino acid involved in dioxygen bond scission, to Thr increased the enzymatic activity by 40%, suggesting that oxygen bond cleavage is not a rate-limiting step in thromboxane A 2 biosynthesis. The present results from site-directed mutagenesis support the overall structure of the TXAS active site predicted by homology modeling and have allowed refinement of the position of bound substrate.Thromboxane A 2 (TXA 2 ) 1 is a potent mediator of platelet aggregation, vasoconstriction, and bronchoconstriction and plays an important role in major human diseases, including myocardial infarction, stroke, septic shock, and asthma (1, 2). TXA 2 biosynthesis is catalyzed in succession by phospholipase A 2 , which liberates arachidonic acid from membrane phospholipids, prostaglandin H synthase, which converts arachidonate to prostaglandin H 2 (PGH 2 ), and TXA 2 synthase (TXAS, EC 5.3.99.5), which converts PGH 2 to TXA 2 as well as to 12L-hydroxy-5,8,10-heptadecatrienoic acid (3). Because PGH 2 can also be converted to several other prostanoids, TXAS plays a pivotal role in determining the amount of TXA 2 synthesized. In mammalian tissues, TXAS is a membrane-bound hemop...

Prostacyclin synthase (PGIS) is a member of the cytochrome P450 family in which the oxyferrous complexes are generally labile in the absence of substrate. At 4 °C, the on‐rate constants and off‐rate constants of oxygen binding to PGIS in solution are 5.9 × 105 m−1·s−1 and 29 s−1, respectively. The oxyferrous complex decays to a ferric form at a rate of 12 s−1. We report, for the first time, a stable oxyferrous complex of PGIS in a transparent sol–gel monolith. The encapsulated ferric PGIS retained the same spectroscopic features as in solution. The binding capabilities of the encapsulated PGIS were demonstrated by spectral changes upon the addition of O‐based, N‐based and C‐based ligands. The peroxidase activity of PGIS in sol–gel was three orders of magnitude slower than that in solution owing to the restricted diffusion of the substrate in sol–gel. The oxyferrous complex in sol–gel was observable for 24 h at room temperature and displayed a much red‐shifted Soret peak. Stabilization of the ferrous–carbon monoxide complex in sol–gel was observed as an enrichment of the 450‐nm species over the 420‐nm species. This result suggests that the sol–gel method may be applied to other P450s to generate a stable intermediate in the di‐oxygen activation.