Structural insights into the role of NahX from Pseudomonas sp. MC1 in the naphthalene degradation pathway

Polycyclic aromatic hydrocarbons (PAHs) are among the most widespread organic pollutants known for their carcinogenic and mutagenic properties. There is a growing interest in understanding the degradation and detoxification processes of these substances using biological approaches. The bacterium Pseudomonas sp. MC1 contains a metabolic plasmid (81 kb) that encodes enzymes involved in the conversion of naphthalene (the simplest and most soluble PAH) to salicylate. Therein, nahX is a part of the lower naphthalene degradation operon and encodes a 140-amino acid protein. However, the function of NahX remains unclear. To understand its function more clearly, we first determined the three-dimensional structure of NahX. It has a fold similar to that of HbpS, which acts as a sensory component in response to oxidative stress. Biochemical studies have also shown that NahX and HbpS exhibit heme degradation activity and bind to iron ions. Heme degradation and iron-sequestering activity protect bacteria against oxidative stress. Previous studies have shown that oxidative stress occurs during naphthalene degradation. Therefore, we postulate that NahX has a defense mechanism against the oxidative stress that may occur during naphthalene metabolism.