title: Structural comparisons of arachidonic acid-induced radicals formed by prostaglandin H synthase-1 and -2.
abstract: Cyclooxygenase catalysis by prostaglandin H synthase (PGHS)-1 and -2 involves reaction of a peroxide-induced Tyr385 radical with arachidonic acid (AA) to form an AA radical that reacts with O(2). The potential for isomeric AA radicals and formation of an alternate tyrosyl radical at Tyr504 complicate analysis of radical intermediates. We compared the EPR spectra of PGHS-1 and -2 reacted with peroxide and AA or specifically deuterated AA in anaerobic, single-turnover experiments. With peroxide-treated PGHS-2, the carbon-centered radical observed after AA addition was consistently a pentadienyl radical; a variable wide-singlet (WS) contribution from mixture of Tyr385 and Tyr504 radicals was also present. Analogous reactions with PGHS-1 produced EPR signals consistent with varying proportions of pentadienyl and tyrosyl radicals, and two additional EPR signals. One, insensitive to oxygen exposure, is the narrow singlet tyrosyl radical with clear hyperfine features found previously in inhibitor-pretreated PGHS-1. The second type of EPR signal is a narrow singlet lacking detailed hyperfine features that disappeared upon oxygen exposure. This signal was previously ascribed to an allyl radical, but high field EPR analysis indicated that ~90% of the signal originates from a novel tyrosyl radical, with a small contribution from a carbon-centered species. The radical kinetics could be resolved by global analysis of EPR spectra of samples trapped at various times during anaerobic reaction of PGHS-1 with a mixture of peroxide and AA. The improved understanding of the dynamics of AA and tyrosyl radicals in PGHS-1 and -2 will be useful for elucidating details of the cyclooxygenase mechanism, particularly the H-transfer between tyrosyl radical and AA.
authors.affiliation: Department of Internal Medicine, University of Texas Health Science Center at Houston, Houston, TX 77030, USA. ah-lim.tsai@uth.tmc.edu
authors.forename: Ah-lim
authors.initials: AL
authors.lastname: Tsai
authors.forename: Gang
authors.initials: G
authors.lastname: Wu
authors.forename: Corina E
authors.initials: CE
authors.lastname: Rogge
authors.forename: Jian-Ming
authors.initials: JM
authors.lastname: Lü
authors.forename: Sheng
authors.initials: S
authors.lastname: Peng
authors.forename: Wilfred A
authors.initials: WA
authors.lastname: van der Donk
authors.forename: Graham
authors.initials: G
authors.lastname: Palmer
authors.forename: Gary J
authors.initials: GJ
authors.lastname: Gerfen
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Kulmacz
mesh_terms: D000466:Alkadienes / Q000737:chemistry / Q000378:metabolism; D016718:Arachidonic Acid / Q000737:chemistry* / Q000378:metabolism; D051545:Cyclooxygenase 1 / Q000737:chemistry* / Q000378:metabolism; D051546:Cyclooxygenase 2 / Q000737:chemistry* / Q000378:metabolism; D016861:Cyclooxygenase Inhibitors / Q000737:chemistry / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D010100:Oxygen / Q000378:metabolism; D010545:Peroxides / Q000378:metabolism; D013379:Substrate Specificity; D014443:Tyrosine / Q000737:chemistry / Q000378:metabolism
chemical_list: D000466:Alkadienes; D016861:Cyclooxygenase Inhibitors; D005609:Free Radicals; D010545:Peroxides; D016718:Arachidonic Acid; D014443:Tyrosine; D051545:Cyclooxygenase 1; D051546:Cyclooxygenase 2; D010100:Oxygen