An active site mutation in 6-hydroxy-l-Nicotine oxidase from Arthrobacter nicotinovorans changes the substrate...

The enzyme 6-Hydroxy-l-Nicotine oxidase (HLNO) is a flavin-dependent enzyme that catalyzes the first step in the pyridine pathway of oxidation of nicotine as a source of energy and nitrogen in several bacteria. Recombinant Arthrobacter nicotinovorans HLNO also catalyzes oxidation of (s)-nicotine at a low but measurable rate (Fitzpatrick et al., 2016, Biochemistry 55, 697-703). Rational design and bioinformatics approaches, based on the known high-resolution structure of this enzyme (RCSB: 3NG7), were employed to further enhance the catalytic turnover and stability of the enzyme using (S)-nicotine as substrate. The active site residue Tyr311 forms a hydrogen bond with the hydroxyl group of (S)-6-OH-nicotine within the catalytic pocket. Its replacement by a tryptophan residue reduced the k_cat for (S)-6-OH-nicotine by more than 6-fold and increased ~1.5-fold. Combining this mutation with two surface mutations that were predicted to enhance enzyme stability, further increased the k_cat for nicotine resulting in a comparatively robust oxidation of (s)-nicotine (k_cat >1 s^-1) at 37 °C, at the same time reducing the specificity for (S)-OH-nicotine (k_cat/K_M) by more than 100-fold and increasing that for (S)-nicotine by more than 2-fold. Interestingly, adding a maltose-binding protein (MBP) tag onto the N-terminus of HLNO markedly increased the thermal stability of the enzyme, extending the half-life at 37 °C from ~2 h to ~22 h. This effect was due almost entirely to increased FAD retention, an observation that may prove useful to improve flavin retention in other flavin-dependent monoamine oxidases.