Archives of biochemistry and biophysics

Protein engineering of thromboxane synthase: conversion of membrane-bound to soluble form.

PMID 12859980


Thromboxane A2 synthase (TXAS) binds to the endoplasmic reticulum membrane and catalyzes both an isomerization of prostaglandin H2 (PGH2) to form thromboxane A2 (TXA2) and a fragmentation of PGH2 to form 12-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and malondialdehyde (MDA). TXAS is a non-classic cytochrome P450 in that it does not require molecular oxygen or an external electron donor for catalysis. Difficulty in obtaining crystals from the membrane-bound TXAS prompted us to modify the protein to a soluble form. Results from site-directed mutagenesis, hydropathy analysis, and homology modeling led us to identify a putative membrane association segment near the end of helix F in TXAS. We report here the generation of a soluble form of TXAS by deletion of the amino-terminal membrane-anchoring domain and replacement of the helix F and F-G loop region with the corresponding region of the structurally characterized microsomal P450 2C5. The resultant TXAS/2C5 chimera is expressed in bacteria as a cytosolic and monomeric protein. Addition of an amino-terminal leader sequence to enhance expression and a tetra-histidine segment at the carboxyl-terminus to facilitate purification yielded approximately 4 mg of nearly homogeneous TXAS/2C5 per liter of bacterial culture. The TXAS/2C5 chimera contains heme at nearly a 1:1 molar ratio and catalyzes the formation of TXA2, MDA, and HHT at a 1:1:1 ratio, although with a reduced catalytic activity compared to wild type TXAS. TXAS/2C5 exhibits electronic absorption spectra similar to wild type TXAS and has similar affinities toward distal heme ligands such as imidazole and U44069. The chimera was mono-dispersive and thus is promising for crystallization trials.

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9,11-Dideoxy-9α,11α-epoxymethanoprostaglandin F