title: A mechanism for hydroxylation by tyrosine hydroxylase based on partitioning of substituted phenylalanines.
abstract: The iron-containing enzyme tyrosine hydroxylase catalyzes the hydroxylation of tyrosine to dihydroxyphenylalanine. A series of 4-X-substituted (X = H, F, Br, Cl, CH3, or CH3O) phenylalanines have been characterized as substrates to gain insight into the mechanism of hydroxylation. Multiple hydroxylated products were formed in most cases. As the size of the substituent at the 4-position increased, the site of hydroxylation switched from the 4- to the 3-position of the aromatic ring. The total amount of product formed with each amino acid showed a very good correlation with the sigma parameter of the substituent, with rho values of -4.3 +/- 0.7 or -5.6 +/- 0.8 when tetrahydrobiopterin or 6-methyltetrahydropterin, respectively, was used as cosubstrate. These values are consistent with a highly electron deficient transition state for hydroxylation. Oxygen addition at the 4-position resulted in either elimination of the substituent to form tyrosine or an NIH shift to form the respective 3-X-tyrosine. The relative amount of the product due to an NIH shift decreased in the order Br > CH3 > Cl >> F approximately CH3O approximately 0. A chemical mechanism for hydroxylation by tyrosine hydroxylase is presented to account for product formation from the various 4-substituted phenylalanines.
authors.affiliation: Department of Biochemistry and Biophysics, Texas A&M University, College Station 77843, USA.
authors.forename: P J
authors.initials: PJ
authors.lastname: Hillas
authors.forename: P F
authors.initials: PF
authors.lastname: Fitzpatrick
mesh_terms: D000818:Animals; D004295:Dihydroxyphenylalanine / Q000378:metabolism; D010134:Fenclonine / Q000378:metabolism; D006900:Hydroxylation; D007700:Kinetics; D008956:Models, Chemical; D015394:Molecular Structure; D010100:Oxygen / Q000378:metabolism; D010649:Phenylalanine / Q000031:analogs & derivatives* / Q000378:metabolism*; D011622:Pterins / Q000378:metabolism; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D013379:Substrate Specificity; D014446:Tyrosine 3-Monooxygenase / Q000378:metabolism*; D010135:p-Fluorophenylalanine / Q000378:metabolism
chemical_list: D011622:Pterins; D011994:Recombinant Proteins; C011552:tetrahydropterin; D010649:Phenylalanine; D010135:p-Fluorophenylalanine; D004295:Dihydroxyphenylalanine; D014446:Tyrosine 3-Monooxygenase; D010134:Fenclonine; D010100:Oxygen