| Abstract
| - The ferriheme protein metmyoglobin (metMb) in buffer solution at physiological pH 7.4 reversiblybinds the biomessenger molecule nitric oxide to yield the nitrosyl adduct (metMb(NO)). The kinetics of theassociation and dissociation processes were investigated by both laser flash photolysis and stopped-flow kineticstechniques at ambient and high pressure, in three laboratories using several different sources of metMb. Theactivation parameters ΔH⧧, ΔS⧧, and ΔV⧧ were calculated from the kinetic effects of varying temperature andhydrostatic pressure. For the “on” reaction of metMb plus NO, reasonable agreement was found between thevarious techniques with ΔHon⧧, ΔSon⧧, and ΔVon⧧ determined to have the respective values ∼65 kJ mol-1, ∼60J mol-1 K-1, and ∼20 cm3 mol-1. The large and positive ΔS⧧ and ΔV⧧ values are consistent with the operationof a limiting dissociative ligand substitution mechanism whereby dissociation of the H2O occupying the sixthdistal coordination site of metMb must precede formation of the Fe−NO bond. While the activation enthalpiesof the “off” reaction displayed reasonable agreement between the various techniques (ranging from 68 to 83kJ mol-1), poorer agreement was found for the ΔSoff⧧ values. For this reason, the kinetics for the “off” reactionwere determined more directly via NO trapping experiments, which gave the respective activation parametersΔHoff⧧ = 76 kJ mol-1, ΔSoff⧧ = ∼41 J mol-1 K-1, and ΔVoff⧧ = 20 cm3 mol-1), again consistent with alimiting dissociative mechanism. These results are discussed in reference to other investigations of the reactionsof NO with both model systems and metalloproteins.
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