title: Chemical rescue of the distal histidine mutants of tryptophan 2,3-dioxygenase.
abstract: Tryptophan 2,3-dioxygenase (TDO) is a heme-dependent enzyme that catalyzes the oxidative degradation of L-tryptophan (L-Trp) to N-formylkynurenine (NFK). A highly conserved histidine residue in the distal heme pocket has attracted great attention in the mechanistic studies of TDO. However, a consensus has not been reached regarding whether and how this distal histidine plays a catalytic role after substrate binding. In this study, three mutant proteins, H72S, H72N, and Q73F were generated to investigate the function of the distal histidine residue in Cupriavidus metallidurans TDO (cmTDO). Spectroscopic characterizations, enzymatic kinetic analysis, and chemical rescue assays were employed to study the biochemical properties of the wild-type enzyme and the mutant proteins. Rapid kinetic methods were utilized to explore the molecular basis for the observed stimulation of catalytic activity by 2-methylimidazole in the His72 variants. The results indicate that the distal histidine plays multiple roles in cmTDO. First, His72 contributes to but is not essential for substrate binding. In addition, it shields the heme center from nonproductive binding of exogenous small ligand molecules (i.e., imidazole and its analogs) via steric hindrance. Most importantly, His72 participates in the subsequent chemical catalytic steps after substrate binding possibly by providing H-bonding interactions to the heme-bound oxygen.
authors: Department of Chemistry & Center for Diagnostics and Therapeutics, Georgia State University, 161 Jesse Hill Jr. Drive, Atlanta, Georgia 30303, USA.
authors: Jiafeng
authors: J
authors: Geng
authors: Kednerlin
authors: K
authors: Dornevil
authors: Aimin
authors: A
authors: Liu
mesh_terms: D001665:Binding Sites; D020134:Catalytic Domain; D049921:Cupriavidus / Q000737:chemistry / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism; D006418:Heme / Q000378:metabolism; D006639:Histidine / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D007093:Imidazoles / Q000737:chemistry / Q000378:metabolism; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D011487:Protein Conformation; D013379:Substrate Specificity; D014366:Tryptophan Oxygenase / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*
chemical_list: D007093:Imidazoles; D006418:Heme; D006639:Histidine; C029899:imidazole; D014366:Tryptophan Oxygenase; C032655:2-methylimidazole