Potential Energy Surface and Molecular Dynamics of MbNO: Existence of an Unsuspected FeON Minimum

Ligands such as CO, O2, or NO are involved in the biol. function of myoglobin.  Here we investigate the energetics and dynamics of NO interacting with the Fe(II) heme group in native myoglobin using ab initio and mol. dynamics simulations.  At the global min. of the ab initio potential energy surface (PES), the binding energy of 23.4 kcal/mol and the Fe-NO structure compare well with the exptl. results.  Interestingly, the PES is found to exhibit two min.: There exists a metastable, linear Fe-O-N min. in addn. to the known, bent Fe-N-O global min. conformation.  Moreover, the T-shaped configuration is found to be a saddle point, in contrast to the corresponding min. for NO interacting with Fe(III).  To use the ab initio results for finite temp. mol. dynamics simulations, an anal. function was fitted to represent the Fe-NO interaction.  The simulations show that the secondary min. is dynamically stable up to 250 K and has a lifetime of several hundred picoseconds at 300 K.  The difference in the topol. of the heme-NO PES from that assumed previously (one deep, single Fe-NO min.) suggests that it is important to use the full PES for a quant. understanding of this system.  Why the metastable state has not been obsd. in the many spectroscopic studies of myoglobin interacting with NO is discussed, and possible approaches to finding it are outlined.