Oxygen-dependent conformation change of hemoglobin modulates methylmercury binding

Methylmercury is a highly toxic that accumulates through the food chain, posing significant health risk to humans, particularly via fish consumption. Methylmercury bioaccumulation in fish is regulated by the ingestion from the environment and food sources and subsequent transport and distribution inside the fish body, with studies lacking for the latter. Hemoglobin is considered one of the key proteins that bind and transport methylmercury in organisms. Due to the function of transporting oxygen, hemoglobin's conformation is closely linked to oxygen content. However, there is limited study on the impact of oxygen content on the binding of methylmercury to hemoglobin. Herein, techniques such as fluorescence spectroscopy, isothermal titration calorimetry, circular dichroism spectroscopy, PyMOL visualization system, and molecular docking were employed to explore the effects of oxygen content on binding of methylmercury to hemoglobin. Significantly lower amounts of methylmercury bound to hemoglobin under hypoxic conditions were measured compared to normoxic conditions, suggesting a reduction in binding capacity. Stern-Volmer fitting and isothermal titration calorimetry results showed that binding of methylmercury to hemoglobin under normoxic condition was more spontaneously than that under hypoxic condition. Finally, circular dichroism spectroscopy, PyMOL's built-in solvent accessible surface area calculation, and molecular docking indicated that conformation changes occurred in hemoglobin which affected binding capacity of methylmercury to hemoglobin under different oxygen conditions. These findings revealed the effects of oxygen content on hemoglobin-methylmercury binding, providing an insight into the transport, distribution, and toxicity of methylmercury in organisms and a new perspective on studying methylmercury accumulation under varying oxygen conditions.