A Novel Degradation Mechanism for Pyridine Derivatives in Alcaligenes faecalis JQ135.

5-Hydroxypicolinic acid (5HPA), a natural pyridine derivative, is microbially degraded in the environment. However, the physiological, biochemical, and genetic foundations of 5HPA metabolism remain unknown. In this study, an operon (), responsible for 5HPA degradation, was cloned from JQ135. HpaM was a monocomponent flavin adenine dinucleotide (FAD)-dependent monooxygenase and shared low identity (only 28 to 31%) with reported monooxygenases. HpaM catalyzed the decarboxylative hydroxylation of 5HPA, generating 2,5-dihydroxypyridine (2,5DHP). The monooxygenase activity of HpaM was FAD and NADH dependent. The apparent values of HpaM for 5HPA and NADH were 45.4 μM and 37.8 μM, respectively. The genes , , and were found to encode 2,5DHP dioxygenase, -formylmaleamic acid deformylase, and maleamate amidohydrolase, respectively; however, the three genes were not essential for 5HPA degradation in JQ135. Furthermore, the gene , which encodes a maleic acid isomerase, was essential for the metabolism of 5HPA, nicotinic acid, and picolinic acid in JQ135, indicating that it might be a key gene in the metabolism of pyridine derivatives. The genes and proteins identified in this study showed a novel degradation mechanism of pyridine derivatives. Unlike the benzene ring, the uneven distribution of the electron density of the pyridine ring influences the positional reactivity and interaction with enzymes; e.g., the and oxidations are more difficult than the oxidations. Hydroxylation is an important oxidation process for the pyridine derivative metabolism. In previous reports, the hydroxylations of pyridine derivatives were catalyzed by multicomponent molybdenum-containing monooxygenases, while the hydroxylations were catalyzed by monocomponent FAD-dependent monooxygenases. This study identified the new monocomponent FAD-dependent monooxygenase HpaM that catalyzed the decarboxylative hydroxylation of 5HPA. In addition, we found that the gene coding for maleic acid isomerase was pivotal for the metabolism of 5HPA, nicotinic acid, and picolinic acid in JQ135. This study provides novel insights into the microbial metabolism of pyridine derivatives.