Reaction mechanism of homoprotocatechuate 2,3-dioxygenase with 4-nitrocatechol: implications for the role of substrate.
The reaction mechanism of the dioxygen activation by homoprotocatechuate 2,3-dioxygenase (HPCD) with the substrate 4-nitrocatechol was investigated by quantum mechanical/molecular mechanical calculations. Our results demonstrated that the experimentally determined side-on iron-oxygen complex in crystallo is a semiquinone substrate radical (SQ(•))-Fe(III)-hydroperoxo species, which could not act as the reactive species. In fact, the Fe(III)-superoxo species with a hydrogen bond between His200 and the proximal oxygen is the reactive oxygen species. The second-sphere His200 residue was found to play an important role in manipulating the orientation of the superoxide in the Fe-O2 adduct for the further reaction. The rate-limiting step is the attack of the superoxo group on the substrate with a barrier of 17.2 kcal/mol, in good agreement with the experimental value of 16.8 kcal/mol. The reaction mechanism was then compared with the one for HPCD with its native substrate homoprotocatechuate studied recently by the same methods, in which a hybrid SQ(•)-Fe(II)-O2(•-)/Fe(III)-O2(•-) was suggested to be the reactive species. Therefore, our studies suggested that the substrate plays important roles in the dioxygen activation by HPCD.
Department of Chemistry, Renmin University of China , Beijing 100872, China.
Geng
G
Dong
Wenzhen
W
Lai
D002384:Catalysis; D002396:Catechols / Q000737:chemistry*; D003198:Computer Simulation; D049308:Dioxygenases / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry; D006860:Hydrogen Bonding; D007700:Kinetics; D008958:Models, Molecular; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry; D011789:Quantum Theory; D017382:Reactive Oxygen Species / Q000737:chemistry; D012984:Software; D013481:Superoxides / Q000737:chemistry
D002396:Catechols; D017382:Reactive Oxygen Species; D013481:Superoxides; D006639:Histidine; D049308:Dioxygenases; C020055:3,4-dihydroxyphenylacetate 2,3-dioxygenase; D010100:Oxygen; C001833:4-nitrocatechol