A theoretical study of myoglobin working as a nitric oxide scavenger

The mechanism for the reaction between nitric oxide (NO) and O(2) bound to the heme iron of myoglobin (Mb), including the following isomerization to nitrate, has been investigated using hybrid density functional theory (B3LYP). Myoglobin working as a NO scavenger could be of importance, since NO reversibly inhibits the terminal enzyme in the respiration chain, cytochrome c oxidase. The concentration of NO in the cell will thus affect the respiration and thereby the synthesis of ATP. The calculations show that the reaction between NO and the heme-bound O(2) gives a peroxynitrite intermediate whose O-O bond undergoes a homolytic cleavage, forming a NO(2) radical and myoglobin in the oxo-ferryl state. The NO(2) radical then recombines with the oxo-ferryl, forming heme-bound nitrate. Nine different models have been used in the present study to examine the effect on the reaction both by the presence and the protonation state of the distal His64, and by the surroundings of the proximal His93. The barriers going from the oxy-Mb and nitric oxide reactant to the peroxynitrite intermediate and further to the oxo-ferryl and NO(2) radical are around 10 and 7 kcal/mol, respectively. Forming the product, nitrate bound to the heme iron has a barrier of less than approximately 7 kcal/mol. The overall reaction going from a free nitric oxide and oxy-Mb to the heme bound nitrate is exergonic by more than 30 kcal/mol.