The conversion between the biological signaling agent NO and its sibling molecule HNO mediated by CuZnSOD has been known for several decades. However, mechanistic details are largely unknown and the reversibility was long debated. A computational study was performed using DFT calculations of active site models with varying sizes and hybrid quantum mechanics and molecular mechanics (QM/MM) calculations of full protein models. The basic HNO to NO conversion mechanism was found to be proton-coupled electron transfer (PCET), with a proton transferred to the bridging His residue and an electron moved to Cu. Besides coordinating residues, the Asp residue linking Cu/Zn sites was also found to be critical in this process, with the computed Gibbs free energy of activation (10.98 kcal/mol) in excellent agreement with experiment (10.62 kcal/mol). This non-heme protein mediated reaction has many significant mechanistic differences compared to the conversions mediated by heme proteins due to geometric and electronic factors. Reasons of the irreversible nature of this conversion and models with the first thermodynamically favorable and kinetically feasible mechanism for the experimental seemingly “reversible” phenomenon were discovered. Such results are the first for non-heme enzyme mediated HNO to NO conversions, which shall facilitate other related studies and HNO probe development.

Mechanistic investigation of HNO to NO conversion via copper zinc superoxide dismutase (CuZnSOD)

ACS Spring Meeting 2021

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