Structural Insights into 6-Hydroxypseudooxynicotine Amine Oxidase from N1, the Key Enzyme Involved in Nicotine Degradation.

Bacteria degrade nicotine mainly using pyridine and pyrrolidine pathways. Previously, we discovered a hybrid of the pyridine and pyrrolidine pathways (the VPP pathway) in N1 and characterized its key enzyme, 6-hydroxypseudooxynicotine amine oxidase (HisD). It catalyzes oxidative deamination of 6-hydroxypseudooxynicotine to 6-hydroxy-3-succinoylsemialdehyde-pyridine, which is the crucial step connecting upstream and downstream portions of the VPP pathway. We determined the crystal structure of wild-type HisD to 2.6 Å. HisD is a monomer that contains a flavin mononucleotide, an iron-sulfur cluster, and ADP. On the basis of sequence alignment and structure comparison, a difference has been found among HisD, closely related trimethylamine dehydrogenase (TMADH), and histamine dehydrogenase (HADH). The flavin mononucleotide (FMN) cofactor is not covalently bound to any residue, and the FMN isoalloxazine ring is planar in HisD compared to TMADH or HADH, which forms a 6--cysteinyl flavin mononucleotide cofactor and has an FMN isoalloxazine ring in a "butterfly bend" conformation. Based on the structure, docking study, and site-directed mutagenesis, the residues Glu60, Tyr170, Asp262, and Trp263 may be involved in substrate binding. The expanded understanding of the substrate binding mode from this study may guide rational engineering of such enzymes for biodegradation of potential pollutants or for bioconversion to generate desired products. Nicotine is a major tobacco alkaloid in tobacco waste. Pyridine and pyrrolidine pathways are the two best-elucidated nicotine metabolic pathways; N1 catabolizes nicotine via a hybrid between the pyridine and pyrrolidine pathways. The crucial enzyme, 6-hydroxypseudooxynicotine amine oxidase (HisD), links the upstream and downstream portions of the VPP pathway; however, there is little structural information about this important enzyme. In this study, we determined the crystal structure of HisD from N1. Its basic insights about the structure may help us to guide the engineering of such enzymes for bioremediation and bioconversion applications.