title: Comparison of the construction of a 3-D model for human thromboxane synthase using P450cam and BM-3 as templates: implications for the substrate binding pocket.
abstract: A 3-D model of human thromboxane A2 synthase (TXAS) was constructed using a homology modeling approach based on information from the 2.0 A crystal structure of the hemoprotein domains of cytochrome P450BM-3 and P450cam. P450BM-3 is a bacterial fatty acid monooxygenase resembling eukaryotic microsomal cytochrome P450s in primary structure and function. TXAS shares 26.4% residue identity and 48.4% residue similarity with the P450BM-3 hemoprotein domain. The homology score between TXAS and P450BM-3 is much higher than that between TXAS and P450cam. Alignment between TXAS and the P450BM-3 hemoprotein domain or P450cam was determined through sequence searches. The P450BM-3 or P450cam main-chain coordinates were applied to the TXAS main chain in those segments where the two sequences were well aligned. These segments were linked to one another using a fragment search method, and the side chains were added to produce a 3-D model for TXAS. A TXAS substrate, prostaglandin H2 (PGH2) was docked into the TXAS cavity corresponding to the arachidonic acid binding pocket in P450BM-3 or camphor binding site in P450cam. Regions of the heme and putative PGH2 binding cavities in the TXAS model were identified and analyzed. The segments and residues involved in the active-site pocket of the TXAS model provide reasonable candidates for TXAS protein engineering and inhibitor design. Comparison of the TXAS model based on P450BM-3 with another TXAS model based on the P450cam structure indicated that P450BM-3 is a more suitable template for homology modeling of TXAS.
authors.affiliation: Department of Internal Medicine, University of Texas Health Science Center at Houston 77030.
authors.forename: K H
authors.initials: KH
authors.lastname: Ruan
authors.forename: K
authors.initials: K
authors.lastname: Milfeld
authors.forename: R J
authors.initials: RJ
authors.lastname: Kulmacz
authors.forename: K K
authors.initials: KK
authors.lastname: Wu
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins; D001665:Binding Sites; D019475:Camphor 5-Monooxygenase; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry; D006418:Heme / Q000737:chemistry; D006801:Humans; D006899:Mixed Function Oxygenases / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009251:NADPH-Ferrihemoprotein Reductase; D044262:Prostaglandin H2; D011463:Prostaglandins H / Q000737:chemistry; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013930:Thromboxane-A Synthase / Q000737:chemistry*
chemical_list: D001426:Bacterial Proteins; D011463:Prostaglandins H; D044262:Prostaglandin H2; D006418:Heme; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D019475:Camphor 5-Monooxygenase; D009251:NADPH-Ferrihemoprotein Reductase; C052423:flavocytochrome P450 BM3 monoxygenases; D013930:Thromboxane-A Synthase