Developing a Mechanistic Proposal for 2,5-Dihydroxypyridine-5,6-Dioxygenase (NicX) from Bordetella bronchiseptica
Several N-heterocyclic aromatic compounds are known environmental contaminants and many are known to be toxic or carcinogenic. The study of a model system of N-heterocyclic aromatic compound degradation is key to understanding the breakdown of this class as a whole in hopes to develop bioremedial strategies to address these environmental issues. The aerobic catabolism of nicotinic acid is considered to be a model system to study these processes. Nicotinic acid breakdown is predicted to be a modulating factor of Bordetella pertussis infection; thus, an understanding of the enzymes involved in this pathway could also provide targets for drug design to prevent these infections. Specifically the role of the predicted nicX gene in the genome of the related Bordetella bronchiseptica was probed in this study. It was shown by the use of mass spectrometry that this gene encodes an enzyme responsible for the conversion of 2,5-dihydroxypyridine to N-formylmaleamic acid by a novel type of dioxygenase, 2,5-dihydroxypyridine-5,6-dioxygenase (NicX). Additionally, it was found using two techniques that NicX exists in an equilibrium between dimer, hexamer and dodecamer oligomeric states (a 24-mer oligomeric state may also exist). The stabilization of NicX activity has presented experimental challenges; hypothesized to be due to the oxidization of active site Fe2+ to Fe3+. This instability makes kinetic and mechanistic studies exceedingly difficult to perform. However, based on mechanisms of extradiol dioxygenases, a mechanistic proposal for the reaction catalyzed by NicX is suggested here. The first several steps of this proposal are similar to the mechanisms for extradioldioxygenases; however, it diverges to potentially novel chemical transformations, stablishing NicX as the founding member of a new class of dioxygenases.
© Copyright 2011 Matthew Henke