Download MendeleyCrystallography Coupled with Kinetic Analysis Provides Mechanistic Underpinnings of a Nicotine-Degrading Enzyme.
Tararina, M.A., Xue, S., Smith, L.C., Muellers, S.N., Miranda, P.O., Janda, K.D., Allen, K.N.(2018) Biochemistry 57: 3741-3751
- PubMed: 29812904
- DOI: https://doi.org/10.1021/acs.biochem.8b00384
- Primary Citation of Related Structures:
6C71 - PubMed Abstract:
Nicotine oxidoreductase (NicA2) is a bacterial flavoenzyme, which catalyzes the first step of nicotine catabolism by oxidizing S-nicotine into N-methyl-myosmine. It has been proposed as a biotherapeutic for nicotine addiction because of its nanomolar substrate binding affinity. The first crystal structure of NicA2 has been reported, establishing NicA2 as a member of the monoamine oxidase (MAO) family. However, substrate specificity and structural determinants of substrate binding and/or catalysis have not been explored. Herein, analysis of the pH-rate profile, single-turnover kinetics, and binding data establish that pH does not significantly affect the catalytic rate and product release is not rate-limiting. The X-ray crystal structure of NicA2 with S-nicotine refined to 2.65 Å resolution reveals a hydrophobic binding site with a solvent exclusive cavity. Hydrophobic interactions predominantly orient the substrate, promoting the binding of a deprotonated species and supporting a hydride-transfer mechanism. Notably, NicA2 showed no activity against neurotransmitters oxidized by the two isoforms of human MAO. To further probe the substrate range of NicA2, enzyme activity was evaluated using a series of substrate analogues, indicating that S-nicotine is the optimal substrate and substitutions within the pyridyl ring abolish NicA2 activity. Moreover, mutagenesis and kinetic analysis of active-site residues reveal that removal of a hydrogen bond between the pyridyl ring of S-nicotine and the hydroxyl group of T381 has a 10-fold effect on K M , supporting the role of this bond in positioning the catalytically competent form of the substrate. Together, crystallography combined with kinetic analysis provides a deeper understanding of this enzyme's remarkable specificity.
Organizational Affiliation:
Program in Biomolecular Pharmacology , Boston University School of Medicine , 72 East Concord Street , Boston , Massachusetts 02118 , United States.
