title: Structures of two histidine ammonia-lyase modifications and implications for the catalytic mechanism.
abstract: Histidine ammonia-lyase (EC 4.3.1.3) catalyzes the nonoxidative elimination of the alpha-amino group of histidine using a 4-methylidene-imidazole-5-one (MIO), which is formed autocatalytically from the internal peptide segment 142Ala-Ser-Gly. The structure of the enzyme inhibited by a reaction with l-cysteine was established at the very high resolution of 1.0 A. Five active center mutants were produced and their catalytic activities were measured. Among them, mutant Tyr280-->Phe could be crystallized and its structure could be determined at 1.7 A resolution. It contains a planar sp2-hybridized 144-N atom of MIO, in contrast to the pyramidal sp3-hybridized 144-N of the wild-type. With the planar 144-N atom, MIO assumes the conformation of a putative intermediate aromatic state of the reaction, demonstrating that the conformational barrier between aromatic and wild-type states is very low. The data led to a new proposal for the geometry for the catalyzed reaction, which also applies to the closely related phenylalanine ammonia-lyase (EC 4.3.1.5). Moreover, it suggested an intermediate binding site for the released ammonia.
authors.affiliation: Institut für Organische Chemie und Biochemie, Albert-Ludwigs-Universität, Freiburg im Breisgau, Germany.
authors.forename: Mathias
authors.initials: M
authors.lastname: Baedeker
authors.forename: Georg E
authors.initials: GE
authors.lastname: Schulz
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D006638:Histidine Ammonia-Lyase / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D016296:Mutagenesis; D011487:Protein Conformation; D014961:X-Ray Diffraction
chemical_list: D006638:Histidine Ammonia-Lyase