title: Theoretical study of the catalytic reaction mechanism of MndD.
abstract: Manganese-dependent homoprotocatechuate 2,3-dioxygenase (MndD) is an enzyme taking part in the catabolism of aromatic compounds in the environment. It uses molecular oxygen to perform an extradiol cleavage of the ring of the ortho-dihydroxylated aromatic compound homoprotocatechuate. A theoretical investigation of the reaction path for MndD was performed using hybrid density functional theory with the B3LYP functional, and a catalytic mechanism has been suggested. Models of different size were built from the crystal structure of the enzyme and were used in the search for intermediates and transition states. It was found that the substrate first binds at the active site as a monoanion. Next the dioxygen is bound, forming a hydroperoxo intermediate. The O-O bond, activated in this way undergoes homolytic cleavage leading to an oxyl and then to an extra epoxide radical with subsequent opening of the aromatic ring. The lactone ring is then hydrolyzed by the Mn-bound OH group, and the final product is obtained in the last reaction steps. Alternative reaction paths were considered, and their calculated barriers were found to be higher than for the suggested mechanism. The selectivity between the extra- and intra-cleavage pathways was found to be determined by the barriers for the decay of the radical state.
authors.affiliation: Department of Physics, Stockholm Center for Physics, Astronomy and Biotechnology, Stockholm University, 106 91, Stockholm, Sweden. valyo@physto.se
authors.forename: Valentin
authors.initials: V
authors.lastname: Georgiev
authors.forename: Tomasz
authors.initials: T
authors.lastname: Borowski
authors.forename: Per E M
authors.initials: PE
authors.lastname: Siegbahn
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D049308:Dioxygenases / Q000737:chemistry*; D008345:Manganese / Q000737:chemistry; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry; D013379:Substrate Specificity
chemical_list: D008345:Manganese; D049308:Dioxygenases; C020055:3,4-dihydroxyphenylacetate 2,3-dioxygenase; D010100:Oxygen