Search

Flavoprotein monooxygenases catalyze reactions, including hydroxylation and epoxidation, involved in the catabolism, detoxification, and biosynthesis of natural substrates and industrial contaminants. Among them, the 6-hydroxy-3-succinoyl-pyridine (HSP) monooxygenase (HspB) from Pseudomonas putida S16 facilitates the hydroxylation and C-C bond cleavage of the pyridine ring in nicotine. However, the mechanism for biodegradation remains elusive. Here, we refined the crystal structure of HspB and elucidated the detailed mechanism behind the oxidative hydroxylation and C-C cleavage processes. Leveraging structural information about domains for binding the cofactor flavin adenine dinucleotide (FAD) and HSP substrate, we used molecular dynamics simulations and quantum/molecular mechanics calculations to demonstrate that the transfer of an oxygen atom from the reactive FAD peroxide species (C4a-hydroperoxyflavin) to the C3 atom in the HSP substrate constitutes a rate-limiting step, with a calculated reaction barrier of about 20 kcal/mol. Subsequently, the hydrogen atom was rebounded to the FAD cofactor, forming C4a-hydroxyflavin. The residue Cys218 then catalyzed the subsequent hydrolytic process of C-C cleavage. Our findings contribute to a deeper understanding of the versatile functions of flavoproteins in the natural transformation of pyridine and HspB in nicotine degradation.IMPORTANCE Pseudomonas putida S16 plays a pivotal role in degrading nicotine, a toxic pyridine derivative that poses significant environmental challenges. This study highlights a key enzyme, HspB (6-hydroxy-3-succinoyl-pyridine monooxygenase), in breaking down nicotine through the pyrrolidine pathway. Utilizing dioxygen and a flavin adenine dinucleotide cofactor, HspB hydroxylates and cleaves the substrate's side chain. Structural analysis of the refined HspB crystal structure, combined with state-of-the-art computations, reveals its distinctive mechanism. The crucial function of Cys218 was never discovered in its homologous enzymes. Our findings not only deepen our understanding of bacterial nicotine degradation but also open avenues for applications in both environmental cleanup and pharmaceutical development., Competing Interests: The authors declare no conflict of interest.

The aim of the study was to assess the relationship between underweight and falls. It was a cross-sectional study in community-based participants from China Hainan Centenarian Cohort Study (CHCCS). A total of 942 centenarians (mean ages were 102.4 years in men and 102.9 years in women, 21.8% were semi-supercentenarians, and 3.2% were supercentenarians) were enrolled from July 2014 to December 2016. Height and weight were measured according to the standard protocol. Participants were interviewed face to face to self-report falls for recent 3 months. The risks of future falls were evaluated by Morse Fall Scale (MFS). The mean body mass index (BMI) was 19.04±2.79 kg/m2 in men and 18.07±3.27 kg/m2 in women. The 3-month incidences of once fall were 10.3% in men and 13.4% in women, recurrent falls were 15.4% in men and 13.4% in women, future high risks of falls were 36.6% in men and 44.3% in women. There were underweight by gender interactions on at least one fall (OR=1.39, 95% CI=1.04-1.86, P=0.026) and future high fall risk (OR=1.39, 95% CI=1.07-1.80, P=0.014). Men with underweight had higher ratios of at least one fall (OR=4.12, 95% CI=1.59-10.70, P=0.004), recurrent falls (OR=6.71, 95% CI=1.69-26.63, P=0.007) and high risk of future falls (OR=2.44, 95% CI=1.27-4.70, P=0.007), while women with underweight was only associated with high risk of future falls (OR=1.37, 95% CI=1.01-1.85, P=0.043). Compared with normal weight and overweight, underweight in men had a higher MFS Score (increased by average 7.6 points, P=0.010), yet this trend was not observed in women counterparts. Underweight was associated with incidences of 3-month and future high fall risk in centenarians, which was more pronounced in men. Underweight could be a marker in identifying potential falls in long-lived people, especially men, who need further measures to gaining weight to normal and preventing falling., Competing Interests: The authors declare that they have no competing interests.