title: Experimental and computational investigations of oxygen reactivity in a heme and tyrosyl radical-containing fatty acid α-(di)oxygenase.
abstract: Rice α-(di)oxygenase mediates the regio- and stereospecific oxidation of fatty acids using a persistent catalytic tyrosyl radical. Experiments conducted in the physiological O(2) concentration range, where initial hydrogen atom abstraction from the fatty acid occurs in a kinetically reversible manner, are described. Our findings indicate that O(2)-trapping of an α-carbon radical is likely to reversibly precede reduction of a 2-(R)-peroxyl radical intermediate in the first irreversible step. A mechanism of concerted proton-coupled electron transfer is proposed on the basis of natural abundance oxygen-18 kinetic isotope effects, deuterium kinetic isotope effects, and calculations at the density functional level of theory, which predict a polarized transition state in which electron transfer is advanced to a greater extent than proton transfer. The approach outlined should be useful for identifying mechanisms of concerted proton-coupled electron transfer in a variety of oxygen-utilizing enzymes.
authors.affiliation: Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States.
authors.forename: Gregory S
authors.initials: GS
authors.lastname: Huff
authors.forename: Irina S
authors.initials: IS
authors.lastname: Doncheva
authors.forename: David W
authors.initials: DW
authors.lastname: Brinkley
authors.forename: Alfredo M
authors.initials: AM
authors.lastname: Angeles-Boza
authors.forename: Arnab
authors.initials: A
authors.lastname: Mukherjee
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Cramer
authors.forename: Justine P
authors.initials: JP
authors.lastname: Roth
mesh_terms: D001667:Binding, Competitive; D004058:Diffusion; D049308:Dioxygenases / Q000737:chemistry* / Q000378:metabolism*; D004579:Electron Transport; D005227:Fatty Acids / Q000737:chemistry / Q000378:metabolism*; D005609:Free Radicals; D006418:Heme; D007700:Kinetics; D008958:Models, Molecular; D012275:Oryza / Q000201:enzymology; D010100:Oxygen / Q000378:metabolism*; D010103:Oxygen Isotopes / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry; D013237:Stereoisomerism; D013379:Substrate Specificity; D014443:Tyrosine; D014783:Viscosity
chemical_list: D005227:Fatty Acids; D005609:Free Radicals; D010103:Oxygen Isotopes; D012997:Solvents; D014443:Tyrosine; D006418:Heme; D049308:Dioxygenases; D010100:Oxygen