HNO to NO Conversion Mechanism with Copper Zinc Superoxide Dismutase, Comparison with Heme Protein Mediated Conversions, and the Origin of Questionable Reversibility

The interconversion of NO and HNO, via copper zinc superoxide dismutase (CuZnSOD), is important in biomedicine and for HNO detection. Many mechanistic questions, including the decades-long debate on reversibility, were resolved in this work. Calculations of various active-site and full-protein models show that the basic mechanism is proton-coupled electron transfer with a computed barrier of 10.98 kcal mol(-1), which is in excellent agreement with experimental results (10.62 kcal mol(-1)), and this nonheme protein-mediated reaction has many significant mechanistic differences compared with the conversions mediated by heme proteins due to geometric and electronic factors. The reasons for the irreversible nature of this conversion and models with the first thermodynamically favorable and kinetically feasible mechanism for the experimental reverse reaction were discovered. Such results are the first for nonheme enzyme mediated HNO to NO conversions, which shall facilitate other related studies and HNO probe development.

Automated summary

This work elucidates the basic mechanism of the nonheme enzyme-mediated conversion of HNO to NO, resolving longstanding debates on reversibility and uncovering the reasons for the irreversible nature of this conversion. The study also identifies the first thermodynamically favorable and kinetically feasible mechanism for the experimental reverse reaction, providing insights that will aid in further related studies and the development of HNO probes.